Gameboy Mimic Shader

/* COMPATIBILITY
   - HLSL compilers
   - Cg   compilers
*/

/*
   Gameboy Mimic Shader
   by Sp00kyFox, 2014

   Calculates luma and rounds it to one of 4 grayscales.

*/


#define TEX(dx,dy) tex2D(decal, VAR.texCoord+float2((dx),(dy))*VAR.t1)

static float3 weights = float3(0.299, 0.587, 0.114);

struct input
{
	float2 video_size;
	float2 texture_size;
	float2 output_size;
};

struct out_vertex {
        float4 position : POSITION;
        float2 texCoord : TEXCOORD0;
        float2 t1;
};


/*    VERTEX_SHADER    */
out_vertex main_vertex
(
	float4 position	: POSITION,
	float2 texCoord : TEXCOORD0,

   	uniform float4x4 modelViewProj,
	uniform input IN
)
{
        out_vertex OUT;

        OUT.position = mul(modelViewProj, position);

        float2 ps = float2(1.0/IN.texture_size.x, 1.0/IN.texture_size.y);
        float dx  = ps.x;
        float dy  = ps.y;

        OUT.texCoord = texCoord;
        OUT.t1       = float2(dx, dy); // F H

        return OUT;
}


/*    FRAGMENT SHADER    */
float3 main_fragment(in out_vertex VAR, uniform sampler2D decal : TEXUNIT0, uniform input IN) : COLOR
{

	float3 C = TEX( 0, 0);

	float luma = (ceil(4.0 * dot(C, weights)) - 1.0) / 3.0;

	return float3(luma);
}