NAME¶

jpeg - jpeg compressor

SYNOPSIS¶

jpeg [options] source target

DESCRIPTION¶

jpeg Copyright (C) 2012-2018 Thomas Richter, University of Stuttgart and Accusoft

For license conditions, see README.license for details.

default is to decode the jpeg input and write a ppm output use -q [1..100] or -p to enforce encoding

-q quality : selects the encoding mode and defines the quality of the base image

-Q quality : defines the quality for the extension layer

-quality q : use a profile and part specific weighting between base and extension

layer quality

-r : enable the residual codestream for HDR and lossless

coding, requires -q and -Q to define base and enhancement layer quality.

-r12 : use a 12 bit residual image instead of an 8 bit residual

image.

-rl : enforce a int-to-int lossless DCT in the residual domain

for lossless coding enabled by -Q 100

-ro : disable the DCT in the residual domain, quantize spatially for

near-lossless coding

-ldr file : specifies a separate file containing the base layer

for encoding.

-R bits : specify refinement bits for the base images.

This works like -r but in the DCT domain.

-rR bits : specify refinement bits for the residual image.

-N : enable noise shaping of the prediction residual

-U : disable automatic upsampling

-l : enable lossless coding without a residual image by an

int-to-int DCT, also requires -c and -q 100 for true lossless

-p : JPEG lossless (predictive) mode

also requires -c for true lossless

-c : disable the RGB to YCbCr decorrelation transformation

-xyz : indicates that the HDR image is in the XYZ colorspace

note that the image is not *converted* to this space, but is assumed to be encoded in this space.

-cxyz : similar to the above, but uses the dedicated C transformation

to implement a XYZ colorspace conversion.

-sp : use separate LUTs for each component.

-md : use the median instead of the center of mass

for constructing the inverse TMO of ISO/IEC 18477-7 profile C.

-ct : use the center of mass instead of the median

for constructing the inverse TMO of ISO/IEC 18477-7 profile C.

-sm iter : use <iter> iterations to smooth out the histogram for

inverse-TMO based algorithms. Default is not to smooth the histogram.

-ncl : disable clamping of out-of-gamut colors.

this is automatically enabled for lossless.

-m maxerr : defines a maximum pixel error for JPEG LS coding

-h : optimize the Huffman tables

-a : use arithmetic coding instead of Huffman coding

available for all coding schemes (-p,-v,-l and default)

-bl : force encoding in the baseline process, default is extended sequential

-v : use progressive instead of sequential encoding

available for all coding schemes (-r,-a,-l and default)

-qv : use a simplified scan pattern for progressive that only

separates AC from DC bands and may improve the performance

-d : encode the DC band only (requires -p)

-y levels : hierarchical JPEG coding with the given number of decomposition

levels. If levels is zero, then a lossless coding mode for hierarchical is used in which the second lossless scan encodes the DCT residuals of the first scan. For that, -c is suggested for true lossless. If levels is one, then the lossy initial scan is downscaled by a power of two.

-g gamma : define the exponent for the gamma for the LDR domain, or rather, for

mapping HDR to LDR. A suggested value is 2.4 for mapping scRGB to sRBG. This option controls the base-nonlinearity that generates the HDR pre-cursor image from the LDR image. It is also used in the absence of -ldr (i.e. no LDR image) to tonemap the HDR input image. Use -g 0 to use an approximate inverse TMO as base-nonlinearity, and for tonemapping with the Reinhard operator if the LDR image is missing.

-gf file : define the inverse one-point L-nonlinearity on decoding from a file

this file contains one (ASCII encoded) digit per line, 256*2^h lines in total, where h is the number of refinement bits. Each line contains an (integer) output value the corresponding input is mapped to.

-z mcus : define the restart interval size, zero disables it

-n : indicate the image height by a DNL marker

-s WxH,... : define subsampling factors for all components

note that these are NOT MCU sizes Default is 1x1,1x1,1x1 (444 subsampling) 1x1,2x2,2x2 is the 420 subsampling often used

-sr WxH,...: define subsampling in the residual domain

-rs : encode the residual image in sequential (rather than the modified residual)

coding mode

-rv : encode the residual image in progressive coding mode

-ol : open loop encoding, residuals are based on original, not reconstructed

-dz : improved deadzone quantizer, may help to improve the R/D performance

-oz : optimize quantizer, may help to improve the R/D performance

-dr : include the optional de-ringing (Gibbs Phenomenon) filter on encoding

EXAMPLES¶

Standard JPEG compression, with 444 (aka "no") subsampling:

$ jpeg -q <quality> infile.ppm outfile.jpg

Standard JPEG compression, with 422 subsampling:

$ jpeg -q <quality> -s 1x1,2x2,2x2 infile.ppm outfile.jpg

Intermediate dynamic range compression, i.e. compression of images of a bit-depth between 8 and 16 bits:

$ jpeg -r -q <base-quality> -Q <extension-quality> -h -r12 infile.ppm outfile.jpg

This type of encoding uses a technology known as "residual scans" which increase the bit-depths in the spatial domain which is enabled by the -r command line switch. The -Q parameter sets the quality of the residual image. To improve the precision in the frequency domain, "refinement scans" can be used. The following encodes a 12-bit image with four additional refinement scans, enabled by the "-R 4" parameter.

