what is colorAttachment[n] in Metal?

Question

Very new to graphics in general, starting off with Metal for immediate needs, will try out OpenGL soon enough.

Was wondering what the question means in lay man terms. Also, what is the extent of 'n', I have just used it as 0 in the 2D triangle I made.

Answer 1

In general, the color attachment(s) are where rendered images are stored (at least temporarily) during a render pass. It is common to only use one color attachment at index 0, so what you're doing is fine. It is also possible to render to multiple color attachments simultaneously, which is why there's an array. It's an advanced technique that you don't have to worry about until you see the need, at which point it should be straightforward how to do it.

There are two places where colorAttachments[n] appears in Metal. First is in MTLRenderPipelineDescriptor. The other is in MTLRenderPassDescriptor.

In both cases, the extent is given in the Metal Implementation Limits table, in the "Render targets" section, in the row labelled "Maximum number of color render targets per render pass descriptor".

For MTLRenderPipelineDescriptor, colorAttachments[n] is a reference to a MTLRenderPipelineColorAttachmentDescriptor. Here, you configure the pixel format, write mask, and color blending operation.

For MTLRenderPassDescriptor, colorAttachments[n] is a reference to a MTLRenderPassColorAttachmentDescriptor. This is a subclass of MTLRenderPassAttachmentDescriptor, which is where most of its properties are defined. Here, you configure which part of which texture you will render to, what should happen to that texture's data when the render pass starts and ends, and, if it's to be cleared, what color it should be cleared to.

Information about color attachments is split across those two objects based on how expensive it is to change. The render pipeline state object is fairly expensive to create from the pipeline descriptor. You would typically create all of your pipeline state objects once in a run and reuse them for the rest of your app's lifetime.

By contrast, you will create render command encoders from render pass descriptors fairly often; at least once per frame. They are relatively inexpensive to create, so you can change the descriptor and create a new one to render elsewhere.

The render pipeline state's color attachment pixel format has to match the pixel format of the texture of the render command encoder's color attachment texture.