Compositing
From Legacy Roblox Wiki
Compositing is combining two images together to create one. This is to prevent a repetitive look in your textures. You can tell how different things can be by looking at this image:
You can clearly see a definitive pattern in the left one. On the right, you can slightly see it, however it is much harder to notice. The top image is just simply a slightly transparent image. Something that you can learn to create on ROBLOX to create effective textures.
In fact, ROBLOX uses compositing already. If you look at the grass and slate and other textures of a part, you'll notice that it looks irregular. That there is no pattern to it. This is because the texture is employing compositing.
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