$ jpeg -q <quality> -R 4 -h infile.ppm outfile.jpg

Both technologies can be combined, and the precision of the residual scan can also be enlarged by using residual refinement scans with the -rR option. The following command line with use a 12-bit residual scan with four refinement scans:

$ jpeg -r -q <base-quality> -Q <extension-quality> -h -rR 4 infile.ppm outfile.jpg

High-dynamic range compression allows three different profiles of varying complexity and performance. The profiles are denoted by "-profile <X>" where <X> is a,b or c. The following encodes an HDR image in profile C:

$ jpeg -r -q <base-quality> -Q <extension-quality> -h -profile c -rR 4 infile.pfm outfile.jpg

HDR images here have to be fed into the command line in "pfm" format. exr or hdr is not supported as input format and requires conversion to pfm first. pfm is the floating-point equivalent of ppm and encodes each pixel by three 32-bit floating point numbers.

Encoding in profiles a and b works likewise, though it is generally advisable to use "open loop" rather than "closed loop" coding for these two profiles by additionally providing the "-ol" parameter. This also works for profile C:

$ jpeg -ol -r -profile a -q <base-quality> -Q <extension-quality> -h infile.pfm out.jpg

similar for profile B.

What is common to profiles A and C is that you may optionally also specify the LDR image, i.e. the image that a legacy JPEG decoder will show. By default, a simple tone mapping algorithm ("global Reinhard") will be used to derive a suitable LDR image from the input image:

$ jpeg -ldr infile.ppm -q <base-quality> -Q <extension-quality> -h -rR 4 infile.pfm out.jpg

The profile is by default profile c, but it also works for profile a:

$ jpeg -ol profile a -ldr infile.ppm -q <base-quality> -Q <extension-quality> infile.pfm out.jpg

It is in general advisable for profile c encoding to enable residual refinement scans, profiles a or b do not require them.

The following options exist for lossless coding integer:

predictive Rec. ITU-T T.81 | ISO/IEC 10918-1 coding. Note, however, that not many implementations are capable of decoding such stream, thus this is probably not a good option for all-day purposes.

$ jpeg -p -c infile.ppm out.jpg

While the result is a valid Rec. ITU-T T.81 | ISO/IEC 10918-1 stream, most other implementations will hick up and break, thus it is not advisable to use it.

A second option for lossless coding is residual coding within profile c:

$ jpeg -q <quality> -Q 100 -h -r infile.ppm out.jpg

This also works for floating point coding. Note that lossless coding is enabled by setting the extension quality to 100.

$ jpeg -q <quality> -Q 100 -h -r infile.pfm out.jpg

However, this is only lossless for 16 bit input samples, i.e. there is a precision loss due to down-converting the 32-bit input to 16 bit. If samples are out of the 601 gamut, the problem also exists that clamping will happen. To avoid that, encode in the XYZ color space (profile C only, currently):

$ jpeg -xyz -q <quality> -Q 100 -h -r infile.pfm out.jpg

A second option for lossless integer coding is to use a lossless 1-1 DCT process. This is enabled with the -l command line option:

$ jpeg -l -q 100 -c infile.ppm out.jpg

Refinement scans can be used to increase the sample precision to up to 12 bits. The "-c" command line option disables the lossy color transformation.

Additionally, this implementation also supports JPEG LS, which is outside of Rec. ITU-T T.81 | ISO/IEC 10918-1 and ISO/IEC 18477. For that, use the command line option -ls:

$ jpeg -ls -c infile.ppm out.jpg

The "-c" command line switch is necessary to disable the color transformation as JPEG LS typically encodes in RGB and not YCbCr space.

Optionally, you may specify the JPEG LS "near" parameter (maximum error) with the -m command line switch:

$ jpeg -ls -m 2 -c infile.ppm out.jpg

JPEG LS also specifies a lossless color transformation that is enabled with -cls:

$ jpeg -ls -cls infile.ppm out.jpg

To encode images with an alpha channel, specify the source image that contains the alpha channel with -al. The alpha channel is a one-component grey-scale image, either integer or floating point. The quality of the alpha channel is specified with -aq, that of the regular image with -q:

$ jpeg -al alpha.pgm -aq 80 -q 85 input.ppm output.jpg

Alpha channels can be larger than 8bpp or can be floating point. In both cases, residual coding is required. To enable residual coding in the alpha channel, use the -ar command line option. Similar to the regular image, where residual coding requires two parameters, -q for the base quality and -Q for the extension quality, an alpha channel that uses residual coding also requires a base and extension quality, the former is given by -aq, the latter with -aQ:

$ jpeg -ar -al alphahigh.pgm -q 85 -Q 90 -aq 80 -aQ 90 input.ppm out.jpg

The alpha channel can be encoded without loss if desired. For that, enable residual coding with -ar and specify an extension quality of 100:

$ jpeg -ar -al alphahigh.pgm -q 85 -Q 90 -aq 80 -aQ 100 input.ppm out.jpg

The alpha channel can use the same technology extensions as the image, namely refinement scans in the base or extension image, or 12-bit residual images. The number of refinement scans is selected with -aR and -arR for the base and residual image, a 12-bit residual image is selected with -ar12.

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Decoding is much simpler:

$ jpeg infile.jpg out.ppm

or, for floating point images:

$ jpeg infile.jpg out.pfm

If you want to decode a JPEG LS image, then you may want to tell the decoder explicitly to disable the color transformation even though the corresponding marker signalling coding in RGB space is typically missing for JPEG LS:

$ jpeg -c infile.jpg out.ppm

If an alpha channel is included in the image, the decoder does not reconstruct this automatically, nor does it attempt to merge the alpha image into the file. Instead, it may optionally be instructed to write the alpha channel into a separate 1-component (grey-scale) file:

$ jpeg -al alpha.pgm infile.jpg outfile.ppm

The -al option for the decoder provides the target file for the alpha channel.

AUTHOR¶

This manual page was written by Mathieu Malaterre <malat@debian.org> for the Debian GNU/Linux system (but may be used by others).