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[devel] Make the 16-to-8 scaling accurate. Dividing by 256 with no rounding is

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wrong (high by one) 25% of the time. Dividing by 257 with rounding is
    wrong in 128 out of 65536 cases. Getting the right answer all the time
    without division is easy.
This commit is contained in:
John Bowler 2011-06-10 23:24:58 -05:00 committed by Glenn Randers-Pehrson
parent cc2770850a
commit b2bee3374c
12 changed files with 81 additions and 55 deletions
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4
ANNOUNCE
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@ -159,6 +159,10 @@ Version 1.5.3beta11 [June 11, 2011]
Revised documentation about png_set_user_limits() to say that it can
only be used to reduce the defined limit, and that it also affects
png writing.
Make the 16-to-8 scaling accurate. Dividing by 256 with no rounding is
wrong (high by one) 25% of the time. Dividing by 257 with rounding is
wrong in 128 out of 65536 cases. Getting the right answer all the time
without division is easy.
Send comments/corrections/commendations to png-mng-implement at lists.sf.net:
(subscription required; visit

4
CHANGES
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@ -3422,6 +3422,10 @@ Version 1.5.3beta11 [June 11, 2011]
Revised documentation about png_set_user_limits() to say that it can
only be used to reduce the defined limit, and that it also affects
png writing.
Make the 16-to-8 scaling accurate. Dividing by 256 with no rounding is
wrong (high by one) 25% of the time. Dividing by 257 with rounding is
wrong in 128 out of 65536 cases. Getting the right answer all the time
without division is easy.
Send comments/corrections/commendations to png-mng-implement at lists.sf.net
(subscription required; visit

6
libpng-manual.txt
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@ -4096,6 +4096,12 @@ In libpng-1.5.3 we reinitialized the zlib stream for each type of data.
We added five png_set_text_*() functions for setting the parameters to
use with textual data.
Prior to libpng-1.5.3, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
option was off by default, and slightly inaccurate scaling occurred.
This option can no longer be turned off, and 16-to-8 scaling is always
accurate. This change will result in some different results while
reading 16-bit images, with some of the pixels no longer being off-by-one.
B. Changes to the build and configuration of libpng
Details of internal changes to the library code can be found in the CHANGES

6
libpng.3
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@ -5047,6 +5047,12 @@ In libpng-1.5.3 we reinitialized the zlib stream for each type of data.
We added five png_set_text_*() functions for setting the parameters to
use with textual data.
Prior to libpng-1.5.3, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
option was off by default, and slightly inaccurate scaling occurred.
This option can no longer be turned off, and 16-to-8 scaling is always
accurate. This change will result in some different results while
reading 16-bit images, with some of the pixels no longer being off-by-one.
B. Changes to the build and configuration of libpng
Details of internal changes to the library code can be found in the CHANGES

4
png.h
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@ -1659,7 +1659,7 @@ PNG_EXPORT(73, void, png_set_compression_method, (png_structp png_ptr,
int method));
#endif
#ifdef PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION
#ifdef PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED
/* Also set zlib parameters for compressing non-IDAT chunks */
PNG_EXPORT(222, void, png_set_text_compression_level,
(png_structp png_ptr, int level));
@ -1678,7 +1678,7 @@ PNG_EXPORT(225, void, png_set_text_compression_window_bits, (png_structp
PNG_EXPORT(226, void, png_set_text_compression_method, (png_structp png_ptr,
int method));
#endif /* PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION */
#endif /* PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED */
/* These next functions are called for input/output, memory, and error
* handling. They are in the file pngrio.c, pngwio.c, and pngerror.c,

85
pngrtran.c
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@ -1439,11 +1439,11 @@ png_init_read_transformations(png_structp png_ptr)
* The PNG_BACKGROUND_EXPAND code above does not expand to 16 bits at
* present, so that case is ok (until do_expand_16 is moved.)
*/
# define CHOP(x) ((png_uint_16)((2*(png_uint_32)(x) + 257)/514))
png_ptr->background.red = CHOP(png_ptr->background.red);
png_ptr->background.green = CHOP(png_ptr->background.green);
png_ptr->background.blue = CHOP(png_ptr->background.blue);
png_ptr->background.gray = CHOP(png_ptr->background.gray);
# define CHOP(x) (x)=((png_uint_16)(((png_uint_32)(x)*255+32895) >> 16))
CHOP(png_ptr->background.red);
CHOP(png_ptr->background.green);
CHOP(png_ptr->background.blue);
CHOP(png_ptr->background.gray);
# undef CHOP
}
#endif
@ -2457,45 +2457,48 @@ png_do_chop(png_row_infop row_info, png_bytep row)
if (row_info->bit_depth == 16)
{
png_bytep sp = row;
png_bytep dp = row;
png_uint_32 i;
png_uint_32 istop = row_info->width * row_info->channels;
png_bytep sp = row; /* source */
png_bytep dp = row; /* destinaton */
png_bytep ep = sp + row_info->rowbytes; /* end+1 */
for (i = 0; i<istop; i++, sp += 2, dp++)
while (sp < ep)
{
#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);
#else
/* Simply discard the low order byte */
*dp = *sp;
#endif
/* The input is an array of 16 bit components, these must be scaled to
* 8 bits each. For a 16 bit value V the required value (from the PNG
* specification) is:
*
* (V * 255) / 65535
*
* This reduces to round(V / 257), or floor((V + 128.5)/257)
*
* Represent V as the two byte value vhi.vlo. Make a guess that the
* result is the top byte of V, vhi, then the correction to this value
* is:
*
* error = floor(((V-vhi.vhi) + 128.5) / 257)
* = floor(((vlo-vhi) + 128.5) / 257)
*
* This can be approximated using integer arithmetic (and a signed
* shift):
*
* error = (vlo-vhi+128) >> 8;
*
* The approximate differs from the exact answer only when (vlo-vhi) is
* 128; it then gives a correction of +1 when the exact correction is
* 0. This gives 128 errors. The exact answer (correct for all 16 bit
* input values) is:
*
* error = (vlo-vhi+128)*65535 >> 24;
*
* An alternative arithmetic calculation which also gives no errors is:
*
* (V * 255 + 32895) >> 16
*/
png_int_32 tmp = *sp++; /* must be signed! */
tmp += (((int)*sp++ - tmp + 128) * 65535) >> 24;
*dp++ = (png_byte)tmp;
}
row_info->bit_depth = 8;
row_info->pixel_depth = (png_byte)(8 * row_info->channels);
row_info->rowbytes = row_info->width * row_info->channels;

