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3e42c81193
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.
56 lines
2.0 KiB
Plaintext
56 lines
2.0 KiB
Plaintext
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Libpng 1.7.0alpha01 - December 10, 2012
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This is not intended to be a public release. It will be replaced
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within a few weeks by a public version or by another test version.
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Files available for download:
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Source files with LF line endings (for Unix/Linux) and with a
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"configure" script
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1.7.0alpha01.tar.xz (LZMA-compressed, recommended)
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1.7.0alpha01.tar.gz
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1.7.0alpha01.tar.bz2
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Source files with CRLF line endings (for Windows), without the
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"configure" script
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lp170a01.7z (LZMA-compressed, recommended)
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lp170a01.zip
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Other information:
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1.7.0alpha01-README.txt
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1.7.0alpha01-LICENSE.txt
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Changes since the last public release (1.6.0):
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Version 1.7.0alpha01 [December 10, 2012]
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Started 1.7.0 branch from libpng-1.6.0beta33.
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Made 8-bit compose and rgb_to_grayscale accuracy improvements. These
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changes cause 16-bit arithmetic to be used for 8-bit data in the gamma
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corrected compose and grayscale operations. The arithmetic errors have
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three sources all of which are fixed in this commit:
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1) 8-bit linear calculations produce massive errors for lower intensity
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values.
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2) The old 16-bit "16 to 8" gamma table code erroneously wrote the lowest
|
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output value into a table entry which corresponded to multiple output
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|
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
|
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for any significant gamma correction.
|
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Send comments/corrections/commendations to png-mng-implement at lists.sf.net
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(subscription required; visit
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https://lists.sourceforge.net/lists/listinfo/png-mng-implement
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to subscribe)
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or to glennrp at users.sourceforge.net
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Glenn R-P
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