#version 330
//fragment shader output
layout(location=0) out vec4 vFragColor;

//input from the vertex shader
smooth in vec2  vUV;      //current texture coordinates

//shader uniforms
uniform sampler2D x0;       //old density or velocity field

//determine texture size
float sizeX = textureSize(x0, 0).x;
float sizeY = textureSize(x0, 0).y;
float stepX = 1.0/sizeX;
float stepY = 1.0/sizeY;


void main(void)
{
    vec4 C  = texture(x0, vUV);
    vec4 N  = texture(x0, vUV + vec2( 0    , -stepY));
    vec4 S  = texture(x0, vUV + vec2( 0    ,  stepY));
    vec4 W  = texture(x0, vUV + vec2(-stepX,  0    ));
    vec4 E  = texture(x0, vUV + vec2( stepX,  0    ));

    vFragColor = -4*C + N + S + W + E;
}