IMAGE(6) IMAGE(6)
NAME
image - external format for images
DESCRIPTION
There are two formats for storing Inferno images in files.
The simpler but deprecated format is identical with the only
bitmap file format originally used in Plan 9. The other is
a compressed format for more efficient storage and
transmission (now also used in Plan 9). From Limbo, either
format may be read but Display.writeimage (see draw-
display(2)) produces only the compressed form.
An image is a rectangular array of pixels. An uncompressed
image 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 number of bits in the image's pixels is
1<<ldepth. The x coordinates of the rectangle range from
r.min.x to r.max.x-1, left to right; y coordinates range
from r.min.y to r.max.y-1, top to bottom. 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 containing pixel
r.max.x-1. A pixel with x-coordinate = x in an image with
ldepth = ld will appear as w = 2^ld 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.
A compressed image file begins with the 11 bytes
"compressed\n", immediately followed by 5 decimal strings
giving the image's ldepth and rectangle, as above. The rest
of the file is 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 com-
fortably in a single Styx 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
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IMAGE(6) IMAGE(6)
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
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.
Page 2 Plan 9 (printed 12/21/24)