How To: Preparing Double Layer Cutout Textures

Introduction

The Problem: You have a single, solid texture and you want to assign some part of it a different color or material property (usually emissive) in your model JSON, while leaving the rest of the texture unmodified.

Solution: A simple way to accomplish this is by splitting the texture into two separate cutout textures, so that transparent pixels in each texture are solid in the other.

This article explains how to do this task using GIMP. If you use a different bitmap editor it will probably still be useful to you. If you have never used GIMP and want to try it, you should probably start here.