6e632af9f6
- Refer to the "slow" [I]DCT algorithms as "accurate" instead, since they are not slow under libjpeg-turbo. - Adjust documentation claims to reflect the fact that the "slow" and "fast" algorithms produce about the same performance on AVX2-equipped CPUs (because of the dual-lane nature of AVX2, it was not possible to accelerate the "fast" algorithm beyond what was achievable with SSE2.) Also adjust the claims to reflect the fact that the "fast" algorithm tends to be ~5-15% faster than the "slow" algorithm on non-AVX2-equipped CPUs, regardless of the use of the libjpeg-turbo SIMD extensions. - Indicate the legacy status of the "fast" and float algorithms in the documentation and cjpeg/djpeg usage info. - Remove obsolete paragraph in the djpeg man page that suggested that the float algorithm could be faster than the "fast" algorithm on some CPUs.
299 lines
9.9 KiB
Groff
299 lines
9.9 KiB
Groff
.TH DJPEG 1 "4 November 2020"
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.SH NAME
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djpeg \- decompress a JPEG file to an image file
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.SH SYNOPSIS
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.B djpeg
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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 djpeg
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decompresses the named JPEG file, or the standard input if no file is named,
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and produces an image file on the standard output. PBMPLUS (PPM/PGM), BMP,
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GIF, Targa, or RLE (Utah Raster Toolkit) output format can be selected.
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(RLE is supported only if the URT library is available.)
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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 \-colors " N"
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Reduce image to at most N colors. This reduces the number of colors used in
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the output image, so that it can be displayed on a colormapped display or
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stored in a colormapped file format. For example, if you have an 8-bit
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display, you'd need to reduce to 256 or fewer colors.
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.TP
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.BI \-quantize " N"
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Same as
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.BR \-colors .
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.B \-colors
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is the recommended name,
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.B \-quantize
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is provided only for backwards compatibility.
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.TP
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.B \-fast
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Select recommended processing options for fast, low quality output. (The
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default options are chosen for highest quality output.) Currently, this is
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equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.
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.TP
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.B \-grayscale
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Force grayscale output even if JPEG file is color. Useful for viewing on
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monochrome displays; also,
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.B djpeg
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runs noticeably faster in this mode.
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.TP
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.B \-rgb
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Force RGB output even if JPEG file is grayscale.
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.TP
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.BI \-scale " M/N"
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Scale the output image by a factor M/N. Currently the scale factor must be
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M/8, where M is an integer between 1 and 16 inclusive, or any reduced fraction
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thereof (such as 1/2, 3/4, etc.) Scaling is handy if the image is larger than
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your screen; also,
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.B djpeg
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runs much faster when scaling down the output.
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.TP
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.B \-bmp
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Select BMP output format (Windows flavor). 8-bit colormapped format is
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emitted if
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.B \-colors
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or
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.B \-grayscale
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is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
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format is emitted.
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.TP
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.B \-gif
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Select GIF output format. Since GIF does not support more than 256 colors,
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.B \-colors 256
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is assumed (unless you specify a smaller number of colors).
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.TP
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.B \-os2
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Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is
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emitted if
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.B \-colors
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or
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.B \-grayscale
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is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
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format is emitted.
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.TP
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.B \-pnm
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Select PBMPLUS (PPM/PGM) output format (this is the default format).
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PGM is emitted if the JPEG file is grayscale or if
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.B \-grayscale
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is specified; otherwise PPM is emitted.
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.TP
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.B \-rle
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Select RLE output format. (Requires URT library.)
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.TP
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.B \-targa
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Select Targa output format. Grayscale format is emitted if the JPEG file is
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grayscale or if
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.B \-grayscale
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is specified; otherwise, colormapped format is emitted if
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.B \-colors
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is specified; otherwise, 24-bit full-color format is emitted.
