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[libpng17] Made 8-bit compose and rgb_to_grayscale accuracy improvements.

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These changes cause 16-bit arithmetic to be used for 8-bit data in the gamma
    corrected compose and grayscale operations.  The arithmetic errors have
    three sources all of which are fixed in this commit:
    1) 8-bit linear calculations produce massive errors for lower intensity
       values.
    2) The old 16-bit "16 to 8" gamma table code erroneously wrote the lowest
       output value into a table entry which corresponded to multiple output
       values (so where the value written should have been the closest to the
       transformed input value.)
    3) In a number of cases the code to access the 16-bit table did not round;
       it did a simple shift, which was wrong and made the side effects of (2)
       even worse.
  The new gamma code does not have the 16-to-8 problem at the cost of slighly
    more calculations and the algorithm used to minimize the number of
    calculations has been extended to all the 16-bit tables; it has advantages
    for any significant gamma correction.
This commit is contained in:
John Bowler
2012-12-10 16:38:43 -06:00
committed by Glenn Randers-Pehrson
parent ac3b4d9b69
commit 3e42c81193
16 changed files with 2318 additions and 1341 deletions
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6
pngrutil.c
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@@ -1832,6 +1832,8 @@ png_handle_tRNS(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
*/
png_set_tRNS(png_ptr, info_ptr, readbuf, png_ptr->num_trans,
&(png_ptr->trans_color));
png_ptr->trans_alpha = info_ptr->trans_alpha;
}
#endif
@@ -4410,7 +4412,7 @@ defined(PNG_USER_TRANSFORM_PTR_SUPPORTED)
png_error(png_ptr, "This image requires a row greater than 64KB");
#endif
if (row_bytes + 48 > png_ptr->old_big_row_buf_size)
if (row_bytes + 48 > png_ptr->big_row_buf_size)
{
png_free(png_ptr, png_ptr->big_row_buf);
png_free(png_ptr, png_ptr->big_prev_row);
@@ -4447,7 +4449,7 @@ defined(PNG_USER_TRANSFORM_PTR_SUPPORTED)
png_ptr->row_buf = png_ptr->big_row_buf + 31;
png_ptr->prev_row = png_ptr->big_prev_row + 31;
#endif
png_ptr->old_big_row_buf_size = row_bytes + 48;
png_ptr->big_row_buf_size = row_bytes + 48;
}
#ifdef PNG_MAX_MALLOC_64K