"ILBM" IFF Interleaved Bitmap Date: January 17, 1986 From: Jerry Morrison, Electronic Arts Status: Released and in use 1. Introduction "EA IFF 85" is Electronic Arts' standard for interchange format files. "ILBM" is a format for a 2 dimensional raster graphics image, specifically an InterLeaved bitplane BitMap image with color map. An ILBM is an IFF "data section" or "FORM type", which can be an IFF file or a part of one. (See the IFF reference.) An ILBM is an archival representation designed for three uses. First, a standalone image that specifies exactly how to display itself (resolution, size, color map, etc.). Second, an image intended to be merged into a bigger picture which has its own depth, color map, and so on. And third, an empty image with a color map selection or "palette" for a paint program. ILBM is also intended as a building block for composite IFF FORMs like "animation sequence" and "structured graphics". Some uses of ILBM will be to preserve as much information as possible across disparate environments. Other uses will be to store data for a single program or highly cooperative programs while maintaining subtle details. So we're trying to accomplish a lot with this one format. This memo is the IFF supplement for FORM ILBM. Section 2 defines the purpose and format of property chunks bitmap header "BMHD", color map "CMAP", hotspot "GRAB", destination merge data "DEST", sprite information "SPRT", and Commodore Amiga viewport mode "CAMG". Section 3 defines the standard data chunk "BODY". These are the "standard" chunks. Section 4 defines the nonstandard color range data chunk "CRNG". Additional specialized chunks like texture pattern can be added later. The ILBM syntax is summarized in Appendix A as a regular expression and in Appendix B as a box diagram. Appendix C explains the optional run encoding scheme. Appendix D names the committee responsible for this FORM ILBM standard. Details of the raster layout are given in part 3, "Standard Data Chunk". Some elements are based on the Commodore Amiga hardware but generalized for use on other computers. An alternative to ILBM would be appropriate for computers with true color data in each pixel. Reference: "EA IFF 85" Standard for Interchange Format Files describes the underlying conventions for all IFF files. Amiga[tm] is a trademark of Commodore-Amiga, Inc. Electronic Arts[tm] is a trademark of Electronic Arts. Macintosh[tm] is a trademark licensed to Apple Computer, Inc. MacPaint[tm] is a trademark of Apple Computer, Inc. 2. Standard Properties The required property "BMHD" and any optional properties must appear before any "BODY" chunk. (Since an ILBM has only one BODY chunk, any following properties are superfluous.) Any of these properties may be shared over a LIST of FORMs IBLM by putting them in a PROP ILBM. (See the "EA IFF 85" memo.) BMHD The required property "BMHD" holds a BitMapHeader as defined in these C declarations and following documentation. It describes the dimensions and encoding of the image, including data necessary to understand the BODY chunk to follow. typedef UBYTE Masking; /* Choice of masking technique. */ #define mskNone 0 #define mskHasMask 1 #define mskHasTransparentColor 2 #define mskLasso 3 typedef UBYTE Compression; /* Choice of compression algorithm applied to the rows of all * source and mask planes. "cmpByteRun1" is the byte run encoding * described in Appendix C. Do not compress across rows! */ #define cmpNone 0 #define cmpByteRun1 1 typedef struct { UWORD w, h; /* raster width & height in pixels */ WORD x, y; /* pixel position for this image */ UBYTE nPlanes; /* # source bitplanes */ Masking masking; Compression compression; UBYTE pad1; /* unused; for consistency, put 0 here */ UWORD transparentColor; /* transparent "color number" (sort of) */ UBYTE xAspect, yAspect; /* pixel aspect, a ratio width : height */ WORD pageWidth, pageHeight; /* source "page" size in pixels */ } BitMapHeader; Fields are filed in the order shown. The UBYTE fields are byte-packed. The fields w and h indicate the size of the image rectangle in pixels. Each row of the image is stored in an integral number of 16 bit words. The number of words per row is Ceiling(w/16). The fields x and y indicate the desired position of this image within the destination picture. Some reader programs may ignore x and y. A safe default for writing an ILBM is (x, y) = (0, 0). The number of source bitplanes in the BODY chunk (see below) is stored in nPlanes. An ILBM with a CMAP but no BODY and nPlanes = 0 is the recommended way to store a color map. Note: Color numbers are color map index values formed by pixels in the destination bitmap, which may be deeper than nPlanes if a DEST chunk calls for merging the image into a deeper image. The field masking indicates what kind of masking is to be used for this image. The value mskNone designates an opaque rectangular image. The value mskHasMask means that a mask plane is