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Demote "fast" [I]DCT algorithms to legacy status

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- Refer to the "slow" [I]DCT algorithms as "accurate" instead, since
  they are not slow under libjpeg-turbo.
- Adjust documentation claims to reflect the fact that the "slow" and
  "fast" algorithms produce about the same performance on AVX2-equipped
  CPUs (because of the dual-lane nature of AVX2, it was not possible to
  accelerate the "fast" algorithm beyond what was achievable with SSE2.)
  Also adjust the claims to reflect the fact that the "fast" algorithm
  tends to be ~5-15% faster than the "slow" algorithm on
  non-AVX2-equipped CPUs, regardless of the use of the libjpeg-turbo
  SIMD extensions.
- Indicate the legacy status of the "fast" and float algorithms in the
  documentation and cjpeg/djpeg usage info.
- Remove obsolete paragraph in the djpeg man page that suggested that
  the float algorithm could be faster than the "fast" algorithm on some
  CPUs.
This commit is contained in:
DRC
2020-11-04 10:13:06 -06:00
parent c3bfbde21d
commit 6e632af9f6
28 changed files with 263 additions and 218 deletions
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135
usage.txt
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@@ -170,35 +170,43 @@ Switches for advanced users:
be unable to view an arithmetic coded JPEG file at
all.
-dct int Use integer DCT method (default).
-dct fast Use fast integer DCT (less accurate).
In libjpeg-turbo, the fast method is generally about
5-15% faster than the int method 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
the fast and the int methods becomes more pronounced.
With quality=97, for instance, the fast method incurs
generally about a 1-3 dB loss (in PSNR) relative to
the int method, but this can be larger for some images.
Do not use the fast method 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. Also, in
libjpeg-turbo, the fast method is not fully accerated
for quality levels above 97, so it will be slower than
the int method.
-dct float Use floating-point DCT method.
The float method is mainly a legacy feature. It does
not produce significantly more accurate results than
the int 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.
-dct int Use accurate integer DCT method (default).
-dct fast Use less accurate integer DCT method [legacy feature].
When the Independent JPEG Group's software was first
released in 1991, the compression time for a
1-megapixel JPEG image on a mainstream PC was measured
in minutes. Thus, the fast integer DCT algorithm
provided noticeable performance benefits. On modern
CPUs running libjpeg-turbo, however, the compression
time for a 1-megapixel JPEG image is measured in
milliseconds, and thus the performance benefits of the
fast algorithm are much less noticeable. On modern
x86/x86-64 CPUs that support AVX2 instructions, the
fast and int methods have similar performance. On
other types of CPUs, the fast method is generally about
5-15% faster than the int method.
For quality levels of 90 and below, there should be
little or no perceptible quality difference between the
two algorithms. For quality levels above 90, however,
the difference between the fast and int methods becomes
more pronounced. With quality=97, for instance, the
fast method incurs generally about a 1-3 dB loss in
PSNR relative to the int method, but this can be larger
for some images. Do not use the fast method 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. Also, in libjpeg-turbo, the fast method
is not fully accelerated for quality levels above 97,
so it will be slower than the int method.
-dct float Use floating-point DCT method [legacy feature].
The float method does not produce significantly more
accurate results than the int 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.
-restart N Emit a JPEG restart marker every N MCU rows, or every
N MCU blocks if "B" is attached to the number.
@@ -315,36 +323,45 @@ The basic command line switches for djpeg are:
Switches for advanced users:
-dct int Use integer DCT method (default).
-dct fast Use fast integer DCT (less accurate).
In libjpeg-turbo, the fast method is generally about
5-15% faster than the int method 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 the fast and int
methods becomes more pronounced. With images
compressed using quality=97, for instance, the fast
method incurs generally about a 4-6 dB loss (in PSNR)
relative to the int method, but this can be larger for
some images. If you can avoid it, do not use the fast
method 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.
-dct float Use floating-point DCT method.
The float method is mainly a legacy feature. It does
not produce significantly more accurate results than
the int 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.
-dct int Use accurate integer DCT method (default).
-dct fast Use less accurate integer DCT method [legacy feature].
When the Independent JPEG Group's software was first
released in 1991, the decompression time for a
1-megapixel JPEG image on a mainstream PC was measured
in minutes. Thus, the fast integer DCT algorithm
provided noticeable performance benefits. On modern
CPUs running libjpeg-turbo, however, the decompression
time for a 1-megapixel JPEG image is measured in
milliseconds, and thus the performance benefits of the
fast algorithm are much less noticeable. On modern
x86/x86-64 CPUs that support AVX2 instructions, the
fast and int methods have similar performance. On
other types of CPUs, the fast method is generally about
5-15% faster than the int method.
If the JPEG image was compressed using a quality level
of 85 or below, then there should be little or no
perceptible quality difference between the two
algorithms. When decompressing images that were
compressed using quality levels above 85, however, the
difference between the fast and int methods becomes
more pronounced. With images compressed using
quality=97, for instance, the fast method incurs
generally about a 4-6 dB loss in PSNR relative to the
int method, but this can be larger for some images. If
you can avoid it, do not use the fast method 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.
-dct float Use floating-point DCT method [legacy feature].
The float method does not produce significantly more
accurate results than the int 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.
-dither fs Use Floyd-Steinberg dithering in color quantization.
-dither ordered Use ordered dithering in color quantization.