filter_kernels: add Magic Kernel Sharp 2013 and 2021 scalers

The quadratic B-spline ("magic kernel") convolved with the respective
discrete sharpening kernels, expressed as single continuous kernels
with a support of radius 2.5 (mks2013) and 4.5 (mks2021).

See https://johncostella.com/magic/ for details.

Author: kasper93

DOCS/interface-changes/mks-scalers.txt

@@ -0,0 +1 @@
+add `mks2013` and `mks2021` (Magic Kernel Sharp) scale filters

DOCS/man/options.rst

@@ -5920,6 +5920,17 @@ them.
         set to ``0.0`` and ``0.5`` respectively. This filter is sharper than
         ``mitchell``, but prone to ringing.
 
+    ``mks2013``
+        Magic Kernel Sharp 2013. The quadratic B-spline ("magic kernel")
+        combined with a sharpening step, expressed as a single kernel with a
+        support of radius 2.5. See https://johncostella.com/magic/ for details.
+
+    ``mks2021``
+        Magic Kernel Sharp 2021. A refinement of ``mks2013`` with a flatter
+        frequency response (support of radius 4.5), making it closer to true
+        interpolation, at a slightly higher cost and with marginally more
+        ringing.
+
     ``oversample``
         A version of nearest neighbour that (naively) oversamples pixels, so
         that pixels overlapping edges get linearly interpolated instead of

video/out/filter_kernels.c

@@ -319,6 +319,28 @@ static double spline64(params *p, double x)
     }
 }
 
+// Magic Kernel Sharp family: the quadratic B-spline ("magic kernel")
+// See https://johncostella.com/magic/
+static double mks2013(params *p, double x)
+{
+    // quadric(x) convolved with {-1/4, +3/2, -1/4}
+    if (x < 0.5)
+        return 17.0/16.0 - 7.0/4.0 * (x * x);
+    if (x < 1.5)
+        return (x * x) - 11.0/4.0 * x + 7.0/4.0;
+    if (x < 2.5) {
+        x -= 5.0/2.0;
+        return -1.0/8.0 * (x * x);
+    }
+    return 0.0;
+}
+
+static double mks2021(params *p, double x)
+{
+    return (mks2013(p, fabs(x - 2.0)) + 34.0 * mks2013(p, x) +
+            mks2013(p, x + 2.0)) / 36.0;
+}
+
 static double gaussian(params *p, double x)
 {
     return exp(-2.0 * x * x / p->params[0]);
@@ -420,6 +442,9 @@ const struct filter_kernel mp_filter_kernels[] = {
     {{SCALER_EWA_ROBIDOUXSHARP, 2,cubic_bc,
       .params = {6 / (13 + 7 * M_SQRT2), 7 / (2 + 12 * M_SQRT2)}},
         .polar = true},
+    // Magic Kernel Sharp filters, see https://johncostella.com/magic/
+    {{SCALER_MKS2013, 2.5, mks2013}},
+    {{SCALER_MKS2021, 4.5, mks2021}},
     // Miscellaneous filters
     {{SCALER_BOX,      1,   box, .resizable = true}},
     {{SCALER_NEAREST,  0.5, box}},

video/out/filter_kernels.h

@@ -42,6 +42,8 @@ enum scaler_filter {
     SCALER_ROBIDOUXSHARP,
     SCALER_EWA_ROBIDOUX,
     SCALER_EWA_ROBIDOUXSHARP,
+    SCALER_MKS2013,
+    SCALER_MKS2021,
     SCALER_BOX,
     SCALER_NEAREST,
     SCALER_TRIANGLE,

video/out/gpu/video.c

@@ -298,6 +298,8 @@ struct gl_video {
     {"mitchell",      SCALER_MITCHELL}, \
     {"robidoux",      SCALER_ROBIDOUX}, \
     {"robidouxsharp", SCALER_ROBIDOUXSHARP}, \
+    {"mks2013",       SCALER_MKS2013}, \
+    {"mks2021",       SCALER_MKS2021}, \
     {"box",           SCALER_BOX}, \
     {"nearest",       SCALER_NEAREST}, \
     {"triangle",      SCALER_TRIANGLE}, \