[PATCH 3/5] wrlib: change default scaling interpolation to Catmull-Rom

[david.m...@gmail.com]

This patch is implementing a new default Catmull-Rom filter
to resize images.
Catmull-Rom is a special case of cubic interpolation with B=0 and
C=0.5 (in the Mitchell-Netravali formulation).
It provides slighlty sharper results than Mitchell with the same
performance.
Catmull-Rom is a better choice for a window manager as it prioritizes
sharpness, making small elements feel crisp.
---
 wrlib/context.c    |  2 +-
 wrlib/scale.c      | 27 ++++++++++++++++++++++++---
 wrlib/wraster.h.in |  3 ++-
 3 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/wrlib/context.c b/wrlib/context.c
index 73044506..6bc4c85a 100644
--- a/wrlib/context.c
+++ b/wrlib/context.c
@@ -554,7 +554,7 @@ RContext *RCreateContext(Display * dpy, int screen_number, const RContextAttribu
 
  if (!(context->attribs->flags & RC_ScalingFilter)) {
  context->attribs->flags |= RC_ScalingFilter;
- context->attribs->scaling_filter = RMitchellFilter;
+ context->attribs->scaling_filter = RCatmullRomFilter;
  }
 
  /* get configuration from environment variables */
diff --git a/wrlib/scale.c b/wrlib/scale.c
index 93c5172d..da715ea5 100644
--- a/wrlib/scale.c
+++ b/wrlib/scale.c
@@ -244,8 +244,25 @@ static double Mitchell_filter(double t)
  return (0.0);
 }
 
-static double (*filterf)(double) = Mitchell_filter;
-static double fwidth = Mitchell_support;
+#define CatmullRom_support (2.0)
+
+static double CatmullRom_filter(double t)
+{
+ double tt;
+
+ tt = t * t;
+ if (t < 0)
+ t = -t;
+ if (t < 1.0) {
+ return ((1.5 * t * tt) - (2.5 * tt) + 1.0);
+ } else if (t < 2.0) {
+ return ((-0.5 * t * tt) + (2.5 * tt) - (4.0 * t) + 2.0);
+ }
+ return (0.0);
+}
+
+static double (*filterf)(double) = CatmullRom_filter;
+static double fwidth = CatmullRom_support;
 
 void wraster_change_filter(RScalingFilter type)
 {
@@ -270,11 +287,15 @@ void wraster_change_filter(RScalingFilter type)
  filterf = Lanczos3_filter;
  fwidth = Lanczos3_support;
  break;
- default:
  case RMitchellFilter:
  filterf = Mitchell_filter;
  fwidth = Mitchell_support;
  break;
+ default:
+ case RCatmullRomFilter:
+ filterf = CatmullRom_filter;
+ fwidth = CatmullRom_support;
+ break;
  }
 }
 
diff --git a/wrlib/wraster.h.in b/wrlib/wraster.h.in
index 6414455e..e197c8d7 100644
--- a/wrlib/wraster.h.in
+++ b/wrlib/wraster.h.in
@@ -143,7 +143,8 @@ typedef enum {
  RBellFilter,
  RBSplineFilter,
  RLanczos3Filter,
- RMitchellFilter
+ RMitchellFilter,
+ RCatmullRomFilter
 } RScalingFilter;
 
 
--
2.43.0