Add support for Magic Kernel Sharp 2021 resampling

Author: drhead

src/PIL/Image.py

@@ -169,6 +169,7 @@ class Resampling(IntEnum):
     HAMMING = 5
     BICUBIC = 3
     LANCZOS = 1
+    MKS2021 = 6
 
 
 _filters_support = {
@@ -177,6 +178,7 @@ class Resampling(IntEnum):
     Resampling.HAMMING: 1.0,
     Resampling.BICUBIC: 2.0,
     Resampling.LANCZOS: 3.0,
+    Resampling.MKS2021: 4.5,
 }
 
 
@@ -2242,6 +2244,7 @@ def resize(
             Resampling.LANCZOS,
             Resampling.BOX,
             Resampling.HAMMING,
+            Resampling.MKS2021,
         ):
             msg = f"Unknown resampling filter ({resample})."
 
@@ -2254,6 +2257,7 @@ def resize(
                     (Resampling.BICUBIC, "Image.Resampling.BICUBIC"),
                     (Resampling.BOX, "Image.Resampling.BOX"),
                     (Resampling.HAMMING, "Image.Resampling.HAMMING"),
+                    (Resampling.MKS2021, "Image.Resampling.MKS2021"),
                 )
             ]
             msg += f" Use {', '.join(filters[:-1])} or {filters[-1]}"
@@ -2904,11 +2908,12 @@ def __transformer(
             Resampling.BILINEAR,
             Resampling.BICUBIC,
         ):
-            if resample in (Resampling.BOX, Resampling.HAMMING, Resampling.LANCZOS):
+            if resample in (Resampling.BOX, Resampling.HAMMING, Resampling.LANCZOS, Resampling.MKS2021):
                 unusable: dict[int, str] = {
                     Resampling.BOX: "Image.Resampling.BOX",
                     Resampling.HAMMING: "Image.Resampling.HAMMING",
                     Resampling.LANCZOS: "Image.Resampling.LANCZOS",
+                    Resampling.MKS2021: "Image.Resampling.MKS2021",
                 }
                 msg = unusable[resample] + f" ({resample}) cannot be used."
             else:

src/libImaging/Imaging.h

@@ -269,6 +269,7 @@ ImagingError_Clear(void);
 #define IMAGING_TRANSFORM_HAMMING 5
 #define IMAGING_TRANSFORM_BICUBIC 3
 #define IMAGING_TRANSFORM_LANCZOS 1
+#define IMAGING_TRANSFORM_MKS2021 6
 
 typedef int (*ImagingTransformMap)(double *X, double *Y, int x, int y, void *data);
 typedef int (*ImagingTransformFilter)(void *out, Imaging im, double x, double y);

src/libImaging/Resample.c

@@ -79,11 +79,30 @@ lanczos_filter(double x) {
     return 0.0;
 }
 
+static inline double
+mks_2021_filter(double x) {
+    /* https://johncostella.com/magic/ */
+    if (x < 0.0)
+        x = -x;
+    if (x < 0.5)
+        return 577.0/576.0 - 239.0/144.0 * pow(x, 2.0);
+    if (x < 1.5)
+        return 35.0/36.0 * (x - 1.0) * (x - 239.0/140.0);
+    if (x < 2.5)
+        return 1.0/6.0 * (x - 2.0) * (65.0/24.0 - x);
+    if (x < 3.5)
+        return 1.0/36.0 * (x - 3.0) * (x - 15.0/4.0);
+    if (x < 4.5)
+        return -1.0/288.0 * pow(x - 9.0/2.0, 2.0);
+    return(0.0);
+}
+
 static struct filter BOX = {box_filter, 0.5};
 static struct filter BILINEAR = {bilinear_filter, 1.0};
 static struct filter HAMMING = {hamming_filter, 1.0};
 static struct filter BICUBIC = {bicubic_filter, 2.0};
 static struct filter LANCZOS = {lanczos_filter, 3.0};
+static struct filter MKS2021 = {mks_2021_filter, 4.5};
 
 /* 8 bits for result. Filter can have negative areas.
    In one cases the sum of the coefficients will be negative,
@@ -693,6 +712,9 @@ ImagingResample(Imaging imIn, int xsize, int ysize, int filter, float box[4]) {
         case IMAGING_TRANSFORM_LANCZOS:
             filterp = &LANCZOS;
             break;
+        case IMAGING_TRANSFORM_MKS2021:
+            filterp = &MKS2021;
+            break;
         default:
             return (Imaging)ImagingError_ValueError("unsupported resampling filter");
     }