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android/extern/libjpeg-turbo/cjpeg.1
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android/extern/libjpeg-turbo/cjpeg.1
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.TH CJPEG 1 "30 November 2021"
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.SH NAME
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cjpeg \- compress an image file to a JPEG file
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.SH SYNOPSIS
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.B cjpeg
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[
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.I options
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]
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[
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.I filename
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]
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.LP
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.SH DESCRIPTION
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.LP
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.B cjpeg
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compresses the named image file, or the standard input if no file is
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named, and produces a JPEG/JFIF file on the standard output.
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The currently supported input file formats are: PPM (PBMPLUS color
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format), PGM (PBMPLUS grayscale format), BMP, GIF, and Targa.
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.SH OPTIONS
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All switch names may be abbreviated; for example,
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.B \-grayscale
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may be written
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.B \-gray
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or
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.BR \-gr .
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Most of the "basic" switches can be abbreviated to as little as one letter.
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Upper and lower case are equivalent (thus
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.B \-BMP
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is the same as
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.BR \-bmp ).
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British spellings are also accepted (e.g.,
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.BR \-greyscale ),
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though for brevity these are not mentioned below.
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.PP
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The basic switches are:
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.TP
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.BI \-quality " N[,...]"
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Scale quantization tables to adjust image quality. Quality is 0 (worst) to
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100 (best); default is 75. (See below for more info.)
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.TP
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.B \-grayscale
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Create monochrome JPEG file from color input. By saying
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.BR \-grayscale,
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you'll get a smaller JPEG file that takes less time to process.
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.TP
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.B \-rgb
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Create RGB JPEG file.
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Using this switch suppresses the conversion from RGB
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colorspace input to the default YCbCr JPEG colorspace.
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.TP
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.B \-optimize
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Perform optimization of entropy encoding parameters. Without this, default
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encoding parameters are used.
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.B \-optimize
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usually makes the JPEG file a little smaller, but
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.B cjpeg
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runs somewhat slower and needs much more memory. Image quality and speed of
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decompression are unaffected by
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.BR \-optimize .
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.TP
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.B \-progressive
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Create progressive JPEG file (see below).
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.TP
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.B \-targa
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Input file is Targa format. Targa files that contain an "identification"
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field will not be automatically recognized by
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.BR cjpeg ;
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for such files you must specify
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.B \-targa
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to make
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.B cjpeg
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treat the input as Targa format.
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For most Targa files, you won't need this switch.
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.PP
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The
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.B \-quality
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switch lets you trade off compressed file size against quality of the
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reconstructed image: the higher the quality setting, the larger the JPEG file,
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and the closer the output image will be to the original input. Normally you
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want to use the lowest quality setting (smallest file) that decompresses into
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something visually indistinguishable from the original image. For this
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purpose the quality setting should generally be between 50 and 95 (the default
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is 75) for photographic images. If you see defects at
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.B \-quality
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75, then go up 5 or 10 counts at a time until you are happy with the output
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image. (The optimal setting will vary from one image to another.)
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.PP
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.B \-quality
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100 will generate a quantization table of all 1's, minimizing loss in the
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quantization step (but there is still information loss in subsampling, as well
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as roundoff error.) For most images, specifying a quality value above
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about 95 will increase the size of the compressed file dramatically, and while
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the quality gain from these higher quality values is measurable (using metrics
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such as PSNR or SSIM), it is rarely perceivable by human vision.
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.PP
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In the other direction, quality values below 50 will produce very small files
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of low image quality. Settings around 5 to 10 might be useful in preparing an
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index of a large image library, for example. Try
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.B \-quality
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2 (or so) for some amusing Cubist effects. (Note: quality
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values below about 25 generate 2-byte quantization tables, which are
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considered optional in the JPEG standard.
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.B cjpeg
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emits a warning message when you give such a quality value, because some
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other JPEG programs may be unable to decode the resulting file. Use
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.B \-baseline
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if you need to ensure compatibility at low quality values.)
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.PP
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The \fB-quality\fR option has been extended in this version of \fBcjpeg\fR to
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support separate quality settings for luminance and chrominance (or, in
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general, separate settings for every quantization table slot.) The principle
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is the same as chrominance subsampling: since the human eye is more sensitive
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to spatial changes in brightness than spatial changes in color, the chrominance
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components can be quantized more than the luminance components without
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incurring any visible image quality loss. However, unlike subsampling, this
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feature reduces data in the frequency domain instead of the spatial domain,
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which allows for more fine-grained control. This option is useful in
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quality-sensitive applications, for which the artifacts generated by
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subsampling may be unacceptable.
