============================================ PARTIAL SPECIFICATION OF THE XCF FILE FORMAT ============================================ Copyright Henning Makholm , 2006-07-11 This is free documentation; you can modify and/or redistribute it according to the terms of the GNU General Public License as published by the Free Software Foundation, either version 2 of the license, or (at your option) any later version. --------------------------------------------- T H I S I S A D R A F T O N L Y ! --------------------------------------------- This is the native image file format of the GIMP. Beware that CinePaint's native file format is called XCF too. While the latter is derived from the format described here, the two formats differ in many details and are not mutually compatible. The XCF format is designed to store the entire part of the state of the Gimp that is specific to one image (i.e., not the cut buffer, tool options, key bindings, etc.) and is not undo data. This makes the full collection of data stored in an XCF file rather heterogeneous and tied to the internals of the Gimp. Use of the format by third-party software is recommended only as a way to get data into and out of the Gimp for which it would be impossible or inconvenient to use a more standard interchange format. Authors of third-party XCF-creating software in particular should take care to write files that are as indistinguishable as possible from ones saved by the Gimp. The Gimp developers take care to make each version of the Gimp able to read XCF files produced by older versions, but they make no special efforts to allow reading of files created by other software that attempt to extrapolate from the Gimp source. The name -------- The name XCF honors the Gimp's origin at the eXperimental Computing Facility of the University of California at Berkeley. Status of this document ----------------------- This specification is an unofficial condensation and extrapolation of the XCF-writing and -reading code in version 2.2.11 of the Gimp. As of this writing, it has not been approved or proofread by any Gimp developer, though it has been written with the intention of contributing it to the Gimp project for use as official documentation. Some of the normative statements made below are enforced by the XCF reader in the Gimp; others are just the author's informed guess about "best practices" that would be likely to maximize interoperability with future versions of the Gimp. 1. BASIC DATA MODEL =================== It is recommended that a software developer who wants to take full advantage of the XCF format be deeply familiar with the Gimp at least as a user. The following high-level overview is meant to help those non-users who just need to extract pixel data from an XCF file get up to speed. In general an XCF file describes a stack of _layers_ and _channels_ on a _canvas_, which is just an abstract rectangular viewport for the layers and channels. Layers ------ A layer is a named rectangular area of pixels which has a definite position with respect to the canvas. A layer may extend beyond the canvas or (more commonly) only cover some of it. Each pixel of the layer has a color which is specified in one of three ways: RGB: Three intensity values for red, green, and blue additive color components, each on a scale from 0 to 255. The exact color space is not specified. The Gimp displays image data directly on PC display hardware without any software correction, so in most cases the intensity values should be considered nonlinear samples that map to physical light intensities using a power function with an exponent ("gamma") of about 2.5. (This is how PC hardware commonly treat bit values in the video buffer, which incidentally also has the property of making each 1/255th step about equally perceptible to the human eye when the monitor is correctly adjusted). Beware, however, that the Gimp's compositing algorithms (as described in Section 8 below) implicitly treat the intensities as _linear_ samples. The XCF file format currently has no support for storing the intended gamma of the samples. Grayscale: One intensity value on a scale from 0 (black) to 255 (white). Gamma considerations as for RIB. Indexed: An 8-bit index into a colormap that is shared between all layers. The colormap maps each index to an RGB triple which is interpreted as in the RGB model. All layers in an image must use the same color model. Exception: if the "floating selection" belongs to a channel or layer mask, it will be represented as grayscale pixels independently of the image's overall color model. Each pixel of a layer also has an alpha component which specifies the opacity of the pixel on a linear scale from 0 (denoting an alpha of 0.0, or completely transparent) to 255 (denoting an alpha of 1.0, or completely opaque). The color values do not use "premultiplied alpha" storage. The color information for pixels with alpha 0 _may_ be meaningful; the Gimp preserves it when parts of a layer are erased and provides (obscure) ways of recovering it in its user interface. The bottommost layer _only_ in an image may not contain alpha information; in this case all pixels in the layer have an alpha value of 255. (Even if the bottommost layer does not cover the entire canvas, it is the only layer that can be without an explicit alpha channel). In images that use the indexed color model, the Gimp does not support partial transparency and interprets alpha values from 0 to 127 as fully transparent and values from 128 to 255 as fully opaque. This behavior _may_ change in future versions of the Gimp. Layers have certain other properties such as a visibility flag, a global opacity (which is multiplied with individual pixel alphas) and various editing state flags. Channels -------- A channel is a named object that contains a single byte of information for each pixel in the canvas area. Channels have a variety of use as intermediate objects during editing; they are not meant to be rendered directly when the final image is displayed or exported to layer-less formats. A channel