interleaved with the bitplanes in the BODY chunk (see below). The value mskHasTransparentColor indicates that pixels in the source planes matching transparentColor are to be considered "transparent". (Actually, transparentColor isn't a "color number" since it's matched with numbers formed by the source bitmap rather than the possibly deeper destination bitmap. Note that having a transparent color implies ignoring one of the color registers. See CMAP, below.) The value mskLasso indicates the reader may construct a mask by lassoing the image as in MacPaint*. To do this, put a 1 pixel border of transparentColor around the image rectangle. Then do a seed fill from this border. Filled pixels are to be transparent. Issue: Include in an appendix an algorithm for converting a transparent color to a mask plane, and maybe a lasso algorithm. A code indicating the kind of data compression used is stored in compression. Beware that using data compression makes your data unreadable by programs that don't implement the matching decompression algorithm. So we'll employ as few compression encodings as possible. The run encoding byteRun1 is documented in Appendix C, below. The field pad1 is a pad byte and must be set to 0 for consistency. This field could get used in the future. The transparentColor specifies which bit pattern means "transparent". This only applies if masking is mskHasTransparentColor or mskLasso (see above). Otherwise, transparentColor should be 0. The pixel aspect ratio is stored as a ratio in the two fields xAspect and yAspect. This may be used by programs to compensate for different aspects or to help interpret the fields w, h, x, y, pageWidth, and pageHeight, which are in units of pixels. The fraction xAspect/yAspect represents a pixel's width/height. It's recommended that your programs store proper fractions in BitMapHeaders, but aspect ratios can always be correctly compared with the the test xAspect%yDesiredAspect = yAspect%xDesiredAspect Typical values for aspect ratio are width : height = 10 : 11 (Amiga 320 x 200 display) and 1 : 1 (Macintosh*). The size in pixels of the source "page" (any raster device) is stored in pageWidth and pageHeight, e.g. (320, 200) for a low resolution Amiga display. This information might be used to scale an image or to automatically set the display format to suit the image. (The image can be larger than the page.) CMAP The optional (but encouraged) property "CMAP" stores color map data as triplets of red, green, and blue intensity values. The n color map entries ("color registers") are stored in the order 0 through n-1, totaling 3n bytes. Thus n is the ckSize/3. Normally, n would equal 2nPlanes. A CMAP chunk contains a ColorMap array as defined below. (These typedefs assume a C compiler that implements packed arrays of 3-byte elements.) typedef struct { UBYTE red, green, blue; /* color intensities 0..255 */ } ColorRegister; /* size = 3 bytes */ typedef ColorRegister ColorMap[n]; /* size = 3n bytes */ The color components red, green, and blue represent fractional intensity values in the range 0 through 255 256ths. White is (255, 255, 255) and black is (0, 0, 0). If your machine has less color resolution, use the high order bits. Shift each field right on reading (or left on writing) and assign it to (from) a field in a local packed format like Color4, below. This achieves automatic conversion of images across environments with different color resolutions. On reading an ILBM, use defaults if the color map is absent or has fewer color registers than you need. Ignore any extra color registers. The example type Color4 represents the format of a color register in working memory of an Amiga computer, which has 4 bit video DACs. (The ":4" tells the C compiler to pack the field into 4 bits.) typedef struct { unsigned pad1 :4, red :4, green :4, blue :4; } Color4; /* Amiga RAM format. Not filed. */ Remember that every chunk must be padded to an even length, so a color map with an odd number of entries would be followed by a 0 byte, not included in the ckSize. GRAB The optional property "GRAB" locates a "handle" or "hotspot" of the image relative to its upper left corner, e.g. when used as a mouse cursor or a "paint brush". A GRAB chunk contains a Point2D. typedef struct { WORD x, y; /* relative coordinates (pixels) */ } Point2D; DEST The optional property "DEST" is a way to say how to scatter zero or more source bitplanes into a deeper destination image. Some readers may ignore DEST. The contents of a DEST chunk is DestMerge structure: typedef struct { UBYTE depth; /* # bitplanes in the original source */ UBYTE pad1; /* unused; for consistency put 0 here */ UWORD planePick; /* how to scatter source bitplanes into destination */ UWORD planeOnOff; /* default bitplane data for planePick */ UWORD planeMask; /* selects which bitplanes to store into */ } DestMerge; The low order depth number of bits in planePick, planeOnOff, and planeMask correspond