NAME

bitmap − external format for bitmaps

SYNOPSIS

­#include <libg.h>

DESCRIPTION

Bitmaps are described in graphics(2). Fonts and bitmaps are stored in external files in machine-independent formats.

Bitmap files are read and written using ­rdbitmapfile and ­wrbitmapfile (see balloc(2)). A bitmap file starts with 5 decimal strings: ldepth, r.min.x, r.min.y, r.max.x, and r.max.y.  Each number is right-justified and blank padded in 11 characters, followed by a blank.  The rest of the file contains the r.max.y−r.min.y rows of bitmap data.  A ­row consists of the byte containing pixel ­r.min.x and all the bytes up to and including the byte containing pixel ­r.min.x−1.  A pixel with x-coordinate = ­x in a bitmap with ­ldepth = ­l will appear as w = 2l contiguous bits in a byte, with the pixel’s high order bit starting at the byte’s bit number w×((x mod 8)/w), where bits within a byte are numbered 0 to 7 from the high order to the low order bit.  If ­w is greater than 8, it is a multiple of 8, so pixel values take up an integral number of bytes.  Rows contain integral number of bytes, so there may be some unused pixels at either end of a row. 

The ­rdbitmap and ­wrbitmap functions described in balloc(2) also deal with rows in this format, stored in user memory.

Some small images, in particular 48×48 face files as used by ­seemail (see mail(1)) and 16×16 cursors, are stored textually, suitable for inclusion in C source. Each line of text represents one scan line as a comma-separated sequence of hexadecimal bytes, shorts, or words in C format. For cursors, each line defines a pair of bytes. (It takes two images to define a cursor; each must be stored separately to be processed by programs such as tweak(1).) Face files of one bit per pixel are stored as a sequence of shorts, those of larger pixel sizes as a sequence of longs. Software that reads these files must deduce the image size from the input; there is no header. These formats reflect history rather than design.

SEE ALSO

tweak(1), graphics(2), bitblt(2), balloc(2), font(6)

Plan 9  —  January 03, 1993