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 e338c80..9d09b81 100644
--- 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 @@
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 @@
.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 7aa061e..1816f59 100644
--- a/cjpeg.c
+++ b/cjpeg.c
@@ -155,7 +155,8 @@
#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 ef8e6d0..27c4f9c 100644
--- a/usage.txt
+++ b/usage.txt
@@ -104,17 +104,17 @@
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