金属着色器输出数据（GPGPU）

Question

I'm trying to implement some image detection algorithms on ios and I need some data preparation that should work as shader (Metal) so it should pass output array with result data.

Small example: SWIFT

...
// Create buffer
var resultBuffer = device.makeBuffer(length: SIZE * MemoryLayout<Int32>.size, options: .storageModeShared)!

// Set buffer to encoder
computeCommandEncoder?.setBuffer(resultBuffer, offset: 0, index: 0)
...
// Call shader
commandBuffer.commit()
commandBuffer.waitUntilCompleted()

// Print first element for test
var p = resultBuffer.contents()
let first = p.load(as: UInt32.self)
print("first = \(first)")

MSL (Metal Shader)

...
kernel void preparation(
  texture2d<float, access::read> inTexture [[texture(0)]], // Input texture

  texture2d<float, access::write> outTexture [[texture(1)]], // Output texture
  device uint* result [[ buffer(0) ]], // Output buffer
                uint2 gid [[thread_position_in_grid]])
{
    result[0] = 5; // Just for test set first element to 5
    float4 colorAtPixel = inTexture.read(gid);
    outTexture.write(colorAtPixel, gid); // Copy color from input texture to output texture
}

So I expect it should print

first = 5

But it prints

first = 0

So it seems shader does not change buffer content? Any ideas how to make it to work?

PS I use iOs 11.2, iPhone 5S, XCode 9.2

Answer 1

You are attempting to write to the same output memory location from every pixel in the thread group. If you are writing to a memory location, you have to take special care to only write to 1 memory location for each pixel identified by each gid element. The better way to do this is to just read from an input texture and write to the output texture for each pixel, just remove the uint*result buffer and use the textures directly and your headaches will go away.