can be edited as if it was a grayscale layer with the same dimensions as the canvas. When it is shown in the Gimp editor UI together with other layers, it is used as if it was the _inverse_ alpha channel of a layer with the same color information in all pixels; this color can be stored in the XCF file as a property of the channel. This "mask" representation is generally thought of as an UI feature rather than an intrinsic semantics of a channel. The current _selection_ in the editor is stored as a channel in the XCF file if it is nonempty. Pixels with a value of 255 belong to the selection; pixels with a value of 0 don't, an pixels with intermediate values are partially selected. A major use of channels is as a store for saved selections. Though the channel data structure in the XCF file contains a height and width field, these must always be the same as the canvas width and height. Related to channels is the _layer mask_ that can be attached to a layer. The layer mask is in fact represented as a channel structure in the XCF file, but it is not listed in the master list of channels. Its dimensions and placement coincide with those of its parent layer. Unless disabled by the PROP_APPLY_MASK property, the layer mask functions as an extra alpha channel for the layer, in that for each pixel the layer's alpha byte and the layer mask byte are multiplied to find the extent to which the layer obscures the background. Thus a layer mask can make parts of the layer more transparent, but never more opaque. 2. FORMAT CONCEPTS AND DATATYPES ================================ An XCF file is a sequence of bytes. It contains a series of data structures, the order of which is in general not significant. The exception to this is that the main image structure must come at the very beginning of the files, and that the tile data blocks for each drawable must follow each other directly. References _between_ structures in the XCF file take the form of 32-bit "pointers" that count the number of bytes between the beginning of the XCF file and the beginning of the pointed-to structure. Each structure is designed to be written and read sequentially; many contain items of variable length and the concept of an offset _within_ a data structure is not often relevant. Basic data types ---------------- A WORD is a 32-bit integer stored as 4 bytes in network byte order, i.e. with the most significant byte first. The word is not necessarily aligned to an offset within the XCF file that is a multiple of 4. Depending on the context the word can be unsigned or (2's complement) signed. In this specification unsigned words are denoted "uint32" and signed words are denoted "int32". A FLOAT is stored as a 32-bit IEEE 754 single-precision floating-point number in network byte order. A STRING is stored as follows: uint32 n+1 The number of bytes that follow, including the zero byte byte[n] ... The string data in Unicode, encoded using UTF-8 byte 0 A terminating zero byte Exception: the empty string is stored simply as an uint32 with the value 0. Properties ---------- As an extension mechanism, most kinds of structures in an XCF file include a variable-length series of variable-length PROPERTY records which have the following general format uint32 t A magic number that identifies the type of property uint32 n They payload length (but BEWARE! see below) byte[n] ... Payload - interpretation depends on the type The authoritative source for property type numbers is the file app/xcf/xcf-private.h in the Gimp sources. The number of properties in a property list is not stored explicitly; the last property in the list is identified by having type 0; it must have length 0. XCF readers must skip and ignore property records of unrecognized type, and the length word is there to support such skipping. However, the Gimp's own XCF reader will _ignore_ the length word of most properties that it _does_ recognize, and instead reads the amount of payload it knows this property to have. This means that a property record is not itself extensible: one cannot piggyback extra data onto an existing property record by increasing its length. Also, some historical versions of the Gimp actually stored the wrong length for some properties, so there are XCF files with misleading property length information in circulation. For maximal compatibility, an XCF reader should endeavor to know the native lengths of as many properties as possible and fall back to the length word only for truly unknown property types. There is not supposed to be more than one instance of each property in a property list, but some versions of the Gimp will erroneously emit duplicate properties. An XCF reader that meets a duplicated property should let the contents of the later instance take precedence, except for properties that contain lists of subitems, in which the lists should generally be concatenated. An XCF writer should never deliberately duplicate properties within a single property list. Parasites --------- A second level of extensibility is provided by the "parasite" concept. A parasite is analogous to a property (and is usually stored in a special property in the XCF file) but is identified by a string rather than a number. This makes a larger namespace available for parasites. Gimp plug-ins can access the parasites of an image component through a generic API and can define their own parasite names which will be ignored by other plug-ins. In contrast, only the Gimp itself should define new property types. A list of known parasites and their data formats can be found in the file devel-doc/parasites.txt of the Gimp source tree. 3. THE MASTER IMAGE STRUCTURE ============================= The image structure always starts at offset 0 in the XCF file. byte[9] "gimp xcf " File type magic byte[4] version XCF version "file" - version 0 "v001" - version 1 "v002" - version 2 byte 0 Zero-terminator for version tag uint32 width With of canvas uint32 height Height of canvas uint32 base_type Color mode of the image; one of 0: RGB color 1: Grayscale 2: Indexed color (enum