Unity - native texture formats explained

Question

I have read the following here:

When you use a Texture compression format that is not supported on the target platform, the Textures are decompressed to RGBA 32 and stored in memory alongside the compressed Textures. When this happens, time is lost decompressing Textures, and memory is lost because you are storing them twice. In addition, all platforms have different hardware, and are optimised to work most efficiently with specific compression formats; choosing non-compatible formats can impact your game’s performance. The table below shows supported platforms for each compression format.

Let's discuss a specific case. Say I stored a .png file on a disc and packaged my game from Android. Now I play that game on an Android device whose GPU requires ETC2 as native texture format. Am I correct that when I enter the game the following should happen:

1. Read PNG file from disk to RAM (RAM is used for storing PNG file data)
2. Decompress PNG to RGBA32 (RAM is used for both PNG and for decompressed data)
3. Compress RGBA32 to ETC2 and upload to GPU (on RAM if I have a texture cash, then I might deallocate memory for PNG file data but I need to store RGBA32 for future reuse or at least I need to store ETC2)

This means I am doing lots of conversions between PNG->RBGA32->ETC2 and during that conversions I not only use CPU resource but also significantly utilize RAM. My question - did I correctly understood what happens when one does not package with native texture formats for targeted platform?

Answer 1

Yes, you kind of correctly understood what's going on here. However you misunderstood something: The way PNG relates to all of this. The compression methods implemented by GPUs to be applied on textures are very different to the filter+deflate method of PNG, so with every kind of GPU you have this kind of behavior.

What the Unity devs are trying to tell you is, that textures can be stored in the very format the GPU works with, and that for optimal performance you should identify which compression formats are supported on your target platform and bundle your asset file for that.

So for a game for platform X identify the compression formats supported by X GPUs, then pack your assets with that and ship the X version with that. Rinse and repeat for other platforms.