cjpeg: Adjust claims RE: image quality settings

Quality values > 95 are not useless.  They just may not provide as good
of a size vs. perceptual quality tradeoff as lower quality values.  This
also displays the default quality value in the cjpeg usage.

Closes #39

cjpeg.1

@@ -1,4 +1,4 @@
-.TH CJPEG 1 "21 November 2014"
+.TH CJPEG 1 "17 December 2015"
 .SH NAME
 cjpeg \- compress an image file to a JPEG file
 .SH SYNOPSIS
@@ -85,8 +85,8 @@ reconstructed image: the higher the quality setting, the larger the JPEG file,
 and the closer the output image will be to the original input.  Normally you
 want to use the lowest quality setting (smallest file) that decompresses into
 something visually indistinguishable from the original image.  For this
-purpose the quality setting should be between 50 and 95; the default of 75 is
+purpose the quality setting should generally be between 50 and 95 (the default
-often about right.  If you see defects at
+is 75) for photographic images.  If you see defects at
 .B \-quality
 75, then go up 5 or 10 counts at a time until you are happy with the output
 image.  (The optimal setting will vary from one image to another.)
@@ -94,11 +94,10 @@ image.  (The optimal setting will vary from one image to another.)
 .B \-quality
 100 will generate a quantization table of all 1's, minimizing loss in the
 quantization step (but there is still information loss in subsampling, as well
-as roundoff error).  This setting is mainly of interest for experimental
+as roundoff error.)  For most images, specifying a quality value above
-purposes.  Quality values above about 95 are
+about 95 will increase the size of the compressed file dramatically, and while
-.B not
+the quality gain from these higher quality values is measurable (using metrics
-recommended for normal use; the compressed file size goes up dramatically for
+such as PSNR or SSIM), it is rarely perceivable by human vision.
-hardly any gain in output image quality.
 .PP
 In the other direction, quality values below 50 will produce very small files
 of low image quality.  Settings around 5 to 10 might be useful in preparing an

cjpeg.c

@@ -155,7 +155,8 @@ usage (void)
 #endif
 
   fprintf(stderr, "Switches (names may be abbreviated):\n");
-  fprintf(stderr, "  -quality N[,...]   Compression quality (0..100; 5-95 is useful range)\n");
+  fprintf(stderr, "  -quality N[,...]   Compression quality (0..100; 5-95 is most useful range,\n");
+  fprintf(stderr, "                     default is 75)\n");
   fprintf(stderr, "  -grayscale     Create monochrome JPEG file\n");
   fprintf(stderr, "  -rgb           Create RGB JPEG file\n");
 #ifdef ENTROPY_OPT_SUPPORTED

usage.txt

@@ -104,17 +104,17 @@ the reconstructed image: the higher the quality setting, the larger the JPEG
 file, and the closer the output image will be to the original input.  Normally
 you want to use the lowest quality setting (smallest file) that decompresses
 into something visually indistinguishable from the original image.  For this
-purpose the quality setting should be between 50 and 95; the default of 75 is
+purpose the quality setting should generally be between 50 and 95 (the default
-often about right.  If you see defects at -quality 75, then go up 5 or 10
+is 75) for photographic images.  If you see defects at -quality 75, then go up
-counts at a time until you are happy with the output image.  (The optimal
+5 or 10 counts at a time until you are happy with the output image.  (The
-setting will vary from one image to another.)
+optimal setting will vary from one image to another.)
 
 -quality 100 will generate a quantization table of all 1's, minimizing loss
 in the quantization step (but there is still information loss in subsampling,
-as well as roundoff error).  This setting is mainly of interest for
+as well as roundoff error.)  For most images, specifying a quality value above
-experimental purposes.  Quality values above about 95 are NOT recommended for
+about 95 will increase the size of the compressed file dramatically, and while
-normal use; the compressed file size goes up dramatically for hardly any gain
+the quality gain from these higher quality values is measurable (using metrics
-in output image quality.
+such as PSNR or SSIM), it is rarely perceivable by human vision.
 
 In the other direction, quality values below 50 will produce very small files
 of low image quality.  Settings around 5 to 10 might be useful in preparing an