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Provide a more thorough description of the trade-offs between the various DCT/IDCT algorithms, based on new resarch

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git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/branches/1.3.x@1286 632fc199-4ca6-4c93-a231-07263d6284db
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DRC
2014-05-11 09:46:28 +00:00
parent b775351012
commit 8940e6ca86
5 changed files with 144 additions and 50 deletions
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55
libjpeg.txt
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@@ -3,7 +3,7 @@ USING THE IJG JPEG LIBRARY
This file was part of the Independent JPEG Group's software:
Copyright (C) 1994-2011, Thomas G. Lane, Guido Vollbeding.
Modifications:
Copyright (C) 2010, D. R. Commander.
Copyright (C) 2010, 2014, D. R. Commander.
For conditions of distribution and use, see the accompanying README file.
@@ -886,14 +886,23 @@ J_DCT_METHOD dct_method
JDCT_FLOAT: floating-point method
JDCT_DEFAULT: default method (normally JDCT_ISLOW)
JDCT_FASTEST: fastest method (normally JDCT_IFAST)
The FLOAT method is very slightly more accurate than the ISLOW method,
but may give different results on different machines due to varying
roundoff behavior. The integer methods should give the same results
on all machines. On machines with sufficiently fast FP hardware, the
floating-point method may also be the fastest. The IFAST method is
considerably less accurate than the other two; its use is not
recommended if high quality is a concern. JDCT_DEFAULT and
JDCT_FASTEST are macros configurable by each installation.
In libjpeg-turbo, JDCT_IFAST is generally about 5-15% faster than
JDCT_ISLOW when using the x86/x86-64 SIMD extensions (results may vary
with other SIMD implementations, or when using libjpeg-turbo without
SIMD extensions.) For quality levels of 90 and below, there should be
little or no perceptible difference between the two algorithms. For
quality levels above 90, however, the difference between JDCT_IFAST and
JDCT_ISLOW becomes more pronounced. With quality=97, for instance,
JDCT_IFAST incurs generally about a 1-3 dB loss (in PSNR) relative to
JDCT_ISLOW, but this can be larger for some images. Do not use
JDCT_IFAST with quality levels above 97. The algorithm often
degenerates at quality=98 and above and can actually produce a more
lossy image than if lower quality levels had been used. JDCT_FLOAT is
mostly a legacy feature. It does not produce significantly more
accurate results than the ISLOW method, and it is much slower. The
FLOAT method may also give different results on different machines due
to varying roundoff behavior, whereas the integer methods should give
the same results on all machines.
J_COLOR_SPACE jpeg_color_space
int num_components
@@ -1170,8 +1179,32 @@ int actual_number_of_colors
Additional decompression parameters that the application may set include:
J_DCT_METHOD dct_method
Selects the algorithm used for the DCT step. Choices are the same
as described above for compression.
Selects the algorithm used for the DCT step. Choices are:
JDCT_ISLOW: slow but accurate integer algorithm
JDCT_IFAST: faster, less accurate integer method
JDCT_FLOAT: floating-point method
JDCT_DEFAULT: default method (normally JDCT_ISLOW)
JDCT_FASTEST: fastest method (normally JDCT_IFAST)
In libjpeg-turbo, JDCT_IFAST is generally about 5-15% faster than
JDCT_ISLOW when using the x86/x86-64 SIMD extensions (results may vary
with other SIMD implementations, or when using libjpeg-turbo without
SIMD extensions.) If the JPEG image was compressed using a quality
level of 85 or below, then there should be little or no perceptible
difference between the two algorithms. When decompressing images that
were compressed using quality levels above 85, however, the difference
between JDCT_IFAST and JDCT_ISLOW becomes more pronounced. With images
compressed using quality=97, for instance, JDCT_IFAST incurs generally
about a 4-6 dB loss (in PSNR) relative to JDCT_ISLOW, but this can be
larger for some images. If you can avoid it, do not use JDCT_IFAST
when decompressing images that were compressed using quality levels
above 97. The algorithm often degenerates for such images and can
actually produce a more lossy output image than if the JPEG image had
been compressed using lower quality levels. JDCT_FLOAT is mostly a
legacy feature. It does not produce significantly more accurate
results than the ISLOW method, and it is much slower. The FLOAT method
may also give different results on different machines due to varying
roundoff behavior, whereas the integer methods should give the same
results on all machines.
boolean do_fancy_upsampling
If TRUE, do careful upsampling of chroma components. If FALSE,