Fix the calculation of the input image shift when using subpixel regions. Includes some more tests which had incorrect results before and now work fine.

stb_image_resize.h

@@ -1004,31 +1004,31 @@ stbir__inline static int stbir__edge_wrap(stbir_edge edge, int n, int max)
 }
 
 // What input pixels contribute to this output pixel?
-static void stbir__calculate_sample_range_upsample(int n, float out_filter_radius, float scale_ratio, float out_shift, int* in_first_pixel, int* in_last_pixel, float* in_center_of_out)
+static void stbir__calculate_sample_range_upsample(int n, float out_filter_radius, float scale_ratio, float in_shift, int* in_first_pixel, int* in_last_pixel, float* in_center_of_out)
 {
     float out_pixel_center = (float)n + 0.5f;
     float out_pixel_influence_lowerbound = out_pixel_center - out_filter_radius;
     float out_pixel_influence_upperbound = out_pixel_center + out_filter_radius;
 
-    float in_pixel_influence_lowerbound = (out_pixel_influence_lowerbound + out_shift) / scale_ratio;
-    float in_pixel_influence_upperbound = (out_pixel_influence_upperbound + out_shift) / scale_ratio;
+    float in_pixel_influence_lowerbound = out_pixel_influence_lowerbound / scale_ratio + in_shift;
+    float in_pixel_influence_upperbound = out_pixel_influence_upperbound / scale_ratio + in_shift;
 
-    *in_center_of_out = (out_pixel_center + out_shift) / scale_ratio;
+    *in_center_of_out = out_pixel_center / scale_ratio + in_shift;
     *in_first_pixel = (int)(floor(in_pixel_influence_lowerbound + 0.5));
     *in_last_pixel = (int)(floor(in_pixel_influence_upperbound - 0.5));
 }
 
 // What output pixels does this input pixel contribute to?
-static void stbir__calculate_sample_range_downsample(int n, float in_pixels_radius, float scale_ratio, float out_shift, int* out_first_pixel, int* out_last_pixel, float* out_center_of_in)
+static void stbir__calculate_sample_range_downsample(int n, float in_pixels_radius, float scale_ratio, float in_shift, int* out_first_pixel, int* out_last_pixel, float* out_center_of_in)
 {
     float in_pixel_center = (float)n + 0.5f;
     float in_pixel_influence_lowerbound = in_pixel_center - in_pixels_radius;
     float in_pixel_influence_upperbound = in_pixel_center + in_pixels_radius;
 
-    float out_pixel_influence_lowerbound = in_pixel_influence_lowerbound * scale_ratio - out_shift;
-    float out_pixel_influence_upperbound = in_pixel_influence_upperbound * scale_ratio - out_shift;
+    float out_pixel_influence_lowerbound = (in_pixel_influence_lowerbound - in_shift) * scale_ratio;
+    float out_pixel_influence_upperbound = (in_pixel_influence_upperbound - in_shift) * scale_ratio;
 
-    *out_center_of_in = in_pixel_center * scale_ratio - out_shift;
+    *out_center_of_in = (in_pixel_center - in_shift) * scale_ratio;
     *out_first_pixel = (int)(floor(out_pixel_influence_lowerbound + 0.5));
     *out_last_pixel = (int)(floor(out_pixel_influence_upperbound - 0.5));
 }
@@ -2229,8 +2229,8 @@ static void stbir__calculate_transform(stbir__info *info, float s0, float t0, fl
         info->horizontal_scale = ((float)info->output_w / info->input_w) / (s1 - s0);
         info->vertical_scale = ((float)info->output_h / info->input_h) / (t1 - t0);
 
-        info->horizontal_shift = s0 * info->input_w / (s1 - s0);
-        info->vertical_shift = t0 * info->input_h / (t1 - t0);
+        info->horizontal_shift = s0 * info->input_w;
+        info->vertical_shift = t0 * info->input_h;
     }
 }
 

tests/resample_test.cpp

@@ -1,4 +1,5 @@
-#include <malloc.h>
+#include <stdlib.h>
+#include <stdio.h>
 
 #if defined(_WIN32) && _MSC_VER > 1200
 #define STBIR_ASSERT(x) \
@@ -356,6 +357,30 @@ void test_subpixel(const char* file, float width_percent, float height_percent,
 	free(output_data);
 }
 
+void test_subpixel_really(const char* file, float width_percent, float height_percent, float s0, float t0, float s1, float t1)
+{
+	int w, h, n;
+	unsigned char* input_data = stbi_load(file, &w, &h, &n, 0);
+
+	if (input_data == NULL)
+		return;
+
+	int new_w = (int)(w * width_percent);
+	int new_h = (int)(h * height_percent);
+
+	unsigned char* output_data = (unsigned char*)malloc(new_w * new_h * n * sizeof(unsigned char));
+
+	stbir_resize_region(input_data, w, h, 0, output_data, new_w, new_h, 0, STBIR_TYPE_UINT8, n, STBIR_ALPHA_CHANNEL_NONE, 0, STBIR_EDGE_CLAMP, STBIR_EDGE_CLAMP, STBIR_FILTER_BOX, STBIR_FILTER_CATMULLROM, STBIR_COLORSPACE_SRGB, &g_context, s0, t0, s1, t1);
+
+	stbi_image_free(input_data);
+
+	char output[200];
+	sprintf(output, "test-output/subpixel-really-%d-%d-%f-%f-%f-%f-%s", new_w, new_h, s0, t0, s1, t1, file);
+	stbi_write_png(output, new_w, new_h, n, output_data, 0);
+
+	free(output_data);
+}
+
 unsigned int* pixel(unsigned int* buffer, int x, int y, int c, int w, int n)
 {
 	return &buffer[y*w*n + x*n + c];
@@ -880,6 +905,19 @@ void test_suite(int argc, char **argv)
 	stbir_resize(image88, 8, 8, 0, output88, 16, 4, 0, STBIR_TYPE_UINT8, 1, STBIR_ALPHA_CHANNEL_NONE, 0, STBIR_EDGE_CLAMP, STBIR_EDGE_CLAMP, STBIR_FILTER_BOX, STBIR_FILTER_CATMULLROM, STBIR_COLORSPACE_SRGB, &g_context);
 	stbir_resize(image88, 8, 8, 0, output88, 16, 4, 0, STBIR_TYPE_UINT8, 1, STBIR_ALPHA_CHANNEL_NONE, 0, STBIR_EDGE_CLAMP, STBIR_EDGE_CLAMP, STBIR_FILTER_CATMULLROM, STBIR_FILTER_BOX, STBIR_COLORSPACE_SRGB, &g_context);
 
+	int res = 10;
+	for (int i = 0; i < res; i++)
+	{
+		float t = (float)i/res / 2;
+		test_subpixel_really(barbara, 0.5f, 0.5f, t, t, t+0.5f, t+0.5f);
+	}
+
+	for (int i = 0; i < res; i++)
+	{
+		float t = (float)i/res / 2;
+		test_subpixel_really(barbara, 2, 2, t, t, t+0.5f, t+0.5f);
+	}
+
 	for (i = 0; i < 10; i++)
 		test_subpixel(barbara, 0.5f, 0.5f, (float)i / 10, 1);