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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
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cjpeg.1
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@@ -1,4 +1,4 @@
.TH CJPEG 1 "18 January 2013"
.TH CJPEG 1 "11 May 2014"
.SH NAME
cjpeg \- compress an image file to a JPEG file
.SH SYNOPSIS
@@ -166,14 +166,25 @@ Use integer DCT method (default).
.TP
.B \-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.
.TP
.B \-dct float
Use floating-point DCT method.
The float method is very slightly more accurate than the int method, but is
much slower unless your machine has very fast floating-point hardware. Also
note that results of the floating-point method may vary slightly across
machines, while the integer methods should give the same results everywhere.
The fast integer method is much less accurate than the other two.
The float method is mostly 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.
.TP
.BI \-restart " N"
Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is