An Alias pix file has a 10-byte header containing 5 short integers (there is no explicit magic number) which is then followed immediately by image data in a simple run-length encoded scheme. This file format has been deprecated, and is no longer created by the software.
Only RGB information is contained in the file. Matte files are similar but exist in a separate file (see Matte File Format for details).
| bytes | header value | notes |
| 0, 1 | width | x resolution in pixels |
| 2, 3 | height | y resolution in pixels |
| 4, 5 | xoffset | unused |
| 6, 7 | yoffset | unused |
| 8, 9 | bits/pixel | 24 for pix files (0x18) |
The pixels are then run-length encoded in 4-byte packets on a per-scanline basis (runs do not extend beyond a single scanline) starting with the top scanline in the image.
| bytes | data range | notes |
| runlength | 1 - 255 | number of pixels in succession with given RGB |
| blue | 0 - 255 | value of blue component |
| green | 0 - 255 | value of green component |
| red | 0 - 255 | value of red component |
Here is the output of od -x for a pix file that is 8 pixels wide and 6 pixels high, representing a ramp that goes from black at the bottom of the image to blue at the top:
0000000 0008 0006 0000 0005 0018 08ff 0000 08cc 0000020 0000 0899 0000 0866 0000 0833 0000 0800 0000040 0000 0000042
This is read as describing an image that is 8 pixels wide [0008] and 6 scanlines high [0006]. The next four bytes describe the obsolete offset information. This is a pix file since there are 24 bits/pixel [0018]. The first (top) scanline is composed of a run of 8 pixels of (B=255, G=0, R=0)[08ff 0000]. The next scanline (since this one is complete) is 8 pixels of (B=204, G=0, R=0) [08cc 0000]. The rest of the scanlines are coded in the same fashion with the last scanline of eight pixels of (B=0, G=0, R=0) [0800 0000].