GimpImageBaseType in libgimpbase/gimpbaseenums.h) property-list Image properties (details below) ,------------------- Repeat once for each layer, topmost layer first: | uint32 layer Pointer to the layer structure `-- uint32 0 Zero marks the end of the array of layer pointers ,------------------- Repeat once for each channel, in no particular order: | uint32 channel1 Pointer to the channel structure `-- uint32 0 Zero marks the end of the array of channel pointers The last four characters of the initial 13-character magic string are a version indicator. The version will be higher than 2 if the correct reconstruction of pixel data from the file requires that the reader understands features not described in this specification. On the other hand, optional extra information that can be safely ignored will not cause the version to increase. The Gimp's XCF writer dynamically selects the lowest version that will allow the image to be represented. Third-party XCF writers should do likewise. Version numbers from v100 upwards have been used by CinePaint, which originated as a 16-bit fork of the Gimp. That format is not described by this specification. Image properties ---------------- The following properties are found only in the property list of the image structure. In addition to these, the list can also contain the properties PROP_TATTOO, PROP_PARASITES and PROP_END, described in section 8 below. PROP_COLORMAP (essential) uint32 1 The type number for PROP_COLORMAP is 1 uint32 3n+4 The payload length uint32 n The number of colors in the colormap (should be <256) ,------------ Repeat n times: | byte r The red component of a colormap color | byte g The green component of a colormap color | byte b The blue component of a colormap color `-- Appears in all indexed images and stores the colormap. The property will be ignored if it is encountered in an RGB or grayscale image. The current GIMP will not write a colormap with RGB or grayscale images, but some older ones occasionally did, and readers should be prepared to gracefully ignore it in those cases. Note that in contrast to the palette data model of, for example, the PNG format, an XCF colormap does not contain alpha components, and there is no colormap entry for "transparent"; the alpha channel of layers that have one is always represented separately. The structure here is that of XCF version >= 1. Comments in the GIMP source code indicate that XCF version 0 could not store indexed images in a sane way; contemporary Gimps will complain and reinterpret the pixel data as a grayscale image if they meet a version-0 indexed image. Beware that the payload length of the PROP_COLORMAP in particular cannot be trusted: some historic releases of the Gimp erroneously wrote n+4 instead of 3n+4 into the length word (but still actually followed it by 3n+4 bytes of payload). PROP_COMPRESSION (essential) uint32 17 The type number for PROP_COMPRESSION is 17 uint32 1 One byte of payload byte c Compression indicator; one of 0: No compression 1: RLE encoding 2: (Never used, but reserved for zlib compression) 3: (Never used, but reserved for some fractal compression) Defines the encoding of pixels in tile data blocks in the entire XCF file. See section 6 below for details. Note that unlike most other properties whose payload is always a small integer, PROP_COMPRESSION does _not_ pad the value to a full 32-bit integer. Contemporary Gimps always write files with c=1. It is unknown to the author of this document whether versions that wrote completely uncompressed (c=0) files ever existed. PROP_GUIDES (editing state) uint32 18 The type number for PROP_GUIDES is 18 uint32 5*n Five bytes of payload per guide ,---------- Repeat n times: | int32 c Guide coordinate | byte o Guide orientation; one of | 1: The guide is horizontal, and c is a y coordinate | 2: The guide is vertical, and c is an x coordinate `-- Appears if any guides have been defined. Some old XCF files define guides with negative coordinates; those should be ignored by readers. PROP_RESOLUTION (not editing state, but not _really_ essential either) uint32 19 The type number for PROP_RESOLUTION is 19 uint32 8 Eight bytes of payload float x Horizontal resolution in pixels per 25.4 mm float y Vertical resolution in pixels per 25.4 mm Gives the intended physical size of the image's pixels. Note that for many images, such as graphics created for the web, the creator does not really have an intended resolution in mind but intends the image to be shown at whatever the natural resolution of the viewer's monitor is. Similarly, photographs commonly do not have a well-defined target size and are intended to be scaled to fit the available space instead. Therefore readers should not interpret the information in this property too rigidly; the Gimp writes it to XCF files unconditionally, even if the user has not explicitly chosen a resolution. PROP_UNIT (editing state) uint32 22 The type number for PROP_UNIT is 21 uint32 4 Four bytes of payload uint32 c Magic number specifying the unit; one of 1: Inches (25.4 mm) 2: Millimeters (1 mm) 3: Points (127/360 mm) 4: Picas (127/30 mm) Specifies the units used to specify resolution in the Scale Image and Print Size dialogs. Note that this is used only in the user interface; the PROP_RESOLUTION property is always stored in pixels per 25.4 mm. Instead of this property, PROP_USER_UNIT can be used to specify a unit not on the list of standard units. PROP_PATHS uint32 23 The type number for PROP_PATHS is 23 uint32 len The total length of the following payload uint32 a The index of the active path uint32 n The number of paths that follow path_1 path_2 ... path_n Each path has one of three formats Format 1: Format 2: Format 3: string string string name uint32 uint32 uint32 locked byte byte byte state 4 if closed, 2 otherwise uint32 uint32 uint32 closed uint32 uint32 uint32 n The number of points uint32=1 uint32=2 uint32=3 v A version indicator uint32 uint32 dummy Ignored; always set to 1 uint32 tattoo 0 if none, or see PROP_TATTOO ,---------- ,---------- ,------------------ Repeat for n