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[master] Fixed error in "ACCURATE" 16-to-8 scaling.

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This commit is contained in:
John Bowler 2011-06-17 21:36:09 -05:00 committed by Glenn Randers-Pehrson
parent cc5226bf2a
commit 484a48e221
3 changed files with 22 additions and 30 deletions
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7
ANNOUNCE
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@ -1,5 +1,5 @@
Libpng 1.4.8beta05 - June 8, 2011
Libpng 1.4.8beta05 - June 18, 2011
This is not intended to be a public release. It will be replaced
within a few weeks by a public version or by another test version.
@ -44,10 +44,13 @@ version 1.4.8beta02 [June 5, 2011]
version 1.4.8beta03 [June 6, 2011]
Check for integer overflow in png_set_rgb_to_gray().
version 1.4.8beta04 [June 8, 2011]
version 1.4.8beta04 [June 7, 2011]
Fixed uninitialized memory read in png_format_buffer() (Bug report by
Frank Busse, related to CVE-2004-0421).
version 1.4.8beta05 [June 18, 2011]
Fixed error in "ACCURATE" 16-to-8 scaling.
Send comments/corrections/commendations to glennrp at users.sourceforge.net
or to png-mng-implement at lists.sf.net (subscription required; visit
https://lists.sourceforge.net/lists/listinfo/png-mng-implement).

5
CHANGES
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@ -2812,10 +2812,13 @@ version 1.4.8beta02 [June 5, 2011]
version 1.4.8beta03 [June 6, 2011]
Check for integer overflow in png_set_rgb_to_gray().
version 1.4.8beta04 [June 8, 2011]
version 1.4.8beta04 [June 7, 2011]
Fixed uninitialized memory read in png_format_buffer() (Bug report by
Frank Busse, related to CVE-2004-0421).
version 1.4.8beta05 [June 18, 2011]
Fixed error in "ACCURATE" 16-to-8 scaling.
Send comments/corrections/commendations to glennrp at users.sourceforge.net
or to png-mng-implement at lists.sf.net (subscription required; visit
https://lists.sourceforge.net/lists/listinfo/png-mng-implement).

40
pngrtran.c
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@ -1,7 +1,7 @@
/* pngrtran.c - transforms the data in a row for PNG readers
*
* Last changed in libpng 1.4.8 [June 8, 2011]
* Last changed in libpng 1.4.8 [June 18, 2011]
* Copyright (c) 1998-2011 Glenn Randers-Pehrson
* (Version 0.96 Copyright (c) 1996, 1997 Andreas Dilger)
* (Version 0.88 Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.)
@ -1799,32 +1799,18 @@ png_do_chop(png_row_infop row_info, png_bytep row)
for (i = 0; i<istop; i++, sp += 2, dp++)
{
#ifdef PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
/* This does a more accurate scaling of the 16-bit color
* value, rather than a simple low-byte truncation.
*
* What the ideal calculation should be:
* *dp = (((((png_uint_32)(*sp) << 8) |
* (png_uint_32)(*(sp + 1))) * 255 + 127)
* / (png_uint_32)65535L;
*
* GRR: no, I think this is what it really should be:
* *dp = (((((png_uint_32)(*sp) << 8) |
* (png_uint_32)(*(sp + 1))) + 128L)
* / (png_uint_32)257L;
*
* GRR: here's the exact calculation with shifts:
* temp = (((png_uint_32)(*sp) << 8) |
* (png_uint_32)(*(sp + 1))) + 128L;
* *dp = (temp - (temp >> 8)) >> 8;
*
* Approximate calculation with shift/add instead of multiply/divide:
* *dp = ((((png_uint_32)(*sp) << 8) |
* (png_uint_32)((int)(*(sp + 1)) - *sp)) + 128) >> 8;
*
* What we actually do to avoid extra shifting and conversion:
*/
*dp = *sp + ((((int)(*(sp + 1)) - *sp) > 128) ? 1 : 0);
/* This does a more accurate scaling of the 16-bit color
* value, rather than a simple low-byte truncation.
*
* Prior to libpng-1.4.8 and 1.5.4, the calculation here was
* incorrect, so if you used ACCURATE_SCALE you will now see
* a slightly different result. In libpng-1.5.4 and
* later you will need to use the new png_set_scale_16_to_8()
* API to obtain accurate 16-to-8 scaling.
*/
png_int_32 tmp = *sp++; /* must be signed! */
tmp += (((int)*sp++ - tmp + 128) * 65535) >> 24;
*dp++ = (png_byte)tmp;
#else
/* Simply discard the low order byte */
*dp = *sp;