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I am working on a painting program where I draw interactive strokes via an MTKView. If I set the renderPassDescriptor loadAction to 'clear': 

renderPassDescriptor?.colorAttachments[0].loadAction = .clear

The frame buffer, as expected, shows the latest contents of renderCommandEncoder?.drawPrimitives, which is this case is the leading edge of the brushstroke.

If I set loadAction to 'load': 

renderPassDescriptor?.colorAttachments[0].loadAction = .load

The frame buffer flashes like crazy and shows a patchy trail of what I've just drawn. I now understand that the flashing is likely caused by MTKView's default triple buffering in place. Thus, each time I write to the currentDrawable, I'm likely writing to one of 3 cycling buffers. Please correct me if I'm wrong.

My question is, what do I need to do to draw a clean brushstroke without the frame buffer flashing as it does now? In other words, is there a way to have a master buffer that gets updated with the latest contents of commandEncoder?

Plutovman
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1 Answers1

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You can use a texture of your own as the color attachment of a render pass. You don't have to use the texture of a drawable. In that way, you can use the .load action without getting garbage or weird flashing or whatever. You will have full control over which texture you're rendering to and what its contents are.

After rendering to that texture for a render pass, you then need to blit that to the drawable's texture for display.

The main complication here is that you won't have the benefits of double- or triple-buffering. You'll lose a certain amount of performance, since everything will have to be synced to that one texture's state. I suspect, though, that you don't need that much performance, since this is interactive and only has to keep up with the speed of a human.

Ken Thomases
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