Document the fact that the fast integer FDCT is not fully accelerated for quality levels above 97 + additional wordsmithing
git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/branches/1.3.x@1289 632fc199-4ca6-4c93-a231-07263d6284db
diff --git a/usage.txt b/usage.txt
index b328a21..38f11b2 100644
--- a/usage.txt
+++ b/usage.txt
@@ -171,7 +171,6 @@
-dct int Use integer DCT method (default).
-dct fast Use fast integer DCT (less accurate).
- -dct float Use floating-point DCT method.
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
@@ -187,13 +186,18 @@
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. 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.
+ 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.
-restart N Emit a JPEG restart marker every N MCU rows, or every
N MCU blocks if "B" is attached to the number.
@@ -310,7 +314,6 @@
-dct int Use integer DCT method (default).
-dct fast Use fast integer DCT (less accurate).
- -dct float Use floating-point DCT method.
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
@@ -330,13 +333,15 @@
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. 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.
+ 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.
-dither fs Use Floyd-Steinberg dithering in color quantization.
-dither ordered Use ordered dithering in color quantization.