feat: Implement NICE/Lanczos-3 filtering

Author: koerismo

include/vtfpp/ImageConversion.h

@@ -375,6 +375,7 @@ enum class ResizeFilter {
 
 	// User-defined
 	KAISER = 100,
+	LANCZOS3 = 101,
 };
 
 enum class ResizeMethod {

src/vtfpp/ImageConversion.cpp

@@ -1942,6 +1942,24 @@ std::vector<std::byte> ImageConversion::resizeImageData(std::span<const std::byt
 				stbir_set_filter_callbacks(&resize, KAISER_FILTER, KAISER_SUPPORT, KAISER_FILTER, KAISER_SUPPORT);
 				break;
 			}
+			case ResizeFilter::LANCZOS3: {
+				static constexpr auto SINC = [](const float x) -> float {
+					if (x == 0.f) return 1.f;
+					const float a = x * math::pi_f32;
+					return sinf(a) / a;
+				};
+				// Normally you would max(1, inv_scale), but stb is weird and immediately un-scales the result when downsampling.
+				// See stb_image_resize2.h L2977
+				static constexpr auto LANCZOS3_SUPPORT = [](float inv_scale, void*) -> float {
+					return inv_scale * 3.f;
+				};
+				static constexpr auto LANCZOS3_FILTER = [](float x, float inv_scale, void*) -> float {
+					if (x >= 3.f || x <= -3.f) return 0.f;
+					return SINC(x) * SINC(x / 3.f);
+				};
+				stbir_set_filter_callbacks(&resize, LANCZOS3_FILTER, LANCZOS3_SUPPORT, LANCZOS3_FILTER, LANCZOS3_SUPPORT);
+				break;
+			}
 		}
 		stbir_resize_extended(&resize);
 	};