letterbox and nearest resampling in a shader

letterboxShader.metal

kernel void
nearestKernel(texture2d<float, access::read>  inTexture  [[texture(AAPLTextureIndexInput)]],
              texture2d<float, access::write> outTexture [[texture(AAPLTextureIndexOutput)]],
              constant ResizeParams &resizeParams [[buffer(0)]],
              uint2 gid [[thread_position_in_grid]])
{
    int input_width = inTexture.get_width();
    int input_height = inTexture.get_height();
    float origin_aspect = float(input_width) / float(input_height);
    
    int output_width = outTexture.get_width();
    int output_height = outTexture.get_height();
    float new_aspect = float(output_width) / float(output_height);

    float u_ratio, v_ratio;
    float u_offset, v_offset;
    if (origin_aspect < new_aspect)
    {
        u_ratio = float(output_width) * float(input_height) / (float(input_width) * float(output_height));
        v_ratio = 1;
        u_offset = (float(output_width) - float(input_width) * float(output_height) / float(input_height)) * 0.5;
        v_offset = 0;
    }
    else
    {
        u_ratio = 1;
        v_ratio = float(output_height) * float(input_width) / (float(input_height) * float(output_width));
        u_offset = 0;
        v_offset = (float(output_height) - float(input_height) * float(output_width) / float(input_width)) * 0.5;
    }

    
    float u = float(gid.x - u_offset) * u_ratio / float(output_width - 1);
    float v = float(gid.y - v_offset) * v_ratio / float(output_height - 1);
    int s = int(u * float(input_width));
    int t = int(v * float(input_height));
    
    if (s < 0 || t < 0 || s >= input_width || t >= input_height)
        outTexture.write(float4(0,0,0,1), gid);
    else
    {
        float4 sample = inTexture.read(uint2(s, t));
        outTexture.write(sample, gid);
    }
}