Metal vertex shader draw points of a Texture - ios

Question

I want to execute Metal (or OpenGLES 3.0) shader that draws Points primitive with blending. To do that, I need to pass all the pixel coordinates of the texture to Vertex shader as vertices which computes the position of the vertex to be passed to fragment shader. The fragment shader simply outputs the color for the point with blending enabled. My problem is if there is an efficient was to pass coordinates of vertices to the vertex shader, since there would be too many vertices for 1920x1080 image, and that needs to be done 30 times in a second? Like we do in a compute shader by using dispatchThreadgroups command, except that compute shader can not draw a geometry with blending enabled.
EDIT: This is what I did -
let vertexFunctionRed = library!.makeFunction(name: "vertexShaderHistogramBlenderRed")
let fragmentFunctionAccumulator = library!.makeFunction(name: "fragmentShaderHistogramAccumulator")
let renderPipelineDescriptorRed = MTLRenderPipelineDescriptor()
renderPipelineDescriptorRed.vertexFunction = vertexFunctionRed
renderPipelineDescriptorRed.fragmentFunction = fragmentFunctionAccumulator
renderPipelineDescriptorRed.colorAttachments[0].pixelFormat = .bgra8Unorm
renderPipelineDescriptorRed.colorAttachments[0].isBlendingEnabled = true
renderPipelineDescriptorRed.colorAttachments[0].rgbBlendOperation = .add
renderPipelineDescriptorRed.colorAttachments[0].sourceRGBBlendFactor = .one
renderPipelineDescriptorRed.colorAttachments[0].destinationRGBBlendFactor = .one
do {
histogramPipelineRed = try device.makeRenderPipelineState(descriptor: renderPipelineDescriptorRed)
} catch {
print("Unable to compile render pipeline state Histogram Red!")
return
}
Drawing code:
let commandBuffer = commandQueue?.makeCommandBuffer()
let renderEncoder = commandBuffer?.makeRenderCommandEncoder(descriptor: renderPassDescriptor!)
renderEncoder?.setRenderPipelineState(histogramPipelineRed!)
renderEncoder?.setVertexTexture(metalTexture, index: 0)
renderEncoder?.drawPrimitives(type: .point, vertexStart: 0, vertexCount: 1, instanceCount: metalTexture!.width*metalTexture!.height)
renderEncoder?.drawPrimitives(type: .point, vertexStart: 0, vertexCount: metalTexture!.width*metalTexture!.height, instanceCount: 1)
and Shaders:
vertex MappedVertex vertexShaderHistogramBlenderRed (texture2d<float, access::sample> inputTexture [[ texture(0) ]],
unsigned int vertexId [[vertex_id]])
{
MappedVertex out;
constexpr sampler s(s_address::clamp_to_edge, t_address::clamp_to_edge, min_filter::linear, mag_filter::linear, coord::pixel);
ushort width = inputTexture.get_width();
ushort height = inputTexture.get_height();
float X = (vertexId % width)/(1.0*width);
float Y = (vertexId/width)/(1.0*height);
int red = inputTexture.sample(s, float2(X,Y)).r;
out.position = float4(-1.0 + (red * 0.0078125), 0.0, 0.0, 1.0);
out.pointSize = 1.0;
out.colorFactor = half3(1.0, 0.0, 0.0);
return out;
}
fragment half4 fragmentShaderHistogramAccumulator ( MappedVertex in [[ stage_in ]]
)
{
half3 colorFactor = in.colorFactor;
return half4(colorFactor*(1.0/256.0), 1.0);
}

Answer 1

Maybe you can draw a single point instanced 1920x1080 times. Something like:
vertex float4 my_func(texture2d<float, access::read> image [[texture(0)]],
constant uint &width [[buffer(0)]],
uint instance_id [[instance_id]])
{
// decompose the instance ID to a position
uint2 pos = uint2(instance_id % width, instance_id / width);
return float4(image.read(pos).r * 255, 0, 0, 0);
}