#pragma MSP stage(compute)
layout(local_size_x=8, local_size_y=8) in;

uniform sampler2D surface_in;
uniform sampler2D bottom_in;
uniform sampler2D normals_in;
layout(r16f) uniform image2D variance_out;

void main()
{
	ivec2 coord = ivec2(gl_GlobalInvocationID.xy)+ivec2(4, 1);
	float variance = 0.0;
	for(int i=-3; i<=3; ++i)
	{
		ivec2 c = coord+ivec2(i, 0);
		float depth = texelFetch(surface_in, c, 0).x-texelFetch(bottom_in, c, 0).x;
		vec2 normal_xy = texelFetch(normals_in, c, 0).xy*2.0-1.0;
		float xy_sq = dot(normal_xy, normal_xy);
		float slope_sq = xy_sq/max(1-xy_sq, 0.001);
		variance += slope_sq*clamp(depth, 0.0, 1.0);
	}
	imageStore(variance_out, coord, vec4(variance/7.0, 0.0, 0.0, 0.0));
}