add Mitchell kernel

see https://github.com/jcupitt/libvips/issues/966

ChangeLog

@@ -28,6 +28,7 @@
 - vips7 C++ interface defaults off
 - make members, getters and operators "const" in cpp API
 - composite has params for x/y position of sub-images [medakk]
+- add Mitchell kernel
 
 12/3/18 started 8.6.4
 - better fitting of fonts with overhanging edges [Adrià]

libvips/conversion/composite.cpp

@@ -98,6 +98,13 @@ typedef struct _VipsCompositeBase {
 	 */
 	gboolean premultiplied;
 
+	/* The x and y positions for each image in the stack. There are n - 1 
+	 * of these, since image 0 is always positioned at (0, 0). Set by
+	 * subclasses. Can be NULL.
+	 */
+	int *x_offset;
+	int *y_offset;
+
 	/* The number of inputs. This can be less than the number of images in
 	 * @in.
 	 */
@@ -112,12 +119,6 @@ typedef struct _VipsCompositeBase {
 	 */
 	double max_band[MAX_BANDS + 1];
 
-	/* The x and y positions for each image in the stack. There are n - 1 
-	 * of these, since image 0 is always positioned at (0, 0).
-	 */
-	int *x_offset;
-	int *y_offset;
-
 #ifdef HAVE_VECTOR_ARITH
 	/* max_band as a vector, for the RGBA case.
 	 */

libvips/include/vips/resample.h

@@ -42,6 +42,7 @@ typedef enum {
 	VIPS_KERNEL_NEAREST,
 	VIPS_KERNEL_LINEAR,
 	VIPS_KERNEL_CUBIC,
+	VIPS_KERNEL_MITCHELL,
 	VIPS_KERNEL_LANCZOS2,
 	VIPS_KERNEL_LANCZOS3,
 	VIPS_KERNEL_LAST

libvips/iofuncs/enumtypes.c

@@ -877,6 +877,7 @@ vips_kernel_get_type( void )
 			{VIPS_KERNEL_NEAREST, "VIPS_KERNEL_NEAREST", "nearest"},
 			{VIPS_KERNEL_LINEAR, "VIPS_KERNEL_LINEAR", "linear"},
 			{VIPS_KERNEL_CUBIC, "VIPS_KERNEL_CUBIC", "cubic"},
+			{VIPS_KERNEL_MITCHELL, "VIPS_KERNEL_MITCHELL", "mitchell"},
 			{VIPS_KERNEL_LANCZOS2, "VIPS_KERNEL_LANCZOS2", "lanczos2"},
 			{VIPS_KERNEL_LANCZOS3, "VIPS_KERNEL_LANCZOS3", "lanczos3"},
 			{VIPS_KERNEL_LAST, "VIPS_KERNEL_LAST", "last"},

libvips/resample/reduceh.cpp

@@ -104,6 +104,7 @@ vips_reduce_get_points( VipsKernel kernel, double shrink )
 		return( rint( 2 * shrink ) + 1 ); 
 
 	case VIPS_KERNEL_CUBIC:
+	case VIPS_KERNEL_MITCHELL:
 		return( rint( 4 * shrink ) + 1 ); 
 
 	case VIPS_KERNEL_LANCZOS2:
@@ -135,7 +136,14 @@ vips_reduce_make_mask( double *c, VipsKernel kernel, double shrink, double x )
 		break;
 
 	case VIPS_KERNEL_CUBIC:
-		calculate_coefficients_adaptive_catmull( c, shrink, x ); 
+		/* Catmull-Rom.
+		 */
+		calculate_coefficients_cubic( c, shrink, x, 0.0, 0.5 );
+		break;
+
+	case VIPS_KERNEL_MITCHELL:
+		calculate_coefficients_cubic( c, shrink, x, 
+			1.0 / 3.0, 1.0 / 3.0 );
 		break;
 
 	case VIPS_KERNEL_LANCZOS2:

libvips/resample/resize.c

@@ -128,6 +128,7 @@ vips_resize_int_shrink( VipsResize *resize, double scale )
 
 		case VIPS_KERNEL_LINEAR:
 		case VIPS_KERNEL_CUBIC:
+		case VIPS_KERNEL_MITCHELL:
 		case VIPS_KERNEL_LANCZOS2:
 		case VIPS_KERNEL_LANCZOS3:
 		default:

libvips/resample/templates.h

@@ -316,7 +316,8 @@ calculate_coefficients_catmull( double c[4], const double x )
  * from the interpolator as well as from the table builder.
  */
 static void inline
-calculate_coefficients_triangle( double *c, const double shrink, const double x )
+calculate_coefficients_triangle( double *c, 
+	const double shrink, const double x )
 {
 	/* Needs to be in sync with vips_reduce_get_points().
 	 */
@@ -342,11 +343,18 @@ calculate_coefficients_triangle( double *c, const double shrink, const double x
 		c[i] /= sum;
 }
 
-/* Calculate a catmull kernel for shrinking. 
+/* Generate a cubic filter. See:
+ *
+ * Mitchell and Netravali, Reconstruction Filters in Computer Graphics 
+ * Computer Graphics, Volume 22, Number 4, August 1988.
+ *
+ * B = 1,   C = 0   - cubic B-spline
+ * B = 1/3, C = 1/3 - Mitchell
+ * B = 0,   C = 1/2 - Catmull-Rom spline
  */
 static void inline
-calculate_coefficients_adaptive_catmull( double *c, 
-	const double shrink, const double x )
+calculate_coefficients_cubic( double *c, 
+	const double shrink, const double x, double B, double C )
 {
 	/* Needs to be in sync with vips_reduce_get_points().
 	 */
@@ -361,18 +369,18 @@ calculate_coefficients_adaptive_catmull( double *c,
 		const double axp = VIPS_FABS( xp ); 
 		const double axp2 = axp * axp;
 		const double axp3 = axp2 * axp;
-		const double a = -0.5;
 
 		double l;
 
 		if( axp <= 1 ) 
-			l = (a + 2) * axp3 - 
-				(a + 3) * axp2 + 1;
+			l = ((12 - 9 * B - 6 * C) * axp3 +
+			     (-18 + 12 * B + 6 * C) * axp2 + 
+			     (6 - 2 * B)) / 6;
 		else if( axp <= 2 )
-			l = a * axp3 - 
-				5 * a * axp2 + 
-				8 * a * axp - 
-				4 * a;
+			l = ((-B - 6 * C) * axp3 +
+			     (6 * B + 30 * C) * axp2 + 
+			     (-12 * B - 48 * C) * axp + 
+			     (8 * B + 24 * C)) / 6;
 		else 
 			l = 0.0;