IMAGE(6) IMAGE(6)
NAME
image - external format for images
SYNOPSIS
#include <draw.h>
DESCRIPTION
Images are described in graphics(2), and the definition of
pixel values is in color(6). Fonts and images are stored in
external files in machine-independent formats.
Image files are read and written using readimage and
writeimage (see allocimage(2)),or readmemimage and
writememimage (see memdraw(2)). An uncompressed image file
starts with 5 strings: chan, r.min.x, r.min.y, r.max.x, and
r.max.y. Each is right-justified and blank padded in 11
characters, followed by a blank. The chan value is a tex-
tual string describing the pixel format (see strtochan in
graphics(2) and the discussion of channel descriptors
below), and the rectangle coordinates are decimal strings.
The rest of the file contains the r.max.y-r.min.y rows of
pixel data. A row consists of the byte containing pixel
r.min.x and all the bytes up to and including the byte con-
taining pixel r.max.x-1. For images with depth d less than
eight, a pixel with x-coordinate = x will appear as d con-
tiguous bits in a byte, with the pixel's high order bit
starting at the byte's bit number d*(x mod (8/d)), where
bits within a byte are numbered 0 to 7 from the high order
to the low order bit. Rows contain integral number of
bytes, so there may be some unused pixels at either end of a
row. If d is greater than 8, the definition of images
requires that it be a multiple of 8, so pixel values take up
an integral number of bytes.
The loadimage and unloadimage functions described in
allocimage(2) also deal with rows in this format, stored in
user memory.
The channel format string is a sequence of two-character
channel descriptions, each comprising a letter (r for red, g
for green, b for blue, a for alpha, m for color-mapped, k
for greyscale, and x for ``don't care'') followed by a num-
ber of bits per pixel. The sum of the channel bits per
pixel is the depth of the image, which must be either a
divisor or a multiple of eight. It is an error to have more
than one of any channel but x. An image must have either a
greyscale channel; a color mapped channel; or red, green,
and blue channels. If the alpha channel is present, it must
be at least as deep as any other channel.
Page 1 Plan 9 (printed 12/21/24)
IMAGE(6) IMAGE(6)
The channel string defines the format of the pixels in the
file, and should not be confused with ordering of bytes in
the file. In particular 'r8g8b8' pixels have byte ordering
blue, green, and red within the file. See color(6) for more
details of the pixel format.
A venerable yet deprecated format replaces the channel
string with a decimal ldepth, which is the base two loga-
rithm of the number of bits per pixel in the image. In this
case, ldepths 0, 1, 2, and 3 correspond to channel descrip-
tors k1, k2, k4, and m8, respectively.
Compressed image files start with a line of text containing
the word compressed, followed by a header as described
above, followed by the image data. The data, when uncom-
pressed, is laid out in the usual form.
The data is represented by a string of compression blocks,
each encoding a number of rows of the image's pixel data.
Compression blocks are at most 6024 bytes long, so that they
fit comfortably in a single 9P message. Since a compression
block must encode a whole number of rows, there is a limit
(about 5825 bytes) to the width of images that may be
encoded. Most wide images are in subfonts, which, at 1 bit
per pixel (the usual case for fonts), can be 46600 pixels
wide.
A compression block begins with two decimal strings of
twelve bytes each. The first number is one more than the y
coordinate of the last row in the block. The second is the
number of bytes of compressed data in the block, not includ-
ing the two decimal strings. This number must not be larger
than 6000.
Pixels are encoded using a version of Lempel & Ziv's sliding
window scheme LZ77, best described in J A Storer & T G Szy-
manski `Data Compression via Textual Substitution', JACM
29#4, pp. 928-951.
The compression block is a string of variable-length code
words encoding substrings of the pixel data. A code word
either gives the substring directly or indicates that it is
a copy of data occurring previously in the pixel stream.
In a code word whose first byte has the high-order bit set,
the rest of the byte indicates the length of a substring
encoded directly. Values from 0 to 127 encode lengths from
1 to 128 bytes. Subsequent bytes are the literal pixel
data.
If the high-order bit is zero, the next 5 bits encode the
length of a substring copied from previous pixels. Values
Page 2 Plan 9 (printed 12/21/24)
IMAGE(6) IMAGE(6)
from 0 to 31 encode lengths from 3 to 34 bytes. The bottom
two bits of the first byte and the 8 bits of the next byte
encode an offset backward from the current position in the
pixel data at which the copy is to be found. Values from 0
to 1023 encode offsets from 1 to 1024. The encoding may be
`prescient', with the length larger than the offset, which
works just fine: the new data is identical to the data at
the given offset, even though the two strings overlap.
Some small images, in particular 48x48 face files as used by
seemail (see faces(1) and face(6)) and 16x16 cursors, can be
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 sepa-
rately 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
jpg(1), tweak(1), graphics(2), draw(2), allocimage(2),
color(6), face(6), font(6)
Page 3 Plan 9 (printed 12/21/24)