struct PS_INPUT {
    float2 uv: TEXCOORD0;
};

struct PS_OUTPUT {
    float4 color_0: SV_TARGET0;
};

uniform float2 texel_size;

float2 unpack_velocity(float4 data) {return float2(data.x + (data.y / 255.0), data.z + (data.w / 255.0));}
float4 pack_velocity(float2 data) {return float4(floor(data.x * 255.0) / 255.0, frac(data.x * 255.0), floor(data.y * 255.0) / 255.0, frac(data.y * 255.0));}
float unpack_divergence(float2 data) {return data.x + (data.y / 255.0);}
float2 pack_divergence(float data) {return float2(floor(data * 255.0) / 255.0, frac(data * 255.0));}

void main(in PS_INPUT IN, out PS_OUTPUT OUT) {
    float2 right = unpack_velocity(tex2D(gm_BaseTexture, IN.uv + float2(texel_size.x, 0.0))) - 128.0 / 255.0;
    float2 left = unpack_velocity(tex2D(gm_BaseTexture, IN.uv - float2(texel_size.x, 0.0))) - 128.0 / 255.0;
    float2 bottom = unpack_velocity(tex2D(gm_BaseTexture, IN.uv + float2(0.0, texel_size.y))) - 128.0 / 255.0;
    float2 top = unpack_velocity(tex2D(gm_BaseTexture, IN.uv - float2(0.0, texel_size.y))) - 128.0 / 255.0;

    float divergence = 0.5 * ((right.x - left.x) + (bottom.y - top.y));

    OUT.color_0 = float4(pack_divergence(saturate(divergence + 128.0 / 255.0)), 0.0, 0.0);
}