points: | int32 | int32 | int32 typ Type of point; one of | | | 0: an anchor | | | 1: a Bezier control point | int32 | float | float x X coordinate | int32 | float | float y Y coordinate `-- `-- `-- This format is used to save path data if all paths in the image are continuous sequences of Bezier strokes. Otherwise PROP_VECTORS is used instead. (Hmmm... PROP_PATHS cannot represent parasites for paths, but the XCF writer does not check whether all paths are parasite-less when choosing which property to use, so path parasites may be lost upon saving). There may be paths that declare a length of 0 points; these should be ignored. PROP_USER_UNIT (editing state) uint32 24 The type number for PROP_USER_UNIT is 24 uint32 length The total length of the following payload float factor 25.4 mm divided by the length of the unit uint32 digits The number of decimal digits used with the unit string id An identifier for the unit string sym The short symbol for the unit string abbr The abbreviation for the unit string sing The name of the unit in singular string plur The name of the unit in plural An alternative to PROP_UNIT which allows the use of units not on the standard list. PROP_VECTORS uint32 25 The type number for PROP_VECTORS is 25 uint32 length The total length of the following payload uint32 1 A version tag; so far always 1 uint32 a The index of the active path uint32 n The number of paths that follow ,---------------------- Repeat n times: | string name Name of the path | uint32 tattoo Tattoo of the path (see PROP_TATTOO), or 0 | uint32 visible | uint32 linked | uint32 m The number of parasites for the path | uint32 k The number of strokes in the first path | ,-------------------- Repeat m times: | | parasite ... In same format as in PROP_PARASITES (q.v.) | `-- | ,-------------------- Repeat k times: | | uint32 1 The stroke is a Bezier stroke (p.t., all are) | | uint32 closed Closed flag | | uint32 na Number of floats given for each point; | | must be >= 2 and <= 6. | | uint32 np Number of control points for this stroke | | ,------------------ Repeat np times: | | | uint32 type Type of the first point; one of | | | 0: an anchor | | | 1: a Bezier control point | | | float x X coordinate | | | float y Y coordinate | | | float pressure Only if na >= 3; otherwise defaults to 1.0 | | | float xtilt Only if na >= 4; otherwise defaults to 0.5 | | | float ytilt Only if na >= 5; otherwise defaults to 0.5 | | | float wheel Only if na == 6; otherwise defaults to 0.5 | | `-- | `-- `-- Appears unless all paths can be stored in the PROP_PATHS format. The Gimp's XCF reader checks that the total size of all path specifications in the property precisely equals the length word, so it is safe for a reader to use the length word to skip the property without parsing the individual parasites. (Note that this is _not_ the case for PROP_PATHS). 4. THE LAYER STRUCTURE ====================== Layer structures are pointed to from a list of layer pointers in the master image structure. uint32 width The width of the layer uint32 height The height of the layer uint32 type Color mode of the layer: one of 0: RGB color without alpha; 3 bytes per pixel 1: RGB color with alpha; 4 bytes per pixel 2: Grayscale without alpha; 1 byte per pixel 3: Grayscale with alpha; 2 bytes per pixel 4: Indexed without alpha; 1 byte per pixel 5: Indexed with alpha; 2 bytes per pixel (enum GimpImageType in libgimpbase/gimpbseenums.h) string name The name of the layer property-list Layer properties (details below) uint32 hptr Pointer to the hierarchy structure containing the pixels uint32 mptr Pointer to the layer mask (a channel structure), or 0 The color mode of a layer must match that of the entire image. All layers except the bottommost one _must_ have an alpha channel. Exception: If the layer is a floating selection (see PROP_FLOATING_SELECTION) and is attached to a channel or layer mask, its color mode must be 3 (grayscale with alpha). Layer properties ---------------- The following properties are found only in the property list of layer structures. In addition to these, the list can also contain the properties PROP_OPACITY, PROP_VISIBLE, PROP_LINKED, PROP_TATTOO, PROP_PARASITES, and PROP_END, defined in section 8 below. PROP_ACTIVE_LAYER (editing state) uint32 2 The type number for PROP_ACTIVE_LAYER is 2 uint32 0 PROP_ACTIVE_LAYER has no payload Appears in the property list for the currently active layer. Only one layer must have this property. PROP_FLOATING_SELECTION (essential) uint32 5 The type number for PROP_FLOATING_SELECTION is 5 uint32 4 Four bytes of payload uint32 ptr Pointer to the layer or channel that the floating selection is attached to. Appears in the property list for the layer that is the floating selection. If a floating selection exists, it must always be the first layer in the layer list, but it is not rendered at that position in the layer stack. Instead it is logically attached to another layer, or a channel or layer mask, and the contents of the floating selection is combined with ("anchored to") that drawable before it is used to render the visible image. The floating selection must not have a layer mask of its own, but if an ordinary (not floating) selection also exists, it will be used as a layer mask for the floating selection. If a floating selection exists, it must also be the active layer. Because floating selections are modal and ephemeral, users rarely save XCF files containing a floating selection. It may be acceptable for third-party XCF consumers to ignore floating selections or explicitly refuse to process them. PROP_MODE (essential) uint32 7 The type number for PROP_MODE is 7 uint32 4 Four bytes of payload unit32 m The layer mode; one of 0: Normal 1: Dissolve (random dithering to discrete alpha) 2: (Behind: not selectable in the Gimp UI) 3: Multiply 4: Screen 5: Overlay 6: Difference 7: Addition 8: Subtract 9: Darken Only 10: Lighten Only 11: Hue (H of HSV) 12: Saturation (S of HSV) 13: Color (H and S of HSL) 14: Value (V of HSV) 15: Divide 16: Dodge 17: Burn 18: Hard Light 19: Soft Light (XCF