flat shading

//-----------------------------------------------------------------------------------------
// Pixel Shader
//-----------------------------------------------------------------------------------------
float4 ColorPS(
    float4 position : TEXCOORD0) : SV_Target
{
    // Hardcode a generic directional light
    float3 L = normalize(float3(0.5, 0.25, -1.0));

    // Compute the surface normal
    float3 dx = ddx(position);
    float3 dy = ddy(position);
    float3 dn = normalize(cross(dx, dy));

    // Calculate a half-lambert diffuse factor
    float NdotL = saturate(-dot(dn, L) * 0.5 + 0.5);

    // Basic colouring
    float4 color = float4(1, 1, 1, 1);
    float3 diffuse = color.xyz * NdotL;

    // Return the result
    return float4(diffuse, 1.0f);
}