Confused about the generation of mipmap in metal?

Question

I set mipmapped to YES in texture2DDescriptorWithPixelFormat and call generateMipmapsForTexture method of MTLBlitCommandEncoder on given texture to automatically generate mipmaps.

The question is if I have set the mipmapped to YES, doesn't it means that the resulting image should be mipmapped, why should I need MTLBlitCommandEncoder to explicit generate mipmaps?

Answer 1

It's a bit confusing, so let's walk through it.

texture2DDescriptorWithPixelFormat takes format, width, height and mipmapped as parameters. The mipmapped parameter is there to tell Metal to calculate the number of mip levels that resulting image will have, since there is no parameter to pass mip level count. Here's how it's described in documentation:

mipmapped

A Boolean indicating whether the resulting image should be mipmapped. If YES, then the mipmapLevelCount property in the returned descriptor is computed from width and height. If NO, then mipmapLevelCount is 1.

If you would use newTextureWithDescriptor with texture descriptor you created explicitly, then there's no mipmapped parameter, since you explicitly pass number of mip levels in mipmapLevelCount property of MTLTextureDescriptor.

Since you create a new texture, there's no reason to generate mipmaps, since the texture is empty.

The generateMipmapsForTexture method is used to generate mipmaps for a texture that already has mip levels and you just need to populate them with mipmaps generated automatically.

So, to get this straight, mipmapped parameter just tells Metal to create a texture that has mip levels, that you can later populate (if you want) with generateMipmapsForTexture (or in some other ways, such as using texture as color attachment in render pass with level specified).