// uniform float gradients[16];
// uniform vec2 gradients[16];
uniform vec2 gradients[16]; // "corner" vectors
uniform int grad_size; // size of a cell
uniform int grad_quant; // how many gradients

#define mat(i,j) gradients[i+grad_quant*j]

float get_value_at(ivec2 pixel_coord) {

    // which gradient is this pixel in?
    ivec2 grad_coord = ivec2(int(pixel_coord.x/float(grad_size)), int(pixel_coord.y/float(grad_size)));

    // what pixel is this gradient pixel?
    ivec2 grad_coord_scaled = grad_coord * grad_size;

    // vector with 0..1 values
    vec2 nm = vec2(pixel_coord - grad_coord_scaled) / float(grad_size);

    vec2 A = mat(grad_coord.x,     grad_coord.y);
    vec2 B = mat(grad_coord.x + 1, grad_coord.y);
    vec2 C = mat(grad_coord.x,     grad_coord.y + 1);
    vec2 D = mat(grad_coord.x + 1, grad_coord.y + 1);

    float dot_a = dot(A, nm);
    float dot_b = dot(B, vec2(nm.x - 1.0f, nm.y));
    float dot_c = dot(C, vec2(nm.x,        nm.y - 1.0f));
    float dot_d = dot(D, vec2(nm.x - 1.0f, nm.y - 1.0f));

    // return
    // mix(
    //     A,B,
    //     vec2(float((pixel_coord.x/10)%4)/4.f, float((pixel_coord.y/10)%4)/4.f)
    //     ).x+0.3;
    return abs(
        mix(
            mix(dot_a, dot_b, nm.x),
            mix(dot_c, dot_d, nm.x),
            nm.y
            ));
    ;
}

void main()
{
    // float val = get_value_at(gl_FragCoord.xy);
    // float val = get_value_at(glTexCoord.xy);
    gl_FragColor = get_value_at(ivec2(glTexCoord.x, glTexCoord.y));
    // gl_FragColor = vec4(0,1,0,1);
    gl_FragColor = vec4(0,val,0,1);

}