RLE(5)

NAME
rle - Run length encoded file format produced by the rle library

DESCRIPTION
The output file format is (note: all words are 16 bits, and in PDP-11 byte order):

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:

    H_CLEARFIRST
        (0x1) If set, clear the frame buffer to background color before restoring.

    H_NO_BACKGROUND
        (0x2) If set, no background color is supplied. If H_CLEARFIRST is also set, it should be ignored (or alternatively, a clear-to-black operation could be performed).

    H_ALPHA
        (0x4) If set, an alpha channel is saved as color channel -1. The alpha channel does not contribute to the count of colors in ncolors.

    H_COMMENT
        (0x8) If set, comments will follow the color map in the header.

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
librle(3)

AUTHOR

Spencer W. Thomas, Todd Fuqua