(Question) Converting BC4 to BMP with texconv about directxtex

[walbourn]

The default decompress format for BC4 maps to DXGI_FORMAT_R8_UNORM which is a single red channel (BC4 encodes just one channel). When DXGI_FORMAT_R8_UNORM is passed to the WIC writer, the default format for WIC maps to GUID_WICPixelFormat8bppGray. For the .BMP codec, this is written as a BI_RLE8 which is a palette mode, and the BMP palette is the identity (0-255 maps to greyscale colors 0-255).

The default decompress format for BC5 maps to DXGI_FORMAT_R8G8_UNORM which is a two channel format (BC5 encodes two channels). When DXGI_FORMAT_R8G8_UNORM is passed to the WIC writer, there is no WIC format that exactly matches so that writer fails. Therefore, you have to provide the target format you want to use.

With that out the way, when you provide -f rgba the target format for the conversion becomes DXGI_FORMAT_R8G8B8A8_UNORM. The tool first decompresses BC4 to R8, then converts from R8 to R8G8B8A8. The default behavior for such conversions is to do a 'greyscale expansion' as you see to set the RED channel to R, GREEN channel to R, and BLUE channel to R, and the ALPHA channel to 1.

If on the other hand you provide a BC5, then it decompresses to R8G8, then converts R8G8 to R8G8B8A8. The default behavior for such conversions is to copy R to red, G to green, and set B to zero, and A to 1.

From my understanding of your question, you want BC4 to write out to R8G8B8A8 where R is red, G is 0, B is zero, and A is 1. You can do this with the -swizzle option. In your case -swizzle r001 which will copy the red channel and then set the other channels to fixed values.