uniform sampler2D sample;
uniform float textureWidth;
uniform float textureHeight;
uniform float opacity;
uniform float brightness;
uniform float saturation;
uniform int u_forceAlpha;

varying vec2 varyingTexCoords;

// Converts pixel coordinates to texture coordinates
vec2 pix2tex( vec2 pix )
{
    return vec2( pix.s / textureWidth, pix.t / textureHeight );
}

void main()
{
    vec4 tex = texture2D( sample, pix2tex( varyingTexCoords ));
    if( saturation != 1.0 )
        {
        vec3 desaturated = tex.rgb * vec3( 0.30, 0.59, 0.11 );
        desaturated = vec3( dot( desaturated, tex.rgb ));
        tex.rgb = tex.rgb * vec3( saturation ) + desaturated * vec3( 1.0 - saturation );
        }
    tex.rgb = tex.rgb * opacity * vec3( brightness );
    tex.rgb = vec3(1.0) - tex.rgb;
    tex.a = tex.a * opacity;
    if (u_forceAlpha > 0) {
        tex.a = 1.0;
    }

    gl_FragColor = tex;
}