one-to-one with destination bitplanes. Bit 0 with bitplane 0, etc. (Any higher order bits should be ignored.) "1" bits in planePick mean "put the next source bitplane into this bitplane", so the number of "1" bits should equal nPlanes. "0" bits mean "put the corresponding bit from planeOnOff into this bitplane". Bits in planeMask gate writing to the destination bitplane: "1" bits mean "write to this bitplane" while "0" bits mean "leave this bitplane alone". The normal case (with no DEST property) is equivalent to planePick = planeMask = 2nPlanesJ- 1. Remember that color numbers are formed by pixels in the destination bitmap (depth planes deep) not in the source bitmap (nPlanes planes deep). SPRT The presence of an "SPRT" chunk indicates that this image is intended as a sprite. It's up to the reader program to actually make it a sprite, if even possible, and to use or overrule the sprite precedence data inside the SPRT chunk: typedef UWORD SpritePrecedence; /* relative precedence, 0 is the highest */ Precedence 0 is the highest, denoting a sprite that is foremost. Creating a sprite may imply other setup. E.g. a 2 plane Amiga sprite would have transparentColor = 0. Color registers 1, 2, and 3 in the CMAP would be stored into the correct hardware color registers for the hardware sprite number used, while CMAP color register 0 would be ignored. CAMG A "CAMG" chunk is specifically for the Commodore Amiga computer. It stores a LONG "viewport mode". This lets you specify Amiga display modes like "dual playfield" and "hold and modify". 3. Standard Data Chunk Raster Layout Raster scan proceeds left-to-right (increasing X) across scan lines, then top-to-bottom (increasing Y) down columns of scan lines. The coordinate system is in units of pixels, where (0,0) is the upper left corner. The raster is typically organized as bitplanes in memory. The corresponding bits from each plane, taken together, make up an index into the color map which gives a color value for that pixel. The first bitplane, plane 0, is the low order bit of these color indexes. A scan line is made of one "row" from each bitplane. A row is one planesU bits for one scan line, but padded out to a word (2 byte) boundary (not necessarily the first word boundary). Within each row, successive bytes are displayed in order and the most significant bit of each byte is displayed first. A "mask" is an optional "plane" of data the same size (w, h) as a bitplane. It tells how to "cut out" part of the image when painting it onto another image."One" bits in the mask mean "copy the corresponding pixel to the destination" while "zero" mask bits mean "leave this destination pixel alone". In other words, "zero" bits designate transparent pixels. The rows of the different bitplanes and mask are interleaved in the file (see below). This localizes all the information pertinent to each scan line. It makes it much easier to transform the data while reading it to adjust the image size or depth. It also makes it possible to scroll a big image by swapping rows directly from the file without random-accessing to all the bitplanes. BODY The source raster is stored in a "BODY" chunk. This one chunk holds all bitplanes and the optional mask, interleaved by row. The BitMapHeader, in a BMHD property chunk, specifies the raster's dimensions w, h, and nPlanes. It also holds the masking field which indicates if there is a mask plane and the compression field which indicates the compression algorithm used. This information is needed to interpret the BODY chunk, so the BMHD chunk must appear first. While reading an ILBM's BODY, a program may convert the image to another size by filling (with transparentColor) or clipping. The BODY's content is a concatenation of scan lines. Each scan line is a concatenation of one row of data from each plane in order 0 through nPlanes-1 followed by one row from the mask (if masking = hasMask ). If the BitMapHeader field compression is cmpNone, all h rows are exactly Ceiling(w/16) words wide. Otherwise, every row is compressed according to the specified algorithm and their stored widths depend on the data compression. Reader programs that require fewer bitplanes than appear in a particular ILBM file can combine planes or drop the high-order (later) planes. Similarly, they may add bitplanes and/or discard the mask plane. Do not compress across rows and don't forget to compress the mask just like the bitplanes. Remember to pad any BODY chunk that contains an odd number of bytes. 