Alpha channels and normal map best practices
In this section of the guide, we look at some more best practices that you can try if you want to use alpha channels in your game.
Be selective when adding an alpha channel to a texture. Adding transparency often makes the texture larger in file size because images are converted to a 32-bit format, increasing the overall bandwidth use.
Another way to store an alpha channel is by using the extra channel in roughness, or metallic textures. In Unity, this texture sometimes uses two channels out of three, roughness (G) and metallic (B), leaving the (R) channel free for you to use.
By using the free channel to store the alpha mask, you can keep the diffuse texture at 16-bit, halving the file size. An ambient occlusion map usually can be baked in the diffuse map.
The following image shows an example of how you could store an opacity map in the red channel:
