#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);
}

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

	if(  pixelCoord.x <= 0
	  || pixelCoord.y <= 0)
	{
		fragColor = vec4(0, 0, 0, 1);
	}
	else
	{
		float tl = x(pixelCoord + ivec2(-1, 1));
		float tr = x(pixelCoord + ivec2(1, 1));
		float bl = x(pixelCoord + ivec2(-1, -1));
		float br = x(pixelCoord + ivec2(1, -1));

		float jacobi = (tl + tr + bl + br + b(pixelCoord))/4.;
		fragColor = vec4(jacobi, 0, 0, 1);
	}
}