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.PP
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The \fB-quality\fR option accepts a comma-separated list of parameters, which
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respectively refer to the quality levels that should be assigned to the
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quantization table slots. If there are more q-table slots than parameters,
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then the last parameter is replicated. Thus, if only one quality parameter is
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given, this is used for both luminance and chrominance (slots 0 and 1,
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respectively), preserving the legacy behavior of cjpeg v6b and prior.
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More (or customized) quantization tables can be set with the \fB-qtables\fR
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option and assigned to components with the \fB-qslots\fR option (see the
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"wizard" switches below.)
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.PP
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JPEG files generated with separate luminance and chrominance quality are fully
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compliant with standard JPEG decoders.
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.PP
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.BR CAUTION:
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For this setting to be useful, be sure to pass an argument of \fB-sample 1x1\fR
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to \fBcjpeg\fR to disable chrominance subsampling. Otherwise, the default
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subsampling level (2x2, AKA "4:2:0") will be used.
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.PP
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The
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.B \-progressive
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switch creates a "progressive JPEG" file. In this type of JPEG file, the data
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is stored in multiple scans of increasing quality. If the file is being
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transmitted over a slow communications link, the decoder can use the first
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scan to display a low-quality image very quickly, and can then improve the
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display with each subsequent scan. The final image is exactly equivalent to a
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standard JPEG file of the same quality setting, and the total file size is
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about the same --- often a little smaller.
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.PP
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Switches for advanced users:
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.TP
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.B \-arithmetic
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Use arithmetic coding.
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.B Caution:
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arithmetic coded JPEG is not yet widely implemented, so many decoders will be
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unable to view an arithmetic coded JPEG file at all.
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.TP
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.B \-dct int
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Use accurate integer DCT method (default).
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.TP
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.B \-dct fast
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Use less accurate integer DCT method [legacy feature].
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When the Independent JPEG Group's software was first released in 1991, the
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compression time for a 1-megapixel JPEG image on a mainstream PC was measured
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in minutes. Thus, the \fBfast\fR integer DCT algorithm provided noticeable
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performance benefits. On modern CPUs running libjpeg-turbo, however, the
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compression time for a 1-megapixel JPEG image is measured in milliseconds, and
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thus the performance benefits of the \fBfast\fR algorithm are much less
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noticeable. On modern x86/x86-64 CPUs that support AVX2 instructions, the
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\fBfast\fR and \fBint\fR methods have similar performance. On other types of
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CPUs, the \fBfast\fR method is generally about 5-15% faster than the \fBint\fR
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method.
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For quality levels of 90 and below, there should be little or no perceptible
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quality difference between the two algorithms. For quality levels above 90,
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however, the difference between the \fBfast\fR and \fBint\fR methods becomes
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more pronounced. With quality=97, for instance, the \fBfast\fR method incurs
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generally about a 1-3 dB loss in PSNR relative to the \fBint\fR method, but
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this can be larger for some images. Do not use the \fBfast\fR method with
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quality levels above 97. The algorithm often degenerates at quality=98 and
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above and can actually produce a more lossy image than if lower quality levels
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had been used. Also, in libjpeg-turbo, the \fBfast\fR method is not fully
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accelerated for quality levels above 97, so it will be slower than the
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\fBint\fR method.
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.TP
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.B \-dct float
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Use floating-point DCT method [legacy feature].
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The \fBfloat\fR method does not produce significantly more accurate results
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than the \fBint\fR method, and it is much slower. The \fBfloat\fR method may
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also give different results on different machines due to varying roundoff
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behavior, whereas the integer methods should give the same results on all
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machines.
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.TP
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.BI \-icc " file"
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Embed ICC color management profile contained in the specified file.
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.TP
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.BI \-restart " N"
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Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
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attached to the number.
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.B \-restart 0
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(the default) means no restart markers.
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.TP
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.BI \-smooth " N"
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Smooth the input image to eliminate dithering noise. N, ranging from 1 to
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100, indicates the strength of smoothing. 0 (the default) means no smoothing.
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.TP
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.BI \-maxmemory " N"
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Set limit for amount of memory to use in processing large images. Value is
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in thousands of bytes, or millions of bytes if "M" is attached to the
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number. For example,
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.B \-max 4m
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selects 4000000 bytes. If more space is needed, an error will occur.
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.TP
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.BI \-outfile " name"
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Send output image to the named file, not to standard output.
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.TP
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.BI \-memdst
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Compress to memory instead of a file. This feature was implemented mainly as a
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way of testing the in-memory destination manager (jpeg_mem_dest()), but it is
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also useful for benchmarking, since it reduces the I/O overhead.
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.TP
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.BI \-report
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Report compression progress.
