"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