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.PP
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Switches for advanced users:
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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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decompression 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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decompression time for a 1-megapixel JPEG image is measured in milliseconds,
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and 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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If the JPEG image was compressed using a quality level of 85 or below, then
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there should be little or no perceptible quality difference between the two
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algorithms. When decompressing images that were compressed using quality
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levels above 85, however, the difference between the \fBfast\fR and \fBint\fR
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methods becomes more pronounced. With images compressed using quality=97, for
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instance, the \fBfast\fR method incurs generally about a 4-6 dB loss in PSNR
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relative to the \fBint\fR method, but this can be larger for some images. If
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you can avoid it, do not use the \fBfast\fR method when decompressing images
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that were compressed using quality levels above 97. The algorithm often
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degenerates for such images and can actually produce a more lossy output image
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than if the JPEG image had been compressed using lower quality levels.
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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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.B \-dither fs
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Use Floyd-Steinberg dithering in color quantization.
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.TP
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.B \-dither ordered
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Use ordered dithering in color quantization.
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.TP
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.B \-dither none
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Do not use dithering in color quantization.
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By default, Floyd-Steinberg dithering is applied when quantizing colors; this
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is slow but usually produces the best results. Ordered dither is a compromise
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between speed and quality; no dithering is fast but usually looks awful. Note
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that these switches have no effect unless color quantization is being done.
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Ordered dither is only available in
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.B \-onepass
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mode.
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.TP
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.BI \-icc " file"
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Extract ICC color management profile to the specified file.
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.TP
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.BI \-map " file"
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Quantize to the colors used in the specified image file. This is useful for
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producing multiple files with identical color maps, or for forcing a
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predefined set of colors to be used. The
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.I file
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must be a GIF or PPM file. This option overrides
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.B \-colors
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and
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.BR \-onepass .
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.TP
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.B \-nosmooth
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Use a faster, lower-quality upsampling routine.
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.TP
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.B \-onepass
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Use one-pass instead of two-pass color quantization. The one-pass method is
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faster and needs less memory, but it produces a lower-quality image.
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.B \-onepass
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is ignored unless you also say
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.B \-colors
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.IR N .
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Also, the one-pass method is always used for grayscale output (the two-pass
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method is no improvement then).
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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 \-memsrc
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Load input file into memory before decompressing. This feature was implemented
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mainly as a way of testing the in-memory source manager (jpeg_mem_src().)
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.TP
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.BI \-skip " Y0,Y1"
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Decompress all rows of the JPEG image except those between Y0 and Y1
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(inclusive.) Note that if decompression scaling is being used, then Y0 and Y1
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are relative to the scaled image dimensions.
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.TP
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.BI \-crop " WxH+X+Y"
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Decompress only a rectangular subregion of the image, starting at point X,Y
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with width W and height H. If necessary, X will be shifted left to the nearest
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iMCU boundary, and the width will be increased accordingly. Note that if
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decompression scaling is being used, then X, Y, W, and H are relative to the
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scaled image dimensions. Currently this option only works with the
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PBMPLUS (PPM/PGM), GIF, and Targa output formats.
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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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.SH EXAMPLES
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.LP
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This example decompresses the JPEG file foo.jpg, quantizes it to
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256 colors, and saves the output in 8-bit BMP format in foo.bmp:
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.IP
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.B djpeg \-colors 256 \-bmp
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.I foo.jpg
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.B >
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.I foo.bmp
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.SH HINTS
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To get a quick preview of an image, use the
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.B \-grayscale
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and/or
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.B \-scale
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switches.
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.B \-grayscale \-scale 1/8
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is the fastest case.
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.PP
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Several options are available that trade off image quality to gain speed.
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.B \-fast
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turns on the recommended settings.
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.PP
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.B \-dct fast
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and/or
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.B \-nosmooth
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gain speed at a small sacrifice in quality.
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When producing a color-quantized image,
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.B \-onepass \-dither ordered
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is fast but much lower quality than the default behavior.
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.B \-dither none
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may give acceptable results in two-pass mode, but is seldom tolerable in
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one-pass mode.
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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 cjpeg (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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Support for compressed GIF output files was removed in djpeg v6b due to
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concerns over the Unisys LZW patent. Although this patent expired in 2006,
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djpeg still lacks compressed GIF support, for these historical reasons.
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(Conversion of JPEG files to GIF is usually a bad idea anyway, since GIF is a
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256-color format.) The uncompressed GIF files that djpeg generates are larger
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than they should be, but they are readable by standard GIF decoders.
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