FM Textures (Fur Maps) - Ezekial711/MonsterHunterWorldModding GitHub Wiki
FM stands for Fur Map. This is an example of a Fur Map from MHW:
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The blue channel determines intensity of the fur effect, red and green determine the directions and application of light on the section. Under extreme material parameters (mrl3) it becomes glass.
It's important to note that the fur effect is not exactly equivalent to length, it's the intensity of the transformations the game applies to the diffuse when fur is applied. In most cases it can be conflated with length however.
Blue determines the presence of fur. Red determines the direction ([0°,180°] mapped to [0,255]). Green similarly to red affects direction but of another layer of the fur. The Green layer of fur is more prominent than the red but both are visible under most circumstances.
The way the game renders the FM is roughly thus:
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The inverse process is how one paints their own furmaps. For simplicity sake only a red-blue fur map is shown but the process is analogous on the green channel.
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Credit to Asterisk for the documentation, and Miralis for the confirmation testing.