image – external format for images
Images are described in
and the definition of pixel values is in
Fonts and images are stored in external files
in machine-independent formats.
Image files are read and written using
An uncompressed image file starts with 5
Each is right-justified and blank padded in 11 characters, followed by a blank.
value is a textual string describing the pixel format
and the discussion of channel descriptors below),
and the rectangle coordinates are decimal strings.
The rest of the file contains the
rows of pixel data.
consists of the byte containing pixel
and all the bytes up to and including the byte containing pixel
For images with depth
less than eight, a pixel with x-coordinate =
will appear as
contiguous 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.
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.
functions described in
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
for “don’t care”)
followed by a number 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
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.
The channel string defines the format of the pixels in the file,
and should not be confused with ordering of bytes in the file.
pixels have byte ordering blue, green, and red within the file.
for more details of the pixel format.
A venerable yet deprecated format replaces the channel string
with a decimal
which is the base two logarithm of the number
of bits per pixel in the image.
In this case,
0, 1, 2, and 3
correspond to channel descriptors
Compressed image files start with a line of text containing the word
followed by a header as described above, followed by the image data.
The data, when uncompressed, 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
coordinate of the last row in the block. The second is the number
of bytes of compressed data in the block, not including 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 Szymanski
‘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.
Some small images, in particular 48×48 face files
as used by
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 separately
to be processed by programs such as
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.