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"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