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

diff --git a/cjpeg.1 b/cjpeg.1
index e338c80c15de2fe1a7690bf73eeb968925242036..9d09b810c302fe3cd187e15d224fa1a345f6fe86 100644 (file)
--- a/cjpeg.1
+++ b/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
-often about right.  If you see defects at
+purpose the quality setting should generally be between 50 and 95 (the default
+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
-purposes.  Quality values above about 95 are
-.B not
-recommended for normal use; the compressed file size goes up dramatically for
-hardly any gain in output image quality.
+as roundoff error.)  For most images, specifying a quality value above
+about 95 will increase the size of the compressed file dramatically, and while
+the quality gain from these higher quality values is measurable (using metrics
+such as PSNR or SSIM), it is rarely perceivable by human vision.
 .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

diff --git a/cjpeg.c b/cjpeg.c
index 7aa061e6dd6bfebf8611277353f33b6ae1f76ff6..1816f59c3c0cba4d2ef976e584969cfc5fccb1ac 100644 (file)
--- a/cjpeg.c
+++ b/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

diff --git a/usage.txt b/usage.txt
index ef8e6d096f0354ed43ced815019a3e984703eec9..27c4f9ce070282e2b86a025aa26829fa613f1034 100644 (file)
--- a/usage.txt
+++ b/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
-often about right.  If you see defects at -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.)
+purpose the quality setting should generally be between 50 and 95 (the default
+is 75) for photographic images.  If you see defects at -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.)
 
 -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 purposes.  Quality values above about 95 are NOT recommended for
-normal use; the compressed file size goes up dramatically for hardly any gain
-in output image quality.
+as well as roundoff error.)  For most images, specifying a quality value above
+about 95 will increase the size of the compressed file dramatically, and while
+the quality gain from these higher quality values is measurable (using metrics
+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