Metal Fragment Shader, access current framebuffer color - fragment-shader

Question

I am writing a metal shader.
All I want is to access the current color of the fragment.
For example.
At the end of my fragment shader, when I put
return currentColor + 0.1, for example
The result will be screen going from black to white at FPS.
This is a basic program that draws a triangle strip that fills the screen.
Ultimate aim is writing a path tracer inside the shader, I have done this with opengl + glsl.
I am having trouble with the buffers. I thought an easy solution is to just pass the current output color back to the shader and average it in there.
These are the shaders:
#include <metal_stdlib>
using namespace metal;
#import "AAPLShaderTypes.h"
vertex float4 vertexShader(uint vertexID [[vertex_id]], constant AAPLVertex *vertices [[buffer(AAPLVertexInputIndexVertices)]], constant vector_uint2 *viewportSizePointer [[buffer(AAPLVertexInputIndexViewportSize)]], constant vector_float4 * seeds) {
float4 clipSpacePosition = vector_float4(0.0, 0.0, 0.0, 1.0);
float2 pixelSpacePosition = vertices[vertexID].position.xy;
vector_float2 viewportSize = vector_float2(*viewportSizePointer);
clipSpacePosition.xy = pixelSpacePosition / (viewportSize / 2.0);
return clipSpacePosition;
}
fragment float4 fragmentShader()
{
// I want to do this
// return currentColor + 0.1;
return float4(1.0, 1.0, 1.0, 1.0);
}

Answer 1

No worries, the answer was staring me in the face the whole time.
Pass in the current color into the fragment shader with float4 color0 [[color(0)]]
With these 2 options set
renderPassDescriptor.colorAttachments[0].storeAction = MTLStoreActionStore;
renderPassDescriptor.colorAttachments[0].loadAction = MTLLoadActionLoad;
My problem was not the color, it was in calculating the average.
In my case I could not do regular average, as in add up all the colors so far and divide by the number of samples.
What I actually needed to do was calculate a Moving average.
https://en.wikipedia.org/wiki/Moving_average#Cumulative_moving_average