// This is a lanczos2 kernel for a nice 2x downsample. I wish I'd documented how I generated it but I didn't so instead
//  this is what you get. Sorry about that.
//static const float k_lanczos2[8] = {-0.009, -0.042, 0.117, 0.434, 0.434, 0.117, -0.042, -0.009};

// However, it's been optimized to take advantage of the linear filtering using the same technique I used for the
//  gaussian filters at https://drilian.com/gaussian-kernel/
// That means it's only 4 texture samples for an 8-tap lanczos, which is, if I did my math correctly, twice as good!

const int k_sampleCount = 4;
const float k_coeffs[k_sampleCount] = float[k_sampleCount](
  -0.051,
   0.551,
   0.551,
  -0.051
);

const float k_offsets[k_sampleCount] = float[k_sampleCount](
  -2.67647052,
  -0.712341249,
   0.712341189,
   2.67647052
);


vec4 Lanczos2xDownsample(
  sampler2D decal,
  vec2 centerTexCoord,
  vec2 filterDir)
{
  ivec2 dim = ivec2(params.SourceSize.xy);

  vec4 v = vec4(0.0, 0.0, 0.0, 0.0);
  for (int i = 0; i < k_sampleCount; i++)
  {
    vec2 c = centerTexCoord + filterDir / float2(dim) * k_offsets[i];
    v += texture(decal, c) * k_coeffs[i];
  }

  return v;
}