wrlib: change default scaling interpolation to Catmull-Rom

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

@@ -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 */

wrlib/scale.c

@@ -244,8 +244,25 @@ static double Mitchell_filter(double t)
 	return (0.0);
 }
 
-static double (*filterf)(double) = Mitchell_filter;
+#define	CatmullRom_support	(2.0)
-static double fwidth = Mitchell_support;
+
+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;
 	}
 }
 

wrlib/wraster.h.in

@@ -143,7 +143,8 @@ typedef enum {
 	RBellFilter,
 	RBSplineFilter,
 	RLanczos3Filter,
-	RMitchellFilter
+	RMitchellFilter,
+	RCatmullRomFilter
 } RScalingFilter;