| .TH JPEGTRAN 1 "15 June 1995" |
| .SH NAME |
| jpegtran \- lossless transcoding of JPEG files |
| .SH SYNOPSIS |
| .B jpegtran |
| [ |
| .I options |
| ] |
| [ |
| .I filename |
| ] |
| .LP |
| .SH DESCRIPTION |
| .LP |
| .B jpegtran |
| translates JPEG files from one variant of JPEG to another, for example |
| from baseline JPEG to progressive JPEG. The transformation is lossless: |
| no image degradation occurs, which would not be true if you used |
| .B djpeg |
| followed by |
| .BR cjpeg . |
| However, you cannot alter the image quality, because that would not be |
| a lossless operation. |
| .B jpegtran |
| reads the named JPEG/JFIF file, or the standard input if no file is |
| named, and produces a JPEG/JFIF file on the standard output. |
| .SH OPTIONS |
| .B jpegtran |
| accepts a subset of the switches recognized by |
| .BR cjpeg . |
| If you specify no switches, you get a plain baseline JPEG output file. |
| .PP |
| All switch names may be abbreviated; for example, |
| .B \-optimize |
| may be written |
| .B \-opt |
| or |
| .BR \-o . |
| Upper and lower case are equivalent. |
| British spellings are also accepted (e.g., |
| .BR \-optimise ), |
| though for brevity these are not mentioned below. |
| .PP |
| The basic switches are: |
| .TP |
| .B \-optimize |
| Perform optimization of entropy encoding parameters. Without this, default |
| encoding parameters are used. |
| .B \-optimize |
| usually makes the JPEG file a little smaller, but at the price of slower |
| compression. Note that |
| .B \-progressive |
| implies |
| .BR \-optimize . |
| .TP |
| .B \-progressive |
| Create progressive JPEG file (see below). |
| .PP |
| The |
| .B \-progressive |
| switch creates a "progressive JPEG" file. In this type of JPEG file, the data |
| is stored in multiple scans of increasing quality. If the file is being |
| transmitted over a slow communications link, the decoder can use the first |
| scan to display a low-quality image very quickly, and can then improve the |
| display with each subsequent scan. The final image is exactly equivalent to a |
| standard JPEG file of the same quality setting, and the total file size is |
| about the same --- often a little smaller. |
| .B Caution: |
| progressive JPEG is not yet widely implemented, so many decoders will be |
| unable to view a progressive JPEG file at all. |
| .PP |
| Switches for advanced users: |
| .TP |
| .BI \-restart " N" |
| Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is |
| attached to the number. |
| .B \-restart 0 |
| (the default) means no restart markers. |
| .TP |
| .BI \-maxmemory " N" |
| Set limit for amount of memory to use in processing large images. Value is |
| in thousands of bytes, or millions of bytes if "M" is attached to the |
| number. For example, |
| .B \-max 4m |
| selects 4000000 bytes. If more space is needed, temporary files will be used. |
| .TP |
| .BI \-outfile " name" |
| Send output image to the named file, not to standard output. |
| .TP |
| .B \-verbose |
| Enable debug printout. More |
| .BR \-v 's |
| give more output. Also, version information is printed at startup. |
| .TP |
| .B \-debug |
| Same as |
| .BR \-verbose . |
| .PP |
| The |
| .B \-restart |
| option inserts extra markers that allow a JPEG decoder to resynchronize after |
| a transmission error. Without restart markers, any damage to a compressed |
| file will usually ruin the image from the point of the error to the end of the |
| image; with restart markers, the damage is usually confined to the portion of |
| the image up to the next restart marker. Of course, the restart markers |
| occupy extra space. We recommend |
| .B \-restart 1 |
| for images that will be transmitted across unreliable networks such as Usenet. |
| .PP |
| Switches for wizards: |
| .TP |
| .BI \-scans " file" |
| Use the scan script given in the specified text file. |
| .PP |
| The "wizard" switches are intended for experimentation with JPEG. If you |
| don't know what you are doing, \fBdon't use them\fR. These switches are |
| documented further in the file wizard.doc. |
| .SH EXAMPLES |
| .LP |
| This example converts a baseline JPEG file to progressive form: |
| .IP |
| .B jpegtran \-progressive |
| .I foo.jpg |
| .B > |
| .I fooprog.jpg |
| .SH ENVIRONMENT |
| .TP |
| .B JPEGMEM |
| If this environment variable is set, its value is the default memory limit. |
| The value is specified as described for the |
| .B \-maxmemory |
| switch. |
| .B JPEGMEM |
| overrides the default value specified when the program was compiled, and |
| itself is overridden by an explicit |
| .BR \-maxmemory . |
| .SH SEE ALSO |
| .BR cjpeg (1), |
| .BR djpeg (1), |
| .BR rdjpgcom (1), |
| .BR wrjpgcom (1) |
| .br |
| Wallace, Gregory K. "The JPEG Still Picture Compression Standard", |
| Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44. |
| .SH AUTHOR |
| Independent JPEG Group |
| .SH BUGS |
| Arithmetic coding is not supported for legal reasons. |
| .PP |
| Still not as fast as we'd like. |
