Metal Texture is not filterable

Question

I am trying to mipmap a texture contained in an MTLTexture object. This texture was loaded from an OpenCV Mat. I can run correctly run kernels on this texture so I know my import process is correct.

Unfortunately, the generate mipmaps function gives this rather opaque error. I get a similar error even if I change temp to be BGRA.

-[MTLDebugBlitCommandEncoder generateMipmapsForTexture:]:1074: 
failed assertion `tex(MTLPixelFormatR8Uint) is not filterable.'

// create an MTL Texture
{
    MTLTextureDescriptor * textureDescriptor = [MTLTextureDescriptor

    texture2DDescriptorWithPixelFormat:MTLPixelFormatR8Uint
                                                   width:cols
                                                   height:rows
                                                   mipmapped:NO];
    textureDescriptor.usage = MTLTextureUsageShaderRead;
    _mImgTex = [_mDevice newTextureWithDescriptor:textureDescriptor];
}

{
    MTLTextureDescriptor * textureDescriptor = [MTLTextureDescriptor

    texture2DDescriptorWithPixelFormat:MTLPixelFormatR8Uint
                                                   width:cols
                                                   height:rows
                                                   mipmapped:YES];
    textureDescriptor.mipmapLevelCount = 5;
    textureDescriptor.usage = MTLTextureUsageShaderRead | MTLTextureUsageShaderWrite;
    _mPyrTex = [_mDevice newTextureWithDescriptor:textureDescriptor];
}


// copy data to GPU
cv::Mat temp;
cv::cvtColor(image, temp, cv::COLOR_BGRA2GRAY);
MTLRegion region = MTLRegionMake2D(0, 0, cols, rows);
const int bytesPerPixel = 1 * 1; // 1 uint * 1 channels
const int bytesPerRow = bytesPerPixel * cols;

[_mImgTex replaceRegion:region mipmapLevel:0 withBytes:temp.data bytesPerRow:bytesPerRow];

// try to mipmap
id<MTLBlitCommandEncoder> blitEncoder = [commandBuffer blitCommandEncoder];
MTLOrigin origin = MTLOriginMake(0, 0, 0);
MTLSize size = MTLSizeMake(cols, rows, 1);
[blitEncoder copyFromTexture:_mImgTex sourceSlice:0 sourceLevel:0 sourceOrigin:origin sourceSize:size toTexture:_mPyrTex destinationSlice:0 destinationLevel:0 destinationOrigin:origin];
[blitEncoder generateMipmapsForTexture:_mPyrTex];
[blitEncoder endEncoding];

Answer 1

The documentation for generateMipmapsForTextures says:

Mipmap generation works only for textures with color-renderable and color-filterable pixel formats.

If you look at the "Pixel Format Capabilities" table here, you can see that R8Uint does not support Filter nor is it colour renderable (Color).

Perhaps R8Unorm (MTLPixelFormatR8Unorm) will work well for your needs. Otherwise you might need to write your own mip generation code with compute (although I'm not sure if there's a use case for mipmaps with non filterable textures).