version >= 2 only) 20: Grain Extract (XCF version >= 2 only) 21: Grain Merge (XCF version >= 2 only) When reading old XCF files that lack this property, assume m=0. The effects of the various layer modes is defined in Section 8, below. Beware that the Gimp ignores all other layer modes than Normal and Dissolve for the bottommost visible layer of the image; if m>=3 has been specified for this layer it will interpreted as m==0 for display and flattening purposes. This effect happens for one layer only: even if the bottommost visible layer covers only some (or none) of the canvas, it will be the only layer to have its mode forced to Normal. PROP_PRESERVE_TRANSPARENCY (editing state) (called PROP_LOCK_ALPHA in Gimp 3.3+) uint32 10 The type number for PROP_PRESERVE_TRANSPARENCY is 10 uint32 4 Four bytes of payload uint32 b 1 if the Preserve Transparency flag is set; 0 if not The Preserve Transparency flag prevents all drawing tools in the Gimp from increasing the alpha of any pixel in the layer. PROP_APPLY_MASK (essential) uint32 11 The type number for PROP_APPLY_MASK is 11 uint32 4 Four bytes of payload uint32 b 1 if the layer mask should be applied, 0 if not If the property does not appear for a layer which has a layer mask, it defaults to true. Robust readers should force this to false if the layer has no layer mask. Writers should never save this as true unless the layer has a layer mask. PROP_EDIT_MASK (editing state) uint32 12 The type number for PROP_EDIT_MASK is 12 uint32 4 Four bytes of payload uint32 b 1 if the layer mask is currently being edited, 0 if not If the property does not appear for a layer which has a layer mask, it defaults to false. Robust readers should force this to false if the layer has no layer mask. Writers should never save this as true unless the layer has a layer mask. PROP_SHOW_MASK (editing state) uint32 13 The type number for PROP_APPLY_MASK is 13 uint32 4 Four bytes of payload uint32 b 1 if the layer mask is visible, 0 if not If the property does not appear for a layer which has a layer mask, it defaults to false. Robust readers should force this to false if the layer has no layer mask. Writers should never save this as true unless the layer has a layer mask. PROP_OFFSETS (essential) uint32 15 The type number for PROP_OFFSETS is 15 uint32 8 Eight bytes of payload int32 dx Horizontal offset int32 dy Vertical offset Gives the coordinates of the topmost leftmost corner of the layer relative to the topmost leftmost corner of the canvas. The coordinates can be negative; this corresponds to a layer that extends to the left of or above the canvas boundary. When reading old XCF files that lack this property, assume (0,0). PROP_TEXT_LAYER_FLAGS uint32 26 The type number for PROP_TEXT_LAYER_FLAGS is 26 uint32 4 Four bytes of payload uint32 f Flags, or'ed together from the following set: 0x00000001 Do _not_ change the layer name if the text contents is changed 0x00000002 The pixel data has been painted to or otherwise modified since the text was rendered Appears in property lists for text layers. The actual text (and other parameters such as font and color) appears as a parasite rather than a property. 5. THE CHANNEL STRUCTURE ======================== Channel structures are pointed to from layer structures (in case of layer masks) or from the master image structure (for all other channels). uint32 width The width of the channel uint32 height The height of the channel string name The name of the channel property-list Layer properties (see below) uint32 hptr Pointer to the hierarchy structure containing the pixels The with and height of the channel must be the same as those of its parent structure (the layer in the case of layer masks; the canvas for all other channels). Channel properties ------------------ The following properties are found only in the property list of channel structures. In addition to these, the list can also contain the properties PROP_OPACITY, PROP_VISIBLE, PROP_LINKED, PROP_TATTOO, PROP_PARASITES, and PROP_END, defined in section 8 below. PROP_ACTIVE_CHANNEL (editing state) uint32 3 The type number for PROP_ACTIVE_CHANNEL is 3 uint32 0 PROP_ACTIVE_CHANNEL has no payload Appears in the property list for the currently active channel. PROP_SELECTION (editing state?) uint32 4 The type number for PROP_SELECTION is 4 uint32 0 PROP_SELECTION has no payload Appears in the property list for the channel structure that represents the selection mask. PROP_SHOW_MASKED (editing state) uint32 14 The type number for PROP_SHOW_MASKED is 14 uint32 4 Four bytes of payload uint32 b 1 if the channel is shown as a mask, 0 if not Appears in the property list for channels. PROP_COLOR uint32 16 The type number for PROP_COLOR is 16 uint32 3 Three bytes of payload byte r Red component of color byte g Green component of color byte b Blue component of color Appears in the property list for channels and gives the color of the screen that is used to represent the channel when it is visible in the UI. (The alpha of the screen is given as the channel's PROP_OPACITY). 6. THE HIERARCHY STRUCTURE ========================== A hierarchy contains data for a rectangular array of pixels. It is pointed to from a layer or channel structure. uint32 width The with of the pixel array uint32 height The height of the pixel array uint32 bpp The number of bytes per pixel given uint32 lptr Pointer to the "level" structure ,--------------- Repeat zero or more times | uint32 dlevel Pointer to an unused level structure `-- uint32 0 A zero ends the list of level pointers The width, height, and bpp values are for consistency checking; their correct values can always be inferred from the context, and are checked when the Gimp reads the XCF file. Levels ------ For some unknown historical reason, the hierarchy structure contains an extra indirection to a series of "level" structures, described below. Only the first level structure is used by the Gimp's XCF reader, except that the reader checks that a terminating zero for the level-pointer list can be found. The Gimp's XCF writer creates a series