NAME

rle_hdr - Structure for communication with RLE functions.

SYNOPSIS

#include <rle.h>

rle_hdr rle_dflt_hdr;

RLE_SET_BIT(the_hdr,bit)
RLE_CLR_BIT(the_hdr,bit)
RLE_BIT(the_hdr,bit)
rle_hdr the_hdr;

DESCRIPTION

This data structure provides communication to and between all the RLE(5) file routines. It describes the parameters of the image being saved or read, and contains some variables describing file state that are private to the routines. The public components are described below.

     typedef unsigned char rle_pixel;
     typedef unsigned short rle_map;
     rle_hdr {
	int	ncolors,	/* Number of colors being saved */
		*bg_color,	/* Background color array */
		alpha,	/* if ≠ 0, save alpha channel (color -1) */
			/* alpha channel background is always 0 */
		background,	/* if = 0, no background processing */
			/* if = 1 or 2, save only non-bg pixels */
			/* If 2, set clear-to-bg flag in file */
		xmin,	/* Min X bound of saved raster */
		xmax,	/* Max X bound */
		ymin,	/* Min Y bound */
		ymax,	/* Max Y bound */
		ncmap,	/* number of color channels in color map */
			/* if = 0, color map is not saved */
		cmaplen;	/* Log2 of the number of entries in */
			/* each channel of the color map */
	rle_map	*cmap;	/* pointer to color map, stored as 16-bit */
			/* words, with values left justified */
	char	**comments;	/* Pointer to array of pointers */
			/* to comment strings. */
	FILE *	rle_file;	/* I/O to this file */
	/* 
	 * Bit map of channels to read/save.  Indexed by (channel mod 256).
	 */
	char    bits[256/8];
    };

A global variable, rle_dflt_hdr, is available, conveniently initialized with default values.

FIELDS

ncolors: The number of colors (exclusive of the alpha channel) in the image. This is one greater than the largest channel index (i.e., ncolors would be 3 if channels 0, 1, and 2 were saved, or if only channel 2 were saved.)
bg_color: A pointer to an array of ncolors integers, defines the background color (if used). The background alpha value is always 0, so is not included in the bg_color array.
alpha: If non-zero, an alpha channel is present as channel -1. This should always be 0 or 1. Rle_get_setup and rle_put_setup enforce this constraint. The alpha channel will only be actually read or written if the corresponding bit in bits is also set.
background: Controls whether background color processing is done. If 0, no background processing is done at all (and bg_color is ignored). If 1 or 2, then runs of 3 or more pixels in the background color are not saved at all. If 2, then these runs will be restored by rle_getrow; if 1, they will not (this can lead to some strange images).
xmin, xmax, ymin, ymax: The bounds of the image. All pixels from xmin to xmax, inclusive, in rows numbered from ymin to ymax, inclusive, will be saved. Thus the dimensions of the image are
(xmax - xmin + 1) × (ymax - ymin + 1)
ncmap, cmaplen: The size of the saved colormap (if any). The color map will have ncmap channels, each 2^cmaplen long. If ncmap is zero, no color map is present.
cmap: A pointer to colormap data, if present. The data is stored in "channel major" order, so that all the values for channel 0 precede all the values for channel 1, etc. Each individual value is left-justified in 16 bits (i.e., the range of values is 0-65535).
comments: A pointer to picture comment data, if present. Use the functions rle_putcom(3), rle_getcom(3), and rle_delcom(3) to manipulate this field.
rle_file: The standard I/O FILE pointer for the file containing this image.
bits: A bitmap that selects the channels that are actually written to/read from the file. The macros below are used to modify this bitmap.

MACROS

The macro RLE_BIT will retrieve the state of one of the bits in the bits map. RLE_SET_BIT, and RLE_CLR_BIT set and clear bits in the bits map. The predefined symbols RLE_RED, RLE_GREEN, RLE_BLUE, and RLE_ALPHA, or an integer value from -1 to 254 may be used in these macros.

AUTHOR