README.md: Mention 4:4:0 math. incomp. vs. jpeg-6b

libjpeg-turbo implements 4:4:0 "fancy" (smooth) upsampling, which is
enabled by default when decompressing JPEG images that use 4:4:0
chrominance subsampling.  libjpeg did not and does not implement fancy
4:4:0 upsampling.

README.md

@@ -283,30 +283,35 @@ Mathematical Compatibility
 ==========================
 
 For the most part, libjpeg-turbo should produce identical output to libjpeg
-v6b.  The one exception to this is when using the floating point DCT/IDCT, in
+v6b.  There are two exceptions:
-which case the outputs of libjpeg v6b and libjpeg-turbo can differ for the
-following reasons:
 
-- The SSE/SSE2 floating point DCT implementation in libjpeg-turbo is ever so
+1. When decompressing a JPEG image that uses 4:4:0 chrominance subsampling, the
-  slightly more accurate than the implementation in libjpeg v6b, but not by
+outputs of libjpeg v6b and libjpeg-turbo can differ because libjpeg-turbo
-  any amount perceptible to human vision (generally in the range of 0.01 to
+implements a "fancy" (smooth) 4:4:0 upsampling algorithm and libjpeg did not.
-  0.08 dB gain in PNSR.)
 
-- When not using the SIMD extensions, libjpeg-turbo uses the more accurate
+2. When using the floating point DCT/IDCT, the outputs of libjpeg v6b and
+libjpeg-turbo can differ for the following reasons:
+
+    - The SSE/SSE2 floating point DCT implementation in libjpeg-turbo is ever
+      so slightly more accurate than the implementation in libjpeg v6b, but not
+      by any amount perceptible to human vision (generally in the range of 0.01
+      to 0.08 dB gain in PNSR.)
+
+    - When not using the SIMD extensions, libjpeg-turbo uses the more accurate
       (and slightly faster) floating point IDCT algorithm introduced in libjpeg
       v8a as opposed to the algorithm used in libjpeg v6b.  It should be noted,
-  however, that this algorithm basically brings the accuracy of the floating
+      however, that this algorithm basically brings the accuracy of the
-  point IDCT in line with the accuracy of the accurate integer IDCT.  The
+      floating point IDCT in line with the accuracy of the accurate integer
-  floating point DCT/IDCT algorithms are mainly a legacy feature, and they do
+      IDCT.  The floating point DCT/IDCT algorithms are mainly a legacy
-  not produce significantly more accuracy than the accurate integer algorithms
+      feature, and they do not produce significantly more accuracy than the
-  (to put numbers on this, the typical difference in PNSR between the two
+      accurate integer algorithms.  (To put numbers on this, the typical
-  algorithms is less than 0.10 dB, whereas changing the quality level by 1 in
+      difference in PNSR between the two algorithms is less than 0.10 dB,
-  the upper range of the quality scale is typically more like a 1.0 dB
+      whereas changing the quality level by 1 in the upper range of the quality
-  difference.)
+      scale is typically more like a 1.0 dB difference.)
 
-- If the floating point algorithms in libjpeg-turbo are not implemented using
+    - If the floating point algorithms in libjpeg-turbo are not implemented
-  SIMD instructions on a particular platform, then the accuracy of the
+      using SIMD instructions on a particular platform, then the accuracy of
-  floating point DCT/IDCT can depend on the compiler settings.
+      the floating point DCT/IDCT can depend on the compiler settings.
 
 While libjpeg-turbo does emulate the libjpeg v8 API/ABI, under the hood it is
 still using the same algorithms as libjpeg v6b, so there are several specific