of dummy level structures (with no tile pointers), each declaring a height and width half of the previous one (rounded down), until the height and with are both less than 64. Thus, for a layer of 3 x 266 pixels, this series of levels will be saved: A level of 3 x 266 pixels, with 5 tiles: the actually used one A level of 1 x 133 pixels with no tiles A level of 0 x 66 pixels with no tiles A level of 0 x 33 pixels with no tiles Third-party XCF writers should probably mimic this entire structure; robust XCF readers should have no reason to even read past the pointer to the first level structure. The level structure is laid out as follows: uint32 width The width of the pixel array uint32 height The height of the pixel array ,-------------- Repeat for each of the ceil(width/64)*ceil(height/64) tiles | uint32 tptr Pointer to tile data `-- uint32 0 A zero marks the end of the array of tile pointers The width and height must be the same as the ones recorded in the hierarchy structure (except for the aforementioned dummy levels). Tiles ----- The pixels in the hierarchy are organized in a grid of "tiles", each with a width and height of up to 64 pixels. The only tiles that have a width less than 64 are those in the rightmost column, and the only tiles that have a height less than 64 are those in the bottommost row. Thus, a layer measuring 200 x 150 pixels will be divided into 12 tiles: +-----------------+-----------------+------------------+-----------------+ | Tile 0: 64 x 64 | Tile 1: 64 x 64 | Tile 2: 64 x 64 | Tile 3: 8 x 64 | +-----------------+-----------------+------------------+-----------------+ | Tile 4: 64 x 64 | Tile 5: 64 x 64 | Tile 6: 64 x 64 | Tile 7: 8 x 64 | +-----------------+-----------------+------------------+-----------------+ | Tile 8: 64 x 22 | Tile 9: 64 x 22 | Tile 10: 64 x 22 | Tile 11: 8 x 22 | +-----------------+-----------------+------------------+-----------------+ As can be seen from this example, the tiles appear in the XCF file in row-major, top-to-bottom, left-to-right order. The dimensions of the individual tiles are not stored explicitly in the XCF file, but must be computed by the reader. The tiles that are pointed to by a single level structure must be contiguous in the XCF file, because the Gimp's XCF reader uses the difference between two subsequent tile pointers to judge the amount of memory it needs to allocate for internal data structures. 7. TILE DATA ORGANIZATION ========================= The format of the data blocks pointed to by the tile pointers in the level structure of the previous section differs according to the value of the PROP_COMPRESSION property of the main image structure. Current Gimps always use RLE compression, but readers should nevertheless be prepared to meet the older uncompressed format. Both formats assume the width, height and byte depth of the tile are known from the context (namely, they are stored explicitly in the hierarchy structure). Both encodings store a linear sequence of with*height pixels, extracted from the tile in row-major, top-to-bottom, left-to-right order (the same as the reading direction of multi-line English text). In color modes with alpha information, the alpha value is the last of the 2 or 4 bytes for each pixel. In RGB color modes, the 3 (first) bytes for each pixel is the red intensity, the green intensity, and the blue intensity, in that order. Uncompressed tile data ---------------------- In the uncompressed format, the file first contains all the bytes for the first pixel, then all the bytes for the second pixel, and so on. RLE compressed tile data ------------------------ In the Run-Length Encoded format, each tile consists of a run-length encoded stream of the first byte of each pixel, then a stream of the second byte of each pixel, and so forth. In each of the streams, multiple occurrences of the same byte value are represented in compressed form. The representation of a stream is a series of operations; the first byte of each operation determines the format and meaning of the operation: byte n For 0 <= n <= 126: a short run of identical bytes byte v Repeat this value n+1 times or byte 127 A long run of identical bytes byte p byte q byte v Repeat this value p*256 + q times or byte 128 A long run of different bytes byte p byte q byte[p*256+q] data Copy these verbatim to the output stream or byte n For 129 <= n <= 255: a short run of different bytes byte[256-n] data Copy these verbatim to the output stream The end of the stream for "the first byte of all pixels" (and the following similar streams) must occur at the end of one of these operations; it is not permitted to have one operation span the boundary between streams. The RLE encoding allows "degenerate" encodings in which the original data stream may double in size (or grow to arbitrarily large sizes if (128,0,0) operations are inserted). Such encodings must be avoided, as the Gimp's XCF reader expects that the size of an encoded tile is never more than 24 KB, which is only 1.5 times the unencoded size of a 64x64 RGBA tile. A simple way for an XCF creator to avoid overflow is a) never using opcode 0 (but instead opcode 255) b) using opcodes 127 and 128 only for lengths larger than 127 c) never emitting two "different bytes" opcodes next to each other in the encoding of a single stream. 