Krrios File Format Reference

Colors
Index Files
Anim.mul
Anim.idx
AnimData.mul
Art.mul
ArtIdx.mul
Fonts.mul
GumpArt.mul
GumpIdx.mul
Hues.mul
Light.mul
LightIdx.mul
Map<n>.mul
MultiMap.rle
Multi.mul
Multi.idx
Palette.mul
RadarCol.mul
SJIS2Uni.mul
SkillGrp.mul
Skills.mul
Skills.idx
Sound.mul
SoundIdx.mul
Speech.mul
StaIdx<n>.mul
Statics<n>.mul
TexIdx.mul
TexMaps.mul
TileData.mul
UniFont[n].mul
VerData.mul

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Gumps

Data File: GumpArt.mul
Index File: GumpIdx.mul

You can compute the following values from the Extra field in the index file.

Sizes:

short width = ((extra >> 16) & 0xFFFF);
short height = (extra & 0xFFFF);

Data:

int[height] lookupTable;
RLE Data...

RLE stands for "run-length encoding", it is a simple form of image compression.
This implementation of the RLE compression is treated as 32-bit values, broken up into two 16-bit values

RLE Packet:

1F1E1D1C1B1A191817161514131211100F0E0D0C0B0A090807060504030201 00
Run Color

The Color field indicates which color should be repeated. The Run field represents the length of pixels:

for ( int i = 0; i < Run; i++ )
{
   SetPixel( x + i, y, Color );
}
x += Run;

See Also:
 - UO Color Format
 - Index Files