4. Nonstandard Data Chunk The following data chunk was defined after various programs began using FORM ILBM so it's a "nonstandard" chunk. That means there's some slight chance of name collisions. CRNG A "CRNG" chunk contains "color register range" information. It's used by Electronic Arts' Deluxe Paint program to identify a contiguous range of color registers for a "shade range" and color cycling. There can be zero or more CRNG chunks in an ILBM, but all should appear before the BODY chunk. Deluxe Paint normally writes 4 CRNG chunks in an ILBM when the user asks it to "Save Picture". typedef struct { WORD pad1; /* reserved for future use; store 0 here */ WORD rate; /* color cycle rate */ WORD active; /* nonzero means cycle the colors */ UBYTE low, high; /* lower and upper color registers selected */ } CRange; The fields low and high indicate the range of color registers (color numbers) selected by this CRange. The field active indicates whether color cycling is on or off. Zero means off. The field rate determines the speed at which the colors will step when color cycling is on. The units are such that a rate of 60 steps per second is represented as 214 = 16384. Slower rates can be obtained by linear scaling: for 30 steps/second, rate = 8192; for 1 step/second, rate = 16384/60 E 273. CCRT Commodore's Graphicraft program uses a similar chunk "CCRT" (for Color Cyling Range and Timing). This chunk contains a CycleInfo structure. typedef struct { WORD direction; /* 0 = don't cycle. 1 = cycle forwards (1, 2, 3). * -1 = cycle backwards (3, 2, 1) */ UBYTE start, end; /* lower and upper color registers selected */ LONG seconds; /* # seconds between changing colors */ LONG microseconds; /* # microseconds between changing colors */ WORD pad; /* reserved for future use; store 0 here */ } CycleInfo; This is pretty similar to a CRNG chunk. A program would probably only use one of these two methods of expressing color cycle data. You could write out both if you want to communicate this information to both Deluxe Paint and Graphicraft. A CCRT chunk expresses the color cycling rate as a number of seconds plus a number of microseconds. Appendix A. ILBM Regular Expression Here's a regular expression summary of the FORM ILBM syntax. This could be an IFF file or a part of one. ILBM ::= "FORM" #{ "ILBM" BMHD [CMAP] [GRAB] [DEST] [SPRT] [CAMG] CRNG* CCRT* [BODY] } BMHD ::= "BMHD" #{ BitMapHeader } CMAP ::= "CMAP" #{ (red green blue)* } [0] GRAB ::= "GRAB" #{ Point2D } DEST ::= "DEST" #{ DestMerge } SPRT ::= "SPRT" #{ SpritePrecendence } CAMG ::= "CAMG" #{ LONG } CRNG ::= "CRNG" #{ CRange } CCRT ::= "CCRT" #{ CycleInfo } BODY ::= "BODY" #{ UBYTE* } [0] The token "#" represents a ckSize LONG count of the following {braced} data bytes. E.g. a BMHD's "#" should equal sizeof(BitMapHeader). Literal strings are shown in "quotes", [square bracket items] are optional, and "*" means 0 or more repetitions. A sometimes-needed pad byte is shown as "[0]". The property chunks (BMHD, CMAP, GRAB, DEST, SPRT, and CAMG) and any CRNG and CCRT data chunks may actually be in any order but all must appear before the BODY chunk since ILBM readers usually stop as soon as they read the BODY. If any of the 6 property chunks are missing, default values are "inherited" from any shared properties (if the ILBM appears inside an IFF LIST with PROPs) or from the reader program's defaults. If any property appears more than once, the last occurrence before the BODY is the one that counts since that's the one that modifies the BODY. Appendix B. ILBM Box Diagram Here's a box diagram for a simple example: an uncompressed image 320 x 200 pixels x 3 bitplanes. The text to the right of the diagram shows the outline that would be printed by the IFFCheck utility program for this particular file. +-----------------------------------+ |'FORM' 24070 | FORM 24070 IBLM +-----------------------------------+ |'ILBM' | +-----------------------------------+ | +-------------------------------+ | | | 'BMHD' 20 | | .BMHD 20 | | 320, 200, 0, 0, 3, 0, 0, ... | | | + ------------------------------+ | | | 'CMAP' 21 | | .CMAP 21 | | 0, 0, 0; 32, 0, 0; 64,0,0; .. | | | +-------------------------------+ | | 0 | +-----------------------------------+ |'BODY' 24000 | .BODY 24000 |0, 0, 0, ... | +-----------------------------------+ The "0" after the CMAP chunk is a pad byte. Appendix C. ByteRun1 Run Encoding The run encoding scheme byteRun1 is best described by psuedo code for the decoder Unpacker (called UnPackBits in the Macintosh* toolbox): UnPacker: LOOP until produced the desired number of bytes Read the next source byte into n SELECT n FROM [0..127] => copy the next n+1 bytes literally [-1..-127] => replicate the next byte -n+1 times -128 => noop ENDCASE; ENDLOOP; In the inverse routine Packer, it's best to encode a 2 byte repeat run as a replicate run except when preceded and followed by a literal run, in which case it's best to merge the three into one literal run. Always encode 3 byte repeats as replicate runs. Remember that each row of each scan line of a raster is separately packed. Appendix D. Standards Committee The following people contributed to the design of this FORM ILBM standard: Bob "Kodiak" Burns, Commodore-Amiga R. J. Mical, Commodore-Amiga Jerry Morrison, Electronic Arts Greg Riker, Electronic Arts Steve Shaw, Electronic Arts Dan Silva, Electronic Arts Barry Walsh, Commodore-Amiga