8. GENERIC PROPERTIES ===================== This section lists the formats of the defined property records that can appear in more than one context in an XCF file. PROP_END uint32 0 The type number for PROP_END is 0 uint32 0 PROP_END has no payload This pseudo-property marks the end of a property list in the XCF file. PROP_OPACITY (essential) uint32 6 The type number for PROP_OPACITY is 6 uint32 4 Four bytes of payload uint32 x The opacity on a scale from 0 (fully transparent) to 255 (fully opaque) Appears in the property list of layers and channels, and records the overall opacity setting for the layer/channel. Note that though the Gimp's user interface displays the opacity as a percentage, it is actually stored on a 0-255 scale. Also note that this opacity value is stored as a 32-bit quantity even though it has been scaled to fit exactly in a single byte. When reading old XCF files that lack this property, full opacity should be assumed. PROP_VISIBLE (essential) uint32 8 The type number for PROP_VISIBLE is 8 uint32 4 Four bytes of payload uint32 b 1 if the layer/channel is visible; 0 if not Appears in the property list for layers and channels. When reading old XCF files that lack this property, assume that layers are visible and channels are not. PROP_LINKED (editing state) uint32 9 The type number for PROP_LINKED is 9 uint32 4 Four bytes of payload uint32 b 1 if the layer is linked; 0 if not Appears in the property list for layers and channels. "Linked" is a UI property: if the Move tool is used to move a linked layer, all other linked layers will be moved in parallel. PROP_TATTOO (internal Gimp state) uint32 20 The type number for PROP_TATTOO is 20 uint32 4 Four bytes of payload uint32 4 The tattoo, a nonzero unsigned integer Appears in the property list of layers, channels, and entire images. A tattoo is a unique and permanent identifier attached to a drawable (or vector element) that can be used to uniquely identify a drawable within an image even between sessions. The PROP_TATTOO property of the entire image stores a "high-water mark" for the entire image; it is greater than OR EQUAL TO any tattoo for an element of the image. It allows efficient generation of new unused tattoo values and also prevents old tattoo numbers from being reused within a single image, lest plug-ins that use the tattoos for bookkeeping get confused. An XCF file must either provide tattoo values for all its elements or for none of them. The Gimp will invent fresh tattoos when it reads in tattoo-less elements, but it does not attempt to keep them different from ones specified explicitly in the file. PROP_PARASITES uint32 21 The type number for PROP_PARASITES is 21 uint32 length Total length of the following payload data ,---------------- Repeat for each parasite: | string name Name of the parasite | uint32 flags ????? | uint32 n Size of the payload data | byte[n] ... Parasite-specific payload `-- This property can appear in any property list. It can contain multiple "parasites" which are named extension records. See "Basic concepts and datatypes" above. The number of parasites is not directly encoded; the list ends when the total length of the parasite data read equals the property payload length. The Gimp's XCF reader checks that the combined size of all parasites in the property precisely equals the length word, so it is safe for a reader to use the length word to skip the property without parsing the individual parasites. The parasite contents may be binary, but often a textual encoding is chosen in order to free the creator/consumer code of having to deal with byte ordering. There can only be one parasite with a given name attached to each element of the image. Some versions of the Gimp will erroneously write some parasites twice in the same property list; XCF readers must be prepared to gracefully ignore all but the last instance of a parasite name in each property list. 9. COMPOSITING AND LAYER MODES =============================== This section describes the "flattening" process that the Gimp applies when a multi-layer image is displayed in the editor or exported to other image file formats. It is present for reference only; an XCF consumer may well elect to do something different with pixel data from the layers than flattening them. Most XCF consumers will need to react to the layer mode property of each layer; such a reaction must be informed by knowledge of how the different layer modes affect the flattening process. In some applications it might be acceptable for an XCF consumer to refuse processing images with layer modes other than "Normal", but such an application will probably not be considered fully XCF capable by its users. In this section we consider primary color (or grayscale) intensities and alpha values for pixels to be real numbers ranging from 0.0 to 1.0. This makes many of the formulas easier; the reader is asked to keep in mind that a (linear) conversion from the integral 0..255 scale of the actual XCF scale is implied whenever data from the XCF file is mentioned. Any practical implementation of the computations specified below may suffer rounding errors; this specification do not detail how these are to be handled. The Gimp itself rounds values to an integral number of 255ths at many points in the computation. This specification does not specify exactly which these points are, and authors of XCF renderers that aim to reproduce the effects of the Gimp's flattening down to the least significant bits are referred to studying its source code. In the description below, the variable letter "a" is used for alpha values. The variable letter "r", "g", "b" are used for primary intensities, "y" is used for grayscale intensities, and "i" is used for colormap indexed. The letter "c" is used for the complete color information for a pixel; depending on the color mode of the image that is either an (r,g,b) triple, a y, or a c. The flattening process works independently for each pixel in the canvas area. The description of some layer modes in the Gimp manual may give the impression that they involve filters that let pixels influence neighbor pixels, but that is not true. This description does not attempt to preserve the color information for completely transparent pixels in a layer. If an application uses this color information, it should document explicitly how it behaves when transparent pixels from several different layers cover the same point of the canvas. Flattening overview ------------------- This is how to compute the flattened result for a single pixel position (in theory, that is - efficient implementations will of course follow this procedure or an equivalent one for many pixels in parallel): 1. Initialize a "working pixel" (a1,c1) to be completely transparent (that is, a1=0.0 and the value of c1 is immaterial). 