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If alpha is zero then sneakily replace it with a very small nonzero so that the color data is preserved.

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Pre-process n1 so that we don't have to do it later on. Can't do n0 since it's used to find the coefficient index.
master
Jorge Rodriguez ago%!(EXTRA string=11 years)
parent d510d70b6a
commit 4b69c0637e
2 changed files with 65 additions and 18 deletions
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  1. 26
      stb_image_resize.h
  2. 57
      tests/resample_test.cpp

26
stb_image_resize.h
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@ -795,12 +795,13 @@ static void stbir__calculate_coefficients_downsample(stbir__info* stbir_info, st
static void stbir__normalize_downsample_coefficients(stbir__info* stbir_info)
{
int num_contributors = stbir__get_horizontal_contributors(stbir_info);
int i;
for (i = 0; i < stbir_info->output_w; i++)
{
float total = 0;
int j;
for (j = 0; j < stbir__get_horizontal_contributors(stbir_info); j++)
for (j = 0; j < num_contributors; j++)
{
if (i >= stbir_info->horizontal_contributors[j].n0 && i <= stbir_info->horizontal_contributors[j].n1)
{
@ -816,7 +817,7 @@ static void stbir__normalize_downsample_coefficients(stbir__info* stbir_info)
float scale = 1 / total;
for (j = 0; j < stbir__get_horizontal_contributors(stbir_info); j++)
for (j = 0; j < num_contributors; j++)
{
if (i >= stbir_info->horizontal_contributors[j].n0 && i <= stbir_info->horizontal_contributors[j].n1)
*stbir__get_coefficient(stbir_info, j, i - stbir_info->horizontal_contributors[j].n0) *= scale;
@ -824,6 +825,15 @@ static void stbir__normalize_downsample_coefficients(stbir__info* stbir_info)
break;
}
}
// Using min to avoid writing into invalid pixels.
for (i = 0; i < num_contributors; i++)
{
stbir__contributors* contributors = &stbir_info->horizontal_contributors[i];
STBIR__DEBUG_ASSERT(contributors->n1 >= contributors->n0);
contributors->n1 = stbir__min(contributors->n1, stbir_info->output_w - 1);
}
}
// Each scan line uses the same kernel values so we should calculate the kernel
@ -1014,6 +1024,10 @@ static void stbir__decode_scanline(stbir__info* stbir_info, int n)
{
int decode_pixel_index = x * channels;
float alpha = decode_buffer[decode_pixel_index + alpha_channel];
if (alpha == 0)
alpha = decode_buffer[decode_pixel_index + alpha_channel] = (float)1 / 17179869184; // 1/2^34 should be small enough that it won't affect anything.
for (c = 0; c < channels; c++)
{
if (c == alpha_channel)
@ -1129,12 +1143,10 @@ static void stbir__resample_horizontal_downsample(stbir__info* stbir_info, int n
int in_x = x - filter_pixel_margin;
int in_pixel_index = in_x * channels;
int max_n = stbir__min(n1, output_w-1);
int max_n = n1;
int coefficient_group = x*kernel_pixel_width;
STBIR__DEBUG_ASSERT(n1 >= n0);
// Using min and max to avoid writing into invalid pixels.
// Using max to avoid writing into invalid pixels.
for (k = stbir__max(n0, 0); k <= max_n; k++)
{
int coefficient_index = (k - n0) + coefficient_group;
@ -1199,8 +1211,8 @@ static void stbir__encode_scanline(stbir__info* stbir_info, int num_pixels, void
int output_pixel_index = x*channels;
int encode_pixel_index = x*channels;
float alpha = encode_buffer[encode_pixel_index + alpha_channel];
STBIR__DEBUG_ASSERT(alpha > 0);
float reciprocal_alpha = alpha ? 1.0f / alpha : 0;
// @TODO: if final alpha=0, we actually want to have ignored alpha... set alpha to sRGB_to_linear(1/255)/(2^24) so floats will discard it?
for (n = 0; n < channels; n++)
if (n != alpha_channel)
encode_buffer[encode_pixel_index + n] *= reciprocal_alpha;

