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The Independent JPEG Group's JPEG software v9a

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Guido Vollbeding
2014-01-19 00:00:00 +00:00
committed by DRC
parent e7f88aec23
commit fc11193e7a
55 changed files with 2665 additions and 1168 deletions
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47
usage.txt
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@@ -158,10 +158,10 @@ file size is about the same --- often a little smaller.
Switches for advanced users:
-arithmetic Use arithmetic coding. CAUTION: arithmetic coded JPEG
is not yet widely implemented, so many decoders will
be unable to view an arithmetic coded JPEG file at
all.
-arithmetic Use arithmetic coding.
CAUTION: arithmetic coded JPEG is not yet widely
implemented, so many decoders will be unable to
view an arithmetic coded JPEG file at all.
-block N Set DCT block size. All N from 1 to 16 are possible.
Default is 8 (baseline format).
@@ -188,6 +188,25 @@ Switches for advanced users:
so many decoders will be unable to view a reversible
color transformed JPEG file at all.
-bgycc Create big gamut YCC JPEG file.
In this type of encoding the color difference
components are quantized further by a factor of 2
compared to the normal Cb/Cr values, thus creating
space to allow larger color values with higher
saturation than the normal gamut limits to be encoded.
In order to compensate for the loss of color fidelity
compared to a normal YCC encoded file, the color
quantization tables can be adjusted accordingly.
For example, cjpeg -bgycc -quality 80,90 will give
similar results as cjpeg -quality 80.
CAUTION: For correct decompression a decoder with big
gamut YCC support (JFIF version 2) is required.
An old decoder may or may not display a big gamut YCC
encoded JPEG file, depending on JFIF version check
and corresponding warning/error configuration.
In case of a granted decompression the old decoder
will display the image with half saturated colors.
-dct int Use integer DCT method (default).
-dct fast Use fast integer DCT (less accurate).
-dct float Use floating-point DCT method.
@@ -387,7 +406,8 @@ quality settings to make very small JPEG files; the percentage improvement
is often a lot more than it is on larger files. (At present, -optimize
mode is always selected when generating progressive JPEG files.)
GIF input files are no longer supported, to avoid the Unisys LZW patent.
GIF input files are no longer supported, to avoid the Unisys LZW patent
(now expired).
(Conversion of GIF files to JPEG is usually a bad idea anyway.)
@@ -415,8 +435,9 @@ it may run out of memory even with -maxmemory 0. In that case you can still
decompress, with some loss of image quality, by specifying -onepass for
one-pass quantization.
To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files. These
are larger than they should be, but are readable by standard GIF decoders.
To avoid the Unisys LZW patent (now expired), djpeg produces uncompressed GIF
files. These are larger than they should be, but are readable by standard GIF
decoders.
HINTS FOR BOTH PROGRAMS
@@ -533,14 +554,20 @@ image region but losslessly preserves what is inside. Like the rotate and
flip transforms, lossless crop is restricted by the current JPEG format: the
upper left corner of the selected region must fall on an iMCU boundary. If
this does not hold for the given crop parameters, we silently move the upper
left corner up and/or left to make it so, simultaneously increasing the region
dimensions to keep the lower right crop corner unchanged. (Thus, the output
image covers at least the requested region, but may cover more.)
left corner up and/or left to make it so, simultaneously increasing the
region dimensions to keep the lower right crop corner unchanged. (Thus, the
output image covers at least the requested region, but may cover more.)
The adjustment of the region dimensions may be optionally disabled.
The image can be losslessly cropped by giving the switch:
-crop WxH+X+Y Crop to a rectangular subarea of width W, height H
starting at point X,Y.
A complementary lossless-wipe option is provided to discard (gray out) data
inside a given image region while losslessly preserving what is outside:
-wipe WxH+X+Y Wipe (gray out) a rectangular subarea of
width W, height H starting at point X,Y.
Other not-strictly-lossless transformation switches are:
-grayscale Force grayscale output.