2. Do the following for each visible layer in the image, starting with the one that comes LAST in the master layer list: 3. Ignore the layer if it is the floating selection, or if it does not overlap the pixel position in question. 4. Let (a2,c2) be the pixel data for the layer at the pixel position in question. If the layer does not have an alpha channel, then set a1 to 1.0. 5. If the layer is the one that the floating selection is attached to and the floating selection overlaps the pixel position in question, then do the following: 6. Let (a3,c3) be the pixel data for the floating selection layer at the pixel position in question. 7. If there is a selection channel, then let x be its value at the pixel position in question, and set a3 to a3*x. 8. Let m3 be the layer mode of the floating selection. 9. Set (a2,c2) to COMPOSITE(a2,c2, a3,c3,m3). The COMPOSITE function is defined below. 10. If the layer has a layer mask and it is enabled, then let x be the value of the layer mask at the pixel position in question, and set a2 to a2*x. 11. Let m2 be the layer mode of the layer. 12. If the layer is the bottommost visible layer (i.e., if it is the last visible layer in the master layer list) and m2 is not "Normal" or "Dissolve", then set m2 to "Normal". 13. Set (a1,c1) to COMPOSITE(a1,c1, a2,c2,m2). The COMPOSITE function is defined below. 14. If the flattened image is to be shown against a background of color c0, then actually visible pixel is COMPOSITE(1.0,c0, a1,c1,Normal). Note that unless all layers have mode Normal, it would give the wrong result to start by initializing (a1,c1) to (1.0,c0). Helper functions ---------------- The following auxiliary functions are used in the definition of COMPOSITE below: MIN(x1,...,xn) is the least value of x1...xn MAX(x1,...,xn) is the largest value of x1..xn MID(x1,...,xn) = (MIN(x1,...,xn)+MAX(x1,...,xn))/2 CLAMP(x) = if x < 0 then 0.0 else if x > 1 then 1.0 else x BLEND(a1,x1, a2,x2) = (1-k)*x1 + k*x2 where k = a2/(1-(1-a1)*(1-a2)) The layer modes --------------- This and the following sections define the COMPOSITE function used in the general flattening algorithm. "Normal" mode for RGB or grayscale images is the usual mode of compositeing in computer graphics with alpha channels. In indexed mode, the alpha value gets rounded to either 1.0 or 0.0 such that no colors outside the colormap get produced: COMPOSITE(a1,y1, a2,y2,Normal) = ( 1-(1-a1)*(1-a2), BLEND(a1,y1, a2,y2) ) COMPOSITE(a1,r1,g1,b1, a2,r2,g2,b2,Normal) = ( 1-(1-a1)*(1-a2), BLEND(a1,r1, a2,r2), BLEND(a1,g1, a2,g2), BLEND(a1,b1, a2,b2) ) COMPOSITE(a1,i1, a2,i2,Normal) = if a2 > 0.5 then (1.0,i2) else (a1,i1) "Dissolve" mode corresponds to randomly dithering the alpha channel to the set {0.0, 1.0}: COMPOSITE(a1,c1, a2,c2,Dissolve) = chose pseudo-randomly between (1.0,c2) with probability a2 (a1,c1) with probability 1-a2 These two modes are the only ones that make sense for all of the RGB, grayscale and indexed color models. In the indexed color model, all layer modes except Dissolve are treated as Normal. Most layer modes belong to the following group, which makes sense for RGB and grayscale images, but not for indexed ones: COMPOSITE(a1,y2, a2,y2,m) = ( a1, BLEND(a1,y1, MIN(a1,a2),f(y1,y2, m)) ) COMPOSITE(a1,r1,g1,b1, a2,r2,g2,b2,m) = ( a1, BLEND(a1,r2, MIN(a1,a2),f(r1,r2, m)), BLEND(a1,g1, MIN(a1,a2),f(g1,g2, m)), BLEND(a1,b1, MIN(a1,a2),f(b1,g2, m)) ) when 3 <= m <= 10 or 15 <= m <= 21. The following table defines f(x1,x2,m): Multiply: f(x1,x2, 3) = x1*x2 Screen: f(x1,x2, 4) = 1-(1-x1)*(1-x2) Overlay: f(x1,x2, 5) = (1-x2)*x1^2 + x2*(1-(1-x2)^2) Difference: f(x1,x2, 6) = if x1 > x2 then x1-x2 else x2-x1 Addition: f(x1,x2, 7) = CLAMP(x1+x2) Subtract: f(x1,x2, 8) = CLAMP(x1-x2) Darken Only: f(x1,x2, 9) = MIN(x1,x2) Lighten Only: f(x1,x2, 10) = MAX(x1,x2) Divide: f(x1,x2, 15) = CLAMP(x1/x2) Dodge: f(x1,x2, 16) = CLAMP(x1/(1-x2)) Burn f(x1,x2, 17) = CLAMP(1-(1-x1)/x2) Hard Light: f(x1,x2, 18) = if x2 < 0.5 then 2*x1*x2 else 1-2*(1-x1)(1-x2) Soft Light: f(x1,x2, 19) = (1-x2)*x1^2 + x2*(1-(1-x2)^2) Grain Extract: f(x1,x2, 20) = CLAMP(x1-x2+0.5) Grain Merge: f(x1,x2, 21) = CLAMP(x1+x2-0.5) Note that the "Overlay" and "Soft Light" modes have identical effects. In the "Divide", "Dodge", and "Burn" modes, division by zero should be considered to produce a number so large that CLAMP(x/0) = 1 unless x=0, in which case CLAMP(0/0) = 0. The remaining four layer modes only make sense in the RGB color model; if the color mode of the image is grayscale or indexed they will be interpreted as Normal. COMPOSITE(a1,r1,g1,b1, a2,r2,g2,b2,m) = ( a1, BLEND(a1,r2, MIN(a1,a2),r0), BLEND(a1,g1, MIN(a1,a2),g0), BLEND(a1,b1, MIN(a1,a2),b0) ) where (r0,g0,b0) = h(r1,g1,b1, r2,g2,b2, m) when 11 <= m <= 14. For defining these modes, we say that (r,g,b) has the _hue_ of (r',g',b') if r' = g' = b' and r >= g = b or there exist p and q such that p>=0 and r=p*r'+q and b=p*b'+q and g=p*g'+q (r,g,b) has the _value_ of (r',g',b') if MAX(r,g,b) = MAX(r',g',b') (r,g,b) has the _HSV-saturation_ of (r',g',b') if r' = g' = b' = 0 and r = g = b or MIN(r,g,b) = MAX(r,g,b)*MIN(r',g',b')/MAX(r',g',b') (r,g,b) has the _luminosity_ of (r',g',b') if MID(r,g,b) = MID(r',g',b') (r,g,b) has the _HSL-saturation_ of (r',g',b') if r' = g' = b' and r = g = b or MAX(r,g,b)-MIN(r,g,b) = MIN(MID(r,g,b),1-MID(r,g,b)) * (MAX(r',g',b')-MIN(r',g',b'))/MIN(MID(r',g',b'),1-MID(r',g',b')) Mode 11: Hue (H of HSV) h(r1,g1,b1, r2,g2,b2, 11) is if r2=g2=b2 then (r1,g1,b1) unchanged otherwise: the color that has the hue of (r1,g2,b2) the value of (r1,g1,b1) the HSV-saturation of (r1,g1,b1) Mode 12: Saturation (S of HSV) h(r1,g1,b1, r2,g2,b2, 12) is the color that has the hue of (r1,g1,b1) the value of (r1,g1,b1) the HSV-saturation of (r2,g2,b2) Mode 13: Color (H and S of HSL) h(r1,g1,b1, r2,g2,b2, 13) is the color that has the hue of (r2,g2,b2) the luminosity of (r1,g1,b1) the HSL-saturation of (r2,g2,b2) Mode 14: Value (V of HSV) h(r1,g1,b1, r2,g2,b2, 14) is the color that has the hue of (r1,g1,b1) the value of (r2,g2,b2) the HSV-saturation of (r1,g1,b1)