Recently I noticed that some strange magenta pixels (1,0,1,1) appeared on objects in Android Packages. After a quick investigation through Renderdoc, I noticed some textures compressed in ASTC6x6 format have there mipmaps lower than 4x4 filled with magenta pixels.

I'm not sure which way is correct to handle the 4x4 mipmaps and lower for ASTC6x6. These small mipmaps should probably be filled with the correct down-sampled pixels (but still take up 6x6 space), or simply not generate them. However, in no way should they be filled with magenta pixels.

Quick Solution

There are two ASTC encoders used in Unreal Engine, one is astcenc provided by Arm and the other is ISPCTextureCompressor provided by Intel. Currently we are using ISPCTextureCompressor.

I found a similar case reported by other developers in the Epic UDN forum and the solution is to use astcenc instead of ISPCTextureCompressor to solve the problem. One of Epic staff replied that astcenc is now the default and recommended ASTC encoder and ISPCTextureCompressor support in Unreal is deprecated and no longer maintained.

// when GASTCCompressor == 0 ,use TextureFormatIntelISPCTexComp instead of this
// @todo Oodle : GASTCCompressor global breaks DDC2.  Need to pass through so TBW can see.
int32 GASTCCompressor = 1;
static FAutoConsoleVariableRef CVarASTCCompressor(
	TEXT("cook.ASTCTextureCompressor"),
	GASTCCompressor,
	TEXT("0: IntelISPC, 1: Arm"),
	ECVF_Default | ECVF_ReadOnly
);

An interesting history of astcenc in UE

The solution reminds me of an interesting background. In the early days of Unreal Engine 4, ISTCTextureCompressor was the default ASTC encoder used in Unreal Engine. Although it has some obvious issues (it doesn't support 10x10 and 12x12), it was still the default encoder for a long time. The main reason, I guess, is that Epic guys grabbed a wrong version of astcenc and got some misleading benchmark results.

In UE4.26, a very old version of astcenc (1.3) was bundled in Unreal Engine. It was built in about 2013, which was a 32bit executable and without any SIMD optimization.

So, it's not surprising that ISPCTextureCompressor completely crushes astcenc in terms of speed.

If you are still using UE4.26 or earlier, the first choice is to grab a latest astcenc3 from internet and replace the old one, and pick some code change from UE5 to make it work(the cmdline options are different). The second choice is to recompile astcenc 1.3 with correct configuration with SIMD optimization and 64bit support and replace the binary in UE directory.

We ran into this problem perhaps also because some configurations were forgotten in the config file during the upgrade of the engine.