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ppmtojpeg - convert portable pixmap into a JPEG/JFIF file
ppmtojpeg converts the named PPM image file, or the standard input if no file is named, to a JPEG/JFIF file on the standard output. It also takes PGM and PBM files as input, by "promoting" them to PPM files before processing.
ppmtojpeg uses the Independent JPEG Group's JPEG library to create the output file. See http://www.ijg.org for information on the library.
The --quality option lets you trade off compressed file size against quality of the reconstructed image: the higher the quality setting, the larger the JPEG file, and the closer the output image will be to the original input. Normally you want to use the lowest quality setting (smallest file) that decompresses into something visually indistinguishable from the original image. For this purpose the quality setting should be between 50 and 95; the default of 75 is often about right. If you see defects at --quality=75, then go up 5 or 10 counts at a time until you are happy with the output image. (The optimal setting will vary from one image to another.)
--quality=100 generates a quantization table of all 1's, minimizing loss in the quantization step (but there is still information loss in subsampling, as well as roundoff error). This setting is mainly of interest for experimental purposes. Quality values above about 95 are not recommended for normal use; the compressed file size goes up dramatically for hardly any gain in output image quality.
In the other direction, quality values below 50 will produce very small files of low image quality. Settings around 5 to 10 might be useful in preparing an index of a large image library, for example. Try --quality=2 (or so) for some amusing Cubist effects. (Note: quality values below about 25 generate 2-byte quantization tables, which are considered optional in the JPEG standard. ppmtojpeg emits a warning message when you give such a quality value, because some other JPEG programs may be unable to decode the resulting file. Use --baseline if you need to ensure compatibility at low quality values.)
The --progressive option 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. Caution: progressive JPEG is not yet widely implemented, so many decoders will be unable to view a progressive JPEG file at all.
Options for advanced users:
The --restart option tells ppmtojpeg to insert 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 --restart=1 for images that will be transmitted across unreliable networks such as Usenet.
The --smooth option filters the input to eliminate fine-scale noise. This is often useful when converting dithered images to JPEG: a moderate smoothing factor of 10 to 50 gets rid of dithering patterns in the input file, resulting in a smaller JPEG file and a better-looking image. Too large a smoothing factor will visibly blur the image, however.
Options for wizards:
The "wizard" options are intended for experimentation with JPEG. If you don't know what you are doing, don't use them. These switches are documented further in the file wizard.doc that comes with the Independent JPEG Group's JPEG library.
This example compresses the PPM file foo.ppm with a quality factor of 60 and saves the output as foo.jpg:
JPEG compression is notable for being a "lossy." This means that, unlike with most graphics conversions, you lose information, which means image quality, when you convert to JPEG. If you convert from PPM to JPEG and back repeatedly, image quality loss will accumulate. After ten or so cycles the image may be noticeably worse than it was after one cycle.
Because of this, you should do all the manipulation you have to do on the image in some other format and convert to JPEG as the last step. And if you can keep a copy in the original format, so much the better. PNG is a good choice for a format that is lossless, yet fairly compact. GIF is another way to go, but chances are you can't create a GIF image without owing a lot of money to Unisys and IBM, holders of patents on the LZW compression used in the GIF format.
The --optimize option to ppmtojpeg is worth using when you are making a "final" version for posting or archiving. It's also a win when you are using low quality settings to make very small JPEG files; the percentage improvement is often a lot more than it is on larger files. (At present, --optimize mode is automatically in effect when you generate a progressive JPEG file).
Another program, cjpeg, is similar. cjpeg is maintained by the Independent JPEG Group and packaged with the JPEG library which ppmtojpeg uses for all its JPEG work. Because of that, you may expect it to exploit more current JPEG features. Also, since you have to have the library to run ppmtojpeg, but not vice versa, cjpeg may be more commonly available.
On the other hand, cjpeg does not use the NetPBM libraries to process its input, as all the NetPBM tools such as ppmtojpeg do. This means it is less likely to be consistent with all the other programs that deal with the NetPBM formats. Also, the command syntax of ppmtojpeg is consistent with that of the other Netpbm tools, unlike cjpeg.
The program could be much faster.
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