#version 300 es

precision highp float;
precision highp sampler2D;

in vec2 texCoord;
out vec4 fragColor;

uniform sampler2D xTex;
uniform sampler2D bTex;

float x(ivec2 coord)
{
	if(  coord.x <= 0
	  || coord.x >= textureSize(xTex, 0).x
	  || coord.y <= 0
	  || coord.y >= textureSize(xTex, 0).y)
	{
		return(0.);
	}
	return(texelFetch(xTex, coord, 0).x);
}

float b(ivec2 coord)
{
	if(  coord.x <= 0
	  || coord.x >= textureSize(bTex, 0).x
	  || coord.y <= 0
	  || coord.y >= textureSize(bTex, 0).y)
	{
		return(0.);
	}
	return(texelFetch(bTex, coord, 0).x);
}

float residual(ivec2 coord)
{
	ivec2 vr = coord + ivec2(1, 0);
	ivec2 vl = coord - ivec2(1, 0);
	ivec2 vt = coord + ivec2(0, 1);
	ivec2 vb = coord - ivec2(0, 1);

	return((x(vl) + x(vr) + x(vt) + x(vb) + b(coord) - 4.*x(coord))*4.);
}

void main()
{
	ivec2 pixelCoord = ivec2(floor(gl_FragCoord.xy));

	float restricted = residual(2*pixelCoord + ivec2(-1, -1))
	                 + residual(2*pixelCoord + ivec2(1, -1))
	                 + residual(2*pixelCoord + ivec2(1, 1))
	                 + residual(2*pixelCoord + ivec2(-1, 1))
	                 + 2.*residual(2*pixelCoord + ivec2(-1, 0))
	                 + 2.*residual(2*pixelCoord + ivec2(1, 0))
	                 + 2.*residual(2*pixelCoord + ivec2(0, -1))
	                 + 2.*residual(2*pixelCoord + ivec2(0, 1))
	                 + 4.*residual(2*pixelCoord);
	restricted /= 16.;
	fragColor = vec4(restricted, 0, 0, 1);
}