2
pngstruct.h
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@ -88,7 +88,7 @@ struct png_struct_def
#endif
/* Added at libpng 1.5.3 */
#if defined(PNG_WRITE_COMPRESSED_TEXT_SUPPORTED) || \
defined(PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION)
defined(PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED)
int zlib_text_level; /* holds zlib compression level */
int zlib_text_method; /* holds zlib compression method */
int zlib_text_window_bits; /* holds zlib compression window bits */

2
pngtest.c
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@ -915,7 +915,7 @@ test_one_file(PNG_CONST char *inname, PNG_CONST char *outname)
# endif
#endif
#ifdef PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION
#ifdef PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED
/* Normally one would use Z_DEFAULT_STRATEGY for text compression.
* This is here just to make pngtest replicate the results from libpng
* versions prior to 1.5.3, and to test this new API.

4
pngwrite.c
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@ -1459,7 +1459,7 @@ png_set_compression_method(png_structp png_ptr, int method)
}
/* The following were added to libpng-1.5.3 */
#ifdef PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION
#ifdef PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED
void PNGAPI
png_set_text_compression_level(png_structp png_ptr, int level)
{
@ -1538,7 +1538,7 @@ png_set_text_compression_method(png_structp png_ptr, int method)
png_ptr->flags |= PNG_FLAG_ZTXT_CUSTOM_METHOD;
png_ptr->zlib_text_method = method;
}
#endif /* PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION */
#endif /* PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED */
/* end of API added to libpng-1.5.3 */
void PNGAPI

4
pngwutil.c
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@ -834,7 +834,7 @@ png_write_IHDR(png_structp png_ptr, png_uint_32 width, png_uint_32 height,
png_ptr->zlib_method = 8;
#ifdef PNG_WRITE_COMPRESSED_TEXT_SUPPORTED
#ifdef PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION
#ifdef PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED
if (!(png_ptr->flags & PNG_FLAG_ZTXT_CUSTOM_STRATEGY))
png_ptr->zlib_text_strategy = Z_DEFAULT_STRATEGY;
@ -855,7 +855,7 @@ png_write_IHDR(png_structp png_ptr, png_uint_32 width, png_uint_32 height,
png_ptr->zlib_text_mem_level = png_ptr->zlib_mem_level;
png_ptr->zlib_text_window_bits = png_ptr->zlib_window_bits;
png_ptr->zlib_text_method = png_ptr->zlib_method;
#endif /* PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION */
#endif /* PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED */
#endif /* PNG_WRITE_COMPRESSED_TEXT_SUPPORTED */
/* Record that the compressor has not yet been initialized. */

11
scripts/pnglibconf.dfa
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@ -346,12 +346,15 @@ option INCH_CONVERSIONS
option BUILD_GRAYSCALE_PALETTE
# This is no longer used - the scaling is always accurate - but the definition
# is left in for older applications (including pngvalid) that check it. Do not
# turn it off!
option READ_16_TO_8_ACCURATE_SCALE requires READ
# IN DEVELOPMENT
# These are currently experimental features, define them if you want
# These are currently experimental features; define them if you want
# Very little testing, not enabled by default.
option READ_16_TO_8_ACCURATE_SCALE requires READ disabled
# NOTHING HERE
# WRITE options

4
scripts/pnglibconf.h.prebuilt
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@ -68,6 +68,7 @@
#define PNG_POINTER_INDEXING_SUPPORTED
#define PNG_PROGRESSIVE_READ_SUPPORTED
#define PNG_READ_16BIT_SUPPORTED
#define PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
#define PNG_READ_16_TO_8_SUPPORTED
#define PNG_READ_ALPHA_MODE_SUPPORTED
#define PNG_READ_ANCILLARY_CHUNKS_SUPPORTED
@ -154,7 +155,7 @@
#define PNG_WRITE_INT_FUNCTIONS_SUPPORTED
#define PNG_WRITE_INVERT_ALPHA_SUPPORTED
#define PNG_WRITE_INVERT_SUPPORTED
#define PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION
#define PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED
#define PNG_WRITE_iTXt_SUPPORTED
#define PNG_WRITE_oFFs_SUPPORTED
#define PNG_WRITE_OPTIMIZE_CMF_SUPPORTED
@ -182,6 +183,5 @@
#define PNG_zTXt_SUPPORTED
#define PNG_WRITE_COMPRESSED_TEXT_SUPPORTED
/*#undef PNG_ERROR_NUMBERS_SUPPORTED*/
/*#undef PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED*/
/* end of options */
#endif /* PNGLCONF_H */