|CJPEG(1)||General Commands Manual||CJPEG(1)|
The basic switches are:
- -quality N[,...]
- Scale quantization tables to adjust image quality. Quality is 0 (worst) to 100 (best); default is 75. (See below for more info.)
- Create monochrome JPEG file from color input. Be sure to use this switch when compressing a grayscale BMP or GIF file, because cjpeg isn't bright enough to notice whether a BMP or GIF file uses only shades of gray. By saying -grayscale, you'll get a smaller JPEG file that takes less time to process.
- Create RGB JPEG file. Using this switch suppresses the conversion from RGB colorspace input to the default YCbCr JPEG colorspace.
- Perform optimization of entropy encoding parameters. Without this, default encoding parameters are used. -optimize usually makes the JPEG file a little smaller, but cjpeg runs somewhat slower and needs much more memory. Image quality and speed of decompression are unaffected by -optimize.
- Create progressive JPEG file (see below).
- Input file is Targa format. Targa files that contain an "identification" field will not be automatically recognized by cjpeg; for such files you must specify -targa to make cjpeg treat the input as Targa format. For most Targa files, you won't need this switch.
The -quality switch 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 generally be between 50 and 95 (the default is 75) for photographic images. 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 will generate 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.) For most images, specifying a quality value above about 95 will increase the size of the compressed file dramatically, and while the quality gain from these higher quality values is measurable (using metrics such as PSNR or SSIM), it is rarely perceivable by human vision.
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. cjpeg 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 -quality option has been extended in this version of cjpeg to support separate quality settings for luminance and chrominance (or, in general, separate settings for every quantization table slot.) The principle is the same as chrominance subsampling: since the human eye is more sensitive to spatial changes in brightness than spatial changes in color, the chrominance components can be quantized more than the luminance components without incurring any visible image quality loss. However, unlike subsampling, this feature reduces data in the frequency domain instead of the spatial domain, which allows for more fine-grained control. This option is useful in quality-sensitive applications, for which the artifacts generated by subsampling may be unacceptable.
The -quality option accepts a comma-separated list of parameters, which respectively refer to the quality levels that should be assigned to the quantization table slots. If there are more q-table slots than parameters, then the last parameter is replicated. Thus, if only one quality parameter is given, this is used for both luminance and chrominance (slots 0 and 1, respectively), preserving the legacy behavior of cjpeg v6b and prior. More (or customized) quantization tables can be set with the -qtables option and assigned to components with the -qslots option (see the "wizard" switches below.)
JPEG files generated with separate luminance and chrominance quality are fully compliant with standard JPEG decoders.
CAUTION: For this setting to be useful, be sure to pass an argument of -sample 1x1 to cjpeg to disable chrominance subsampling. Otherwise, the default subsampling level (2x2, AKA "4:2:0") will be used.
The -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.
Switches for advanced users:
- Use arithmetic coding. Caution: arithmetic coded JPEG is not yet widely implemented, so many decoders will be unable to view an arithmetic coded JPEG file at all.
- -dct int
- Use accurate integer DCT method (default).
- -dct fast
- Use less accurate integer DCT method [legacy feature]. When the
Independent JPEG Group's software was first released in 1991, the
compression time for a 1-megapixel JPEG image on a mainstream PC was
measured in minutes. Thus, the fast integer DCT algorithm provided
noticeable performance benefits. On modern CPUs running libjpeg-turbo,
however, the compression time for a 1-megapixel JPEG image is measured in
milliseconds, and thus the performance benefits of the fast
algorithm are much less noticeable. On modern x86/x86-64 CPUs that support
AVX2 instructions, the fast and int methods have similar
performance. On other types of CPUs, the fast method is generally
about 5-15% faster than the int method.
For quality levels of 90 and below, there should be little or no perceptible quality difference between the two algorithms. For quality levels above 90, however, the difference between the fast and int methods becomes more pronounced. With quality=97, for instance, the fast method incurs generally about a 1-3 dB loss in PSNR relative to the int method, but this can be larger for some images. Do not use the fast method with quality levels above 97. The algorithm often degenerates at quality=98 and above and can actually produce a more lossy image than if lower quality levels had been used. Also, in libjpeg-turbo, the fast method is not fully accelerated for quality levels above 97, so it will be slower than the int method.
- -dct float
- Use floating-point DCT method [legacy feature]. The float method does not produce significantly more accurate results than the int method, and it is much slower. The float method may also give different results on different machines due to varying roundoff behavior, whereas the integer methods should give the same results on all machines.
- -icc file
- Embed ICC color management profile contained in the specified file.
- -restart N
- Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is attached to the number. -restart 0 (the default) means no restart markers.
- -smooth N
- Smooth the input image to eliminate dithering noise. N, ranging from 1 to 100, indicates the strength of smoothing. 0 (the default) means no smoothing.
- -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, -max 4m selects 4000000 bytes. If more space is needed, an error will occur.
- -outfile name
- Send output image to the named file, not to standard output.
- Compress to memory instead of a file. This feature was implemented mainly as a way of testing the in-memory destination manager (jpeg_mem_dest()), but it is also useful for benchmarking, since it reduces the I/O overhead.
- Report compression progress.
- Enable debug printout. More -v's give more output. Also, version information is printed at startup.
- Same as -verbose.
- Print version information and exit.
The -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 -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.
Switches for wizards:
- Force baseline-compatible quantization tables to be generated. This clamps quantization values to 8 bits even at low quality settings. (This switch is poorly named, since it does not ensure that the output is actually baseline JPEG. For example, you can use -baseline and -progressive together.)
- -qtables file
- Use the quantization tables given in the specified text file.
- -qslots N[,...]
- Select which quantization table to use for each color component.
- -sample HxV[,...]
- Set JPEG sampling factors for each color component.
- -scans file
- Use the scan script given in the specified text file.
The "wizard" switches 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.txt.
- cjpeg -quality 60 foo.ppm > foo.jpg
Avoid running an image through a series of JPEG compression/decompression cycles. Image quality loss will accumulate; after ten or so cycles the image may be noticeably worse than it was after one cycle. It's best to use a lossless format while manipulating an image, then convert to JPEG format when you are ready to file the image away.
The -optimize option to cjpeg 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 always selected when generating progressive JPEG files.)
- If this environment variable is set, its value is the default memory limit. The value is specified as described for the -maxmemory switch. JPEGMEM overrides the default value specified when the program was compiled, and itself is overridden by an explicit -maxmemory.
Wallace, Gregory K. "The JPEG Still Picture Compression Standard", Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
This file was modified by The libjpeg-turbo Project to include only information relevant to libjpeg-turbo, to wordsmith certain sections, and to describe features not present in libjpeg.
The -targa switch is not a bug, it's a feature. (It would be a bug if the Targa format designers had not been clueless.)
|4 November 2020|