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Use an even smaller epsilon. Allow the user to avoid the epsilon business if they want to preserve their alpha values.

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This commit is contained in:
Jorge Rodriguez 2014-09-10 15:03:55 -07:00
parent c9d67446d2
commit 6922628106
1 changed files with 16 additions and 3 deletions
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17
stb_image_resize.h
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@ -241,6 +241,15 @@ STBIRDEF int stbir_resize_region( const void *input_pixels , int input_w , int
#define STBIR_PROGRESS_REPORT(float_0_to_1) #define STBIR_PROGRESS_REPORT(float_0_to_1)
#endif #endif
// This value is added to alpha just before premultiplication to avoid
// zeroing out color values. It is equivalent to 2^-80. If you don't want
// that behavior (it may interfere if you have floating point images with
// very small alpha values) then you can define STBIR_NO_ALPHA_EPSILON to
// disable it.
#ifndef STBIR_EPSILON
#define STBIR_EPSILON ((float)1 / (1 << 20) / (1 << 20) / (1 << 20) / (1 << 20))
#endif
// //
// //
//// end header file ///////////////////////////////////////////////////// //// end header file /////////////////////////////////////////////////////
@ -1035,6 +1044,7 @@ static void stbir__decode_scanline(stbir__info* stbir_info, int n)
break; break;
} }
#ifndef STBIR_NO_ALPHA_EPSILON
if (!(stbir_info->flags & STBIR_FLAG_PREMULTIPLIED_ALPHA)) if (!(stbir_info->flags & STBIR_FLAG_PREMULTIPLIED_ALPHA))
{ {
for (x = -stbir__get_filter_pixel_margin_horizontal(stbir_info); x < max_x; x++) for (x = -stbir__get_filter_pixel_margin_horizontal(stbir_info); x < max_x; x++)
@ -1042,7 +1052,7 @@ static void stbir__decode_scanline(stbir__info* stbir_info, int n)
int decode_pixel_index = x * channels; int decode_pixel_index = x * channels;
// If the alpha value is 0 it will clobber the color values. Make sure it's not. // If the alpha value is 0 it will clobber the color values. Make sure it's not.
float alpha = (decode_buffer[decode_pixel_index + alpha_channel] += (float)1 / 17179869184); // 1/2^34 should be small enough that it won't affect anything. float alpha = (decode_buffer[decode_pixel_index + alpha_channel] += STBIR_EPSILON);
for (c = 0; c < channels; c++) for (c = 0; c < channels; c++)
{ {
@ -1053,6 +1063,7 @@ static void stbir__decode_scanline(stbir__info* stbir_info, int n)
} }
} }
} }
#endif
if (edge_horizontal == STBIR_EDGE_ZERO) if (edge_horizontal == STBIR_EDGE_ZERO)
{ {
@ -1220,6 +1231,7 @@ static void stbir__encode_scanline(stbir__info* stbir_info, int num_pixels, void
int x; int x;
int n; int n;
#ifndef STBIR_NO_ALPHA_EPSILON
if (!(stbir_info->flags&STBIR_FLAG_PREMULTIPLIED_ALPHA)) if (!(stbir_info->flags&STBIR_FLAG_PREMULTIPLIED_ALPHA))
{ {
for (x=0; x < num_pixels; ++x) for (x=0; x < num_pixels; ++x)
@ -1234,9 +1246,10 @@ static void stbir__encode_scanline(stbir__info* stbir_info, int num_pixels, void
encode_buffer[pixel_index + n] *= reciprocal_alpha; encode_buffer[pixel_index + n] *= reciprocal_alpha;
// We added in a small epsilon to prevent the color channel from being deleted with zero alpha. Remove it now. // We added in a small epsilon to prevent the color channel from being deleted with zero alpha. Remove it now.
encode_buffer[pixel_index + alpha_channel] -= (float)1 / 17179869184; // 1/2^34 should be small enough that it won't affect anything. encode_buffer[pixel_index + alpha_channel] -= STBIR_EPSILON;
} }
} }
#endif
switch (decode) switch (decode)
{ {