Add a bicubic filter.

stb_resample.h

@@ -27,8 +27,9 @@ Initial implementation by Jorge L Rodriguez
 
 typedef enum
 {
-	STBR_FILTER_NEAREST = 1,
-	STBR_FILTER_LINEAR = 2,
+	STBR_FILTER_NEAREST     = 1,
+	STBR_FILTER_BILINEAR    = 2,
+	STBR_FILTER_BICUBIC     = 3,  // A cubic b spline
 } stbr_filter;
 
 typedef enum
@@ -212,13 +213,15 @@ static stbr_inline int stbr__max(int a, int b)
 
 static float stbr__filter_nearest(float x)
 {
-	if (fabs(x) <= 0.5)
+	x = (float)fabs(x);
+
+	if (x <= 0.5)
 		return 1;
 	else
 		return 0;
 }
 
-static float stbr__filter_linear(float x)
+static float stbr__filter_bilinear(float x)
 {
 	x = (float)fabs(x);
 
@@ -228,10 +231,25 @@ static float stbr__filter_linear(float x)
 		return 0;
 }
 
+static float stbr__filter_bicubic(float x)
+{
+	x = (float)fabs(x);
+
+	float xx = x*x;
+
+	if (x < 1.0f)
+		return 0.5f * (x * xx) - xx + 0.66666666666f;
+	else if (x < 2.0f)
+		return -0.16666666f * (x * xx) + xx - 2 * x + 1.3333333333f;
+
+	return (0.0f);
+}
+
 static stbr__filter_info stbr__filter_info_table[] = {
-		{ NULL,                 0.0f },
-		{ stbr__filter_nearest, 0.5f },
-		{ stbr__filter_linear,  1.0f },
+		{ NULL,                  0.0f },
+		{ stbr__filter_nearest,  0.5f },
+		{ stbr__filter_bilinear, 1.0f },
+		{ stbr__filter_bicubic,  2.0f },
 };
 
 stbr_inline static int stbr__use_width_upsampling_noinfo(int output_w, int input_w)

tests/resample_test.cpp

@@ -48,7 +48,7 @@ int main(int argc, char** argv)
 	int in_w = 512;
 	int in_h = 512;
 
-	size_t memory_required = stbr_calculate_memory(in_w, in_h, w*n, out_w, out_h, out_stride, n, STBR_FILTER_LINEAR);
+	size_t memory_required = stbr_calculate_memory(in_w, in_h, w*n, out_w, out_h, out_stride, n, STBR_FILTER_BICUBIC);
 	void* extra_memory = malloc(memory_required);
 
 	// Cut out the outside 64 pixels all around to test the stride.
@@ -56,7 +56,7 @@ int main(int argc, char** argv)
 	STBR_ASSERT(in_w + border <= w);
 	STBR_ASSERT(in_h + border <= h);
 
-	stbr_resize_arbitrary(input_data + w * border * n + border * n, in_w, in_h, w*n, output_data, out_w, out_h, out_stride, n, STBR_TYPE_UINT8, STBR_FILTER_LINEAR, STBR_EDGE_CLAMP, extra_memory, memory_required);
+	stbr_resize_arbitrary(input_data + w * border * n + border * n, in_w, in_h, w*n, output_data, out_w, out_h, out_stride, n, STBR_TYPE_UINT8, STBR_FILTER_BICUBIC, STBR_EDGE_CLAMP, extra_memory, memory_required);
 
 	free(extra_memory);