Because this format is sourced on IRIX, the word format is bit[15..0]. Fetched words will have to be swapped for PC based readers.
First, a 10-byte header:
BYTES DESCRIPTION
0, 1 Width -- X res in pixels.
2, 3 Height -- Y res in pixels.
4, 5 X offset -- unused (obsolete, but still present).
6, 7 Y offset -- unused (obsolete, but still present).
8, 9 Bits per pixel -- always 0x18, or 8 bits each of red, green,
and blue (they have no other supported bit depth at this time)
The pixels are then run-length encoded in 4 byte packets on a per-scanline basis, starting with the top scanline in the image. Individual runs do not extend beyond a single scanline.
PACKET
BYTE RANGE DESCRIPTION
0 0x01 - 0xFF Run length - number of pixels in succession
with given RGB.
1 0x00 - 0xFF Blue component.
2 0x00 - 0xFF Green component.
3 0x00 - 0xFF Red component.
Here's a sample:
00000000 0008 0006 0000 0005 0018 08FF 0000 08CC 00000020 0000 0899 0000 0866 0000 0833 0000 0800 00000040 0000
This sample describes an 8 x 6 ramp from blue (at the top of the image) to black. The 10 byte header is easy enough (note the obsolete offset info of 0, 5): [0008 0006 0000 0005 0018]. This is followed by [08FF 0000], which is 8 pixels of [FF 00 00], or pure blue. All 8 bytes of the first line are satisfied, so the next packet begins the next scan line: [08CC 0000], and so on.
Alias has recently been purchased by SGI, merged with another company, Wavefront, and is now Alias | Wavefront. PowerAnimator, their SGI-based 3D modeler / renderer package uses a simple file format called PIX for individual images. There is a more complex format that references multiple PIX files as layers of a larger file (along with alpha channel matte files) that is used by their StudioPaint product, but it's so hopelessly huge and complex that it's better to merge all the layers using their software and then decode the resulting PIX.
[This information was kindly provided by Rob Neve]
