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Man Pages


Manual Reference Pages  -  RLE (5)

NAME

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

CONTENTS

Description
See Also
Author

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

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5 RLE (5) 9/14/82

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