precision mediump float;

attribute vec2 aVertexPosition;
attribute vec2 aTextureCoord;
attribute vec4 aColor;

uniform mat3 projectionMatrix;
uniform vec2 resolution;

varying vec2 vTextureCoord;
varying vec4 vColor;

varying vec2 vResolution;

//texcoords computed in vertex step
//to avoid dependent texture reads
varying vec2 v_rgbNW;
varying vec2 v_rgbNE;
varying vec2 v_rgbSW;
varying vec2 v_rgbSE;
varying vec2 v_rgbM;


void texcoords(vec2 fragCoord, vec2 resolution,
            out vec2 v_rgbNW, out vec2 v_rgbNE,
            out vec2 v_rgbSW, out vec2 v_rgbSE,
            out vec2 v_rgbM) {
    vec2 inverseVP = 1.0 / resolution.xy;
    v_rgbNW = (fragCoord + vec2(-1.0, -1.0)) * inverseVP;
    v_rgbNE = (fragCoord + vec2(1.0, -1.0)) * inverseVP;
    v_rgbSW = (fragCoord + vec2(-1.0, 1.0)) * inverseVP;
    v_rgbSE = (fragCoord + vec2(1.0, 1.0)) * inverseVP;
    v_rgbM = vec2(fragCoord * inverseVP);
}

void main(void){
   gl_Position = vec4((projectionMatrix * vec3(aVertexPosition, 1.0)).xy, 0.0, 1.0);
   vTextureCoord = aTextureCoord;
   vColor = vec4(aColor.rgb * aColor.a, aColor.a);
   vResolution = resolution;

   //compute the texture coords and send them to varyings
   texcoords(aTextureCoord * resolution, resolution, v_rgbNW, v_rgbNE, v_rgbSW, v_rgbSE, v_rgbM);
}