57
tests/resample_test.cpp
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@ -384,18 +384,21 @@ void test_subpixel(const char* file, float width_percent, float height_percent,
free(output_data);
}
unsigned char* pixel(unsigned char* buffer, int x, int y, int c, int w, int n)
unsigned int* pixel(unsigned int* buffer, int x, int y, int c, int w, int n)
{
return &buffer[y*w*n + x*n + c];
}
void test_premul()
{
unsigned char input[2 * 2 * 4];
unsigned char output[1 * 1 * 4];
unsigned int input[2 * 2 * 4];
unsigned int output[1 * 1 * 4];
unsigned int output2[2 * 2 * 4];
memset(input, 0, sizeof(input));
// First a test to make sure premul is working properly.
// Top left - solid red
*pixel(input, 0, 0, 0, 2, 4) = 255;
*pixel(input, 0, 0, 3, 2, 4) = 255;
@ -412,18 +415,50 @@ void test_premul()
*pixel(input, 1, 1, 1, 2, 4) = 255;
*pixel(input, 1, 1, 3, 2, 4) = 25;
stbir_resize_uint8_generic(input, 2, 2, 0, output, 1, 1, 0, 4, 3, 0, STBIR_EDGE_CLAMP, STBIR_FILTER_BOX, STBIR_COLORSPACE_LINEAR, &g_context);
stbir_resize(input, 2, 2, 0, output, 1, 1, 0, STBIR_TYPE_UINT32, 4, 3, 0, STBIR_EDGE_CLAMP, STBIR_EDGE_CLAMP, STBIR_FILTER_BOX, STBIR_FILTER_BOX, STBIR_COLORSPACE_LINEAR, &g_context);
float r = 1.0f;
float g = 1.0f;
float ra = 1.0;
float ga = (float)25 / 255;
float r = (float)255 / 4294967296;
float g = (float)255 / 4294967296;
float ra = (float)255 / 4294967296;
float ga = (float)25 / 4294967296;
float a = (ra + ga) / 2;
STBIR_ASSERT(output[0] == (int)(r * ra / 2 / a * 255 + 0.5f)); // 232
STBIR_ASSERT(output[1] == (int)(g * ga / 2 / a * 255 + 0.5f)); // 23
STBIR_ASSERT(output[0] == (int)(r * ra / 2 / a * 4294967296 + 0.5f)); // 232
STBIR_ASSERT(output[1] == (int)(g * ga / 2 / a * 4294967296 + 0.5f)); // 23
STBIR_ASSERT(output[2] == 0);
STBIR_ASSERT(output[3] == (int)(a * 255 + 0.5f)); // 140
STBIR_ASSERT(output[3] == (int)(a * 4294967296 + 0.5f)); // 140
// Now a test to make sure it doesn't clobber existing values.
// Top right - completely transparent green
*pixel(input, 1, 0, 1, 2, 4) = 255;
*pixel(input, 1, 0, 3, 2, 4) = 0;
// Bottom right - completely transparent green
*pixel(input, 1, 1, 1, 2, 4) = 255;
*pixel(input, 1, 1, 3, 2, 4) = 0;
stbir_resize(input, 2, 2, 0, output2, 2, 2, 0, STBIR_TYPE_UINT32, 4, 3, 0, STBIR_EDGE_CLAMP, STBIR_EDGE_CLAMP, STBIR_FILTER_BOX, STBIR_FILTER_BOX, STBIR_COLORSPACE_LINEAR, &g_context);
STBIR_ASSERT(*pixel(output2, 0, 0, 0, 2, 4) == 255);
STBIR_ASSERT(*pixel(output2, 0, 0, 1, 2, 4) == 0);
STBIR_ASSERT(*pixel(output2, 0, 0, 2, 2, 4) == 0);
STBIR_ASSERT(*pixel(output2, 0, 0, 3, 2, 4) == 255);
STBIR_ASSERT(*pixel(output2, 0, 1, 0, 2, 4) == 255);
STBIR_ASSERT(*pixel(output2, 0, 1, 1, 2, 4) == 0);
STBIR_ASSERT(*pixel(output2, 0, 1, 2, 2, 4) == 0);
STBIR_ASSERT(*pixel(output2, 0, 1, 3, 2, 4) == 255);
STBIR_ASSERT(*pixel(output2, 1, 0, 0, 2, 4) == 0);
STBIR_ASSERT(*pixel(output2, 1, 0, 1, 2, 4) == 255);
STBIR_ASSERT(*pixel(output2, 1, 0, 2, 2, 4) == 0);
STBIR_ASSERT(*pixel(output2, 1, 0, 3, 2, 4) == 0);
STBIR_ASSERT(*pixel(output2, 1, 1, 0, 2, 4) == 0);
STBIR_ASSERT(*pixel(output2, 1, 1, 1, 2, 4) == 255);
STBIR_ASSERT(*pixel(output2, 1, 1, 2, 2, 4) == 0);
STBIR_ASSERT(*pixel(output2, 1, 1, 3, 2, 4) == 0);
}
// test that splitting a pow-2 image into tiles produces identical results

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