image – external format for images

#include <draw.h>

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 textual 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 containing pixel r.max.x–1. For images with depth d less than eight, a pixel with x–coordinate = x will appear as d contiguous bits in a byte, with the pixel's high order bit starting at the byte's bit number (x mod (8/w)), 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 will 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 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 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.

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 logarithm of the number of bits per pixel in the image. In this case, ldepths 0, 1, 2, and 3 correspond to channel descriptors 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 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 y 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 seemail (see faces(1) and face(6)) and 16×16 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 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.

jpg(1), tweak(1), graphics(2), draw(2), allocimage(2), color(6), face(6), font(6)
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