NAME

DESCRIPTION

Word 0

A "magic" number 0xcc52. (Byte order 0x52, 0xcc.)

Words 1-4

The structure (chars saved in PDP-11 order)

{
    short   xpos,                       /* Lower left corner
            ypos,
            xsize,                      /* Size of saved box
            ysize;
}
    

Byte 10

(flags) The following flags are defined:

Byte 11

(ncolors) Number of color channels present. 0 means load only the color map (if present), 1 means a B&W image, 3 means a normal color image.

Byte 12

(pixelbits) Number of bits per pixel, per color channel. Values greater than 8 currently will not work.

Byte 13

(ncmap) Number of color map channels present. Need not be identical to ncolors. If this is non-zero, the color map follows immediately after the background colors.

Byte 14

(cmaplen) Log base 2 of the number of entries in the color map for each color channel. I.e., would be 8 for a color map with 256 entries.

Bytes 15-...

The background color. There are ncolors bytes of background color. If ncolors is even, an extra padding byte is inserted to end on a 16 bit boundary. The background color is only present if H_NO_BACKGROUND is not set in flags. IF H_NO BACKGROUND is set, there is a single filler byte. Background color is ignored, but present, if H_CLEARFIRST is not set in flags.

If ncmap is non-zero, then the color map will follow as ncmap*2^cmaplen 16 bit words. The color map data is left justified in each word.

If the H_COMMENT flag is set, a set of comments will follow. The first 16 bit word gives the length of the comments in bytes. If this is odd, a filler byte will be appended to the comments. The comments are interpreted as a sequence of null terminated strings which should be, by convention, of the form name=value, or just name.

Following the setup information is the Run Length Encoded image. Each instruction consists of an opcode, a datum and possibly one or more following words (all words are 16 bits). The opcode is encoded in the first byte of the instruction word. Instructions come in either a short or long form. In the short form, the datum is in the second byte of the instruction word; in the long form, the datum is a 16 bit value in the word following the instruction word. Long form instructions are distinguished by having the 0x40 bit set in the opcode byte. The instruction opcodes are:

SkipLines (1)

The datum is an unsigned number to be added to the current Y position.

SetColor (2)

The datum indicates which color is to be loaded with the data described by the following ByteData and RunData instructions. Typically, 0→red, 1→green, 2→blue. The operation also resets the X position to the initial X (i.e. a carriage return operation is performed).

SkipPixels (3)

The datum is an unsigned number to be added to the current X position.

ByteData (5)

The datum is one less than the number of bytes of color data following. If the number of bytes is odd, a filler byte will be appended to the end of the byte string to make an integral number of 16-bit words. The X position is incremented to follow the last byte of data.

RunData (6)

The datum is one less than the run length. The following word contains (in its lower 8 bits) the color of the run. The X position is incremented to follow the last byte in the run.

EOF (7)

This opcode indicates the logical end of image data. A physical end-of-file will also serve as well. The EOF opcode may be used to concatenate several images in a single file.

SEE ALSO

AUTHOR