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.TP
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.BI \-strict
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Treat all warnings as fatal. Enabling this option will cause the compressor to
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abort if an LZW-compressed GIF input image contains incomplete or corrupt image
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data.
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.TP
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.B \-verbose
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Enable debug printout. More
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.BR \-v 's
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give more output. Also, version information is printed at startup.
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.TP
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.B \-debug
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Same as
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.BR \-verbose .
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.TP
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.B \-version
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Print version information and exit.
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.PP
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The
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.B \-restart
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option inserts extra markers that allow a JPEG decoder to resynchronize after
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a transmission error. Without restart markers, any damage to a compressed
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file will usually ruin the image from the point of the error to the end of the
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image; with restart markers, the damage is usually confined to the portion of
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the image up to the next restart marker. Of course, the restart markers
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occupy extra space. We recommend
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.B \-restart 1
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for images that will be transmitted across unreliable networks such as Usenet.
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.PP
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The
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.B \-smooth
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option filters the input to eliminate fine-scale noise. This is often useful
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when converting dithered images to JPEG: a moderate smoothing factor of 10 to
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50 gets rid of dithering patterns in the input file, resulting in a smaller
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JPEG file and a better-looking image. Too large a smoothing factor will
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visibly blur the image, however.
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.PP
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Switches for wizards:
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.TP
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.B \-baseline
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Force baseline-compatible quantization tables to be generated. This clamps
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quantization values to 8 bits even at low quality settings. (This switch is
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poorly named, since it does not ensure that the output is actually baseline
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JPEG. For example, you can use
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.B \-baseline
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and
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.B \-progressive
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together.)
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.TP
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.BI \-qtables " file"
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Use the quantization tables given in the specified text file.
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.TP
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.BI \-qslots " N[,...]"
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Select which quantization table to use for each color component.
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.TP
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.BI \-sample " HxV[,...]"
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Set JPEG sampling factors for each color component.
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.TP
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.BI \-scans " file"
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Use the scan script given in the specified text file.
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.PP
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The "wizard" switches are intended for experimentation with JPEG. If you
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don't know what you are doing, \fBdon't use them\fR. These switches are
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documented further in the file wizard.txt.
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.SH EXAMPLES
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.LP
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This example compresses the PPM file foo.ppm with a quality factor of
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60 and saves the output as foo.jpg:
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.IP
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.B cjpeg \-quality
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.I 60 foo.ppm
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.B >
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.I foo.jpg
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.SH HINTS
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Color GIF files are not the ideal input for JPEG; JPEG is really intended for
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compressing full-color (24-bit) images. In particular, don't try to convert
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cartoons, line drawings, and other images that have only a few distinct
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colors. GIF works great on these, JPEG does not. If you want to convert a
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GIF to JPEG, you should experiment with
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.BR cjpeg 's
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.B \-quality
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and
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.B \-smooth
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options to get a satisfactory conversion.
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.B \-smooth 10
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or so is often helpful.
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.PP
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Avoid running an image through a series of JPEG compression/decompression
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cycles. Image quality loss will accumulate; after ten or so cycles the image
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may be noticeably worse than it was after one cycle. It's best to use a
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lossless format while manipulating an image, then convert to JPEG format when
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you are ready to file the image away.
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.PP
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The
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.B \-optimize
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option to
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.B cjpeg
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is worth using when you are making a "final" version for posting or archiving.
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It's also a win when you are using low quality settings to make very small
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JPEG files; the percentage improvement is often a lot more than it is on
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larger files. (At present,
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.B \-optimize
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mode is always selected when generating progressive JPEG files.)
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.SH ENVIRONMENT
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.TP
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.B JPEGMEM
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If this environment variable is set, its value is the default memory limit.
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The value is specified as described for the
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.B \-maxmemory
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switch.
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.B JPEGMEM
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overrides the default value specified when the program was compiled, and
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itself is overridden by an explicit
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.BR \-maxmemory .
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.SH SEE ALSO
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.BR djpeg (1),
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.BR jpegtran (1),
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.BR rdjpgcom (1),
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.BR wrjpgcom (1)
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.br
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.BR ppm (5),
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.BR pgm (5)
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.br
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Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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.SH AUTHOR
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Independent JPEG Group
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.PP
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This file was modified by The libjpeg-turbo Project to include only information
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relevant to libjpeg-turbo, to wordsmith certain sections, and to describe
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features not present in libjpeg.
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.SH ISSUES
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Not all variants of BMP and Targa file formats are supported.
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.PP
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The
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.B \-targa
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switch is not a bug, it's a feature. (It would be a bug if the Targa format
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designers had not been clueless.)
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