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

git-svn-id: svn://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1287 632fc199-4ca6-4c93-a231-07263d6284db

1 files changed, 44 insertions, 11 deletions

diff --git a/libjpeg.txt b/libjpeg.txt
index d110738..afc002b 100644
--- a/libjpeg.txt
+++ b/libjpeg.txt
@@ -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,