iOS Metal Shader - Texture read and write access?

Question

I'm using a metal shader to draw many particles onto the screen. Each particle has its own position (which can change) and often two particles have the same position. How can I check if the texture2d I write into does not have a pixel at a certain position yet? (I want to make sure that I only draw a particle at a certain position if there hasn't been drawn a particle yet, because I get an ugly flickering if many particles are drawn at the same positon)

I've tried outTexture.read(particlePosition), but this obviously doesn't work, because of the texture access qualifier, which is access::write.

Is there a way I can have read and write access to a texture2d at the same time? (If there isn't, how could I still solve my problem?)

Answer 1

There are several approaches that could work here. In concurrent systems programming, what you're talking about is termed first-write wins.

1) If the particles only need to preclude other particles from being drawn (and aren't potentially obscured by other elements in the scene in the same render pass), you can write a special value to the depth buffer to signify that a fragment has already been written to a particular coordinate. For example, you'd turn on depth test (using the depth compare function Equal), clear the depth buffer to some distant value (like 1.0), and then write a value of 0.0 to the depth buffer in the fragment function. Any subsequent write to a given pixel will fail to pass the depth test and will not be drawn.

2) Use framebuffer read-back. On iOS, Metal allows you to read from the currently-bound primary renderbuffer by attributing a parameter to your fragment function with [[color(0)]]. This parameter will contain the current color value in the renderbuffer, which you can test against to determine whether it has been written to. This does require you to clear the texture to a predetermined color that will never otherwise be produced by your fragment function, so it is more limited than the above approach, and possibly less performant.

All of the above applies whether you're rendering to a drawable's texture for direct presentation to the screen, or to some offscreen texture.

Answer 2

To answer the read and write part : you can specify a read/write access for the output texture as such :

texture2d<float, access::read_write> outTexture [[texture(1)]],

Also, your texture descriptor must specify usage :

textureDescriptor?.usage = [.shaderRead, .shaderWrite]