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/trunk@1290 632fc199-4ca6-4c93-a231-07263d6284db

cjpeg.1

@@ -176,11 +176,13 @@ 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.
+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.
 .TP
 .B \-dct float
 Use floating-point DCT method.
-The float method is mostly a legacy feature.  It does not produce significantly
+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

djpeg.1

@@ -132,7 +132,7 @@ compressed using lower quality levels.
 .TP
 .B \-dct float
 Use floating-point DCT method.
-The float method is mostly a legacy feature.  It does not produce significantly
+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

libjpeg.txt

@@ -897,12 +897,14 @@ J_DCT_METHOD dct_method
         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.
+        lossy image than if lower quality levels had been used.  Also, in
+        libjpeg-turbo, JDCT_IFAST is not fully accelerated for quality levels
+        above 97, so it will be slower than JDCT_ISLOW.  JDCT_FLOAT is mainly 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
@@ -1199,7 +1201,7 @@ J_DCT_METHOD dct_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.  JDCT_FLOAT is mostly a
+        been compressed using lower quality levels.  JDCT_FLOAT is mainly 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

usage.txt

@@ -171,7 +171,6 @@ Switches for advanced users:
 
         -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 @@ Switches for advanced users:
                         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 @@ Switches for advanced users:
 
         -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 @@ Switches for advanced users:
                         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.