JPEG

Section: User Commands (1)
Updated: 14 June 1993
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NAME

jpeg - JPEG compression and decompression

SYNOPSIS

jpeg -iw ImageWidth -ih ImageHeight [-JFIF] [-q(l) Q-Factor]
     [-a] [-b] [-d] [-k predictortype] [-n] [-o] [-y] [-z]
     [-p PrecisionValue] [-t pointtransform]
     [-r ResyncInterval] [-s StreamName]
     [[-ci ComponentIndex1] [-fw FrameWidth1] [-fh FrameHeight1]
      [-hf HorizontalFrequency1] [-vf VerticalFrequency1]
      ComponentFile1]
     [[-ci ComponentIndex2] [-fw FrameWidth2] [-fh FrameHeight2]
      [-hf HorizontalFrequency2] [-vf VerticalFrequency2]
      ComponentFile2]
     ....

DESCRIPTION

jpeg is a still-image compression/decompression program that performs JPEG encoding and decoding of multiple raster-scanned files.

OPTIONS

ImageWidth

specifies the width of the original image. This should correspond to the width of the widest component and, thus, the width of the ``original image''. All components have widths roughly corresponding to an integer decimation ratio from this specification.

ImageHeight

specifies the height of the tallest component. This corresponds to the height of the ``original image''.

-JFIF

specifies that a JFIF header is placed on the encoded stream. This is unnecessary for decoding.

Q-Factor

option specifies a multiplicative factor for the quantization: each quantization coefficient of the default matrix is scaled by (Q-Factor/50). A Q-Factor of 0 is the same thing as a Q-Factor of 50 because it disables this function. -q specifies an 8 bit quantization matrix; -ql specifies a 16 bit quantization matrix, useful for 12 bit data.

-a

enables the double-precision floating point Reference DCT. (Default is Chen DCT.)

-b

enables the Lee DCT. (Default is Chen DCT.)

-d

enables decoding. See below.

-k predictortype

The lossless predictor type, specified as an integer between 1-7. If specified, then lossless mode is used.

-n

This option specifies that the files should not be transmitted in interleaved format.

-o

signals that the command interpreter will read from the standard input.

-p

Specifies the precision. Normally should be between 2-16 for lossless; 8 or 12 for DCT. If it is specified as a number greater than 8 then the input is considered to be unsigned shorts (16 bits, msb first). Not aggressively checked.

-t pointtransform

Specifies the shifting (right) upon loading input and shifting (left) upon writing input. Generally used by the lossless mode only. Can be used by the DCT mode to add or subtract bits.

-y

for decoding only, signals that no resynchronization is enabled, thus ignore any markers found in the data stream.

-z

enables use of default Huffman tables. This converts the coding from a two-pass system using the first pass to generate custom tables to a one-pass system using internal default tables. With this option, the compression speed is nearly doubled, but because the internal tables are not custom to the image, the compressed file size increases slightly.

ResyncInterval

specifies a resync (restart) interval for the input file--if set to 0 (default), resynchronization is disabled; otherwise it signifies the number of MDU between a resync marker.

StreamName

is the place to load(decoder)/store(encoder) the coded image--if unspecified it defaults to ComponentFile1.jpg.

For every component in the image we have:

ComponentIndex

describes the component index where the file data should be associated with. The possible values are between 0 and 255. As a rule Y is in 1; U is in 2; V is in 3. The file specfications, if left undisturbed, will result in component location of 1 for the first component file, 2 for the second component file, and so on. If -ci is specified for the previous component file, then the next component index defaults to the previous component index plus 1.

FrameWidth

describes the actual width of the component. This should be determinable by the size of the original image (ImageHeight and ImageWidth) and the frequency sampling of that component. This program assumes that the sampling component will be round up to the nearest integer and other programs may not necessarily follow that convention, we allow precise specification of the FrameWidth. The program will notify the user if the framewidth and frameheight specifications do not correspond to a logical MDU pattern and thus will refuse to take the input (in fact, sometimes rounding down will not result in a logical MDU pattern).

FrameHeight

describes the actual height of the component. Multiplied together with FrameWidth, this should equal the file size of the component. See the above discussion on the actual specification.

Hor-Frequency

specifies the block sampling frequency of the component in the horizontal direction for every MDU transmitted.

Ver-Frequency

specifies the block sampling frequency of the component in the vertical direction. When multiplied together with the Horizontal frequency, it corresponds to the number of blocks of that component in the MDU.

ComponentFilen

represents the directory path location of the nth component file.

EXAMPLES

In order to encode a set of raster-scanned files: 128x128 in image.Y; 64x128 in image.U; and 64x128 in image.V into the file image.jpg, the command is

jpeg -iw 128 -ih 128 -hf 2 image.Y image.U image.V -s image.jpg

In order to decode a compressed file in image.jpg, type

jpeg -d -s image.jpg

The three output files will be in image.jpg.1 image.jpg.2 image.jpg.3. The images can be displayed by the cv program. The images can also be converted to ppm and back through the programs cyuv2ppm and ppm2cyuv Those utility programs available by anonymous ftp from havefun.stanford.edu:pub/cv/CVv1.2.1.tar.Z.
There are many more options within an internal command interpreter. Please see the accompanying documentation in doc.ps for more details.

FTP

jpeg is available by anonymous ftp from havefun.stanford.edu:pub/jpeg/JPEGv1.2.tar.Z.

BUGS

Somewhat slower than many commercial implementations, some bugs are probably lurking around. Lossless coding and decoding are especially slow. Please inform the author at achung@cs.stanford.edu if any bugs are found.

AUTHOR

Andy Hung

This document was created by man2html, using the manual pages.
Time: 20:51:04 GMT, February 12, 2023