| NOTE: This file was modified by The libjpeg-turbo Project to include only |
| information relevant to libjpeg-turbo and to wordsmith certain sections. |
| |
| USAGE instructions for the Independent JPEG Group's JPEG software |
| ================================================================= |
| |
| This file describes usage of the JPEG conversion programs cjpeg and djpeg, |
| as well as the utility programs jpegtran, rdjpgcom and wrjpgcom. (See |
| the other documentation files if you wish to use the JPEG library within |
| your own programs.) |
| |
| If you are on a Unix machine you may prefer to read the Unix-style manual |
| pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1. |
| |
| |
| INTRODUCTION |
| |
| These programs implement JPEG image encoding, decoding, and transcoding. |
| JPEG (pronounced "jay-peg") is a standardized compression method for |
| full-color and grayscale images. |
| |
| |
| GENERAL USAGE |
| |
| We provide two programs, cjpeg to compress an image file into JPEG format, |
| and djpeg to decompress a JPEG file back into a conventional image format. |
| |
| On Unix-like systems, you say: |
| cjpeg [switches] [imagefile] >jpegfile |
| or |
| djpeg [switches] [jpegfile] >imagefile |
| The programs read the specified input file, or standard input if none is |
| named. They always write to standard output (with trace/error messages to |
| standard error). These conventions are handy for piping images between |
| programs. |
| |
| On most non-Unix systems, you say: |
| cjpeg [switches] imagefile jpegfile |
| or |
| djpeg [switches] jpegfile imagefile |
| i.e., both the input and output files are named on the command line. This |
| style is a little more foolproof, and it loses no functionality if you don't |
| have pipes. (You can get this style on Unix too, if you prefer, by defining |
| TWO_FILE_COMMANDLINE when you compile the programs; see install.txt.) |
| |
| You can also say: |
| cjpeg [switches] -outfile jpegfile imagefile |
| or |
| djpeg [switches] -outfile imagefile jpegfile |
| This syntax works on all systems, so it is useful for scripts. |
| |
| The currently supported image file formats are: PPM (PBMPLUS color format), |
| PGM (PBMPLUS grayscale format), BMP, GIF, and Targa. cjpeg recognizes the |
| input image format automatically, with the exception of some Targa files. You |
| have to tell djpeg which format to generate. |
| |
| JPEG files are in the defacto standard JFIF file format. There are other, |
| less widely used JPEG-based file formats, but we don't support them. |
| |
| All switch names may be abbreviated; for example, -grayscale may be written |
| -gray or -gr. Most of the "basic" switches can be abbreviated to as little as |
| one letter. Upper and lower case are equivalent (-BMP is the same as -bmp). |
| British spellings are also accepted (e.g., -greyscale), though for brevity |
| these are not mentioned below. |
| |
| |
| CJPEG DETAILS |
| |
| The basic command line switches for cjpeg 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.) |
| |
| -grayscale 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. |
| |
| -rgb Create RGB JPEG file. |
| Using this switch suppresses the conversion from RGB |
| colorspace input to the default YCbCr JPEG colorspace. |
| |
| -optimize 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. |
| |
| -progressive Create progressive JPEG file (see below). |
| |
| -targa 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: |
| |
| -arithmetic 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. |
| |
| -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. |
| |
| -verbose Enable debug printout. More -v's give more printout. |
| or -debug Also, version information is printed at startup. |
| |
| 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: |
| |
| -baseline 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. |
| |
| |
| DJPEG DETAILS |
| |
| The basic command line switches for djpeg are: |
| |
| -colors N Reduce image to at most N colors. This reduces the |
| or -quantize N number of colors used in the output image, so that it |
| can be displayed on a colormapped display or stored in |
| a colormapped file format. For example, if you have |
| an 8-bit display, you'd need to reduce to 256 or fewer |
| colors. (-colors is the recommended name, -quantize |
| is provided only for backwards compatibility.) |
| |
| -fast Select recommended processing options for fast, low |
| quality output. (The default options are chosen for |
| highest quality output.) Currently, this is equivalent |
| to "-dct fast -nosmooth -onepass -dither ordered". |
| |
| -grayscale Force grayscale output even if JPEG file is color. |
| Useful for viewing on monochrome displays; also, |
| djpeg runs noticeably faster in this mode. |
| |
| -rgb Force RGB output even if JPEG file is grayscale. |
| |
| -scale M/N Scale the output image by a factor M/N. Currently |
| the scale factor must be M/8, where M is an integer |
| between 1 and 16 inclusive, or any reduced fraction |
| thereof (such as 1/2, 3/4, etc. Scaling is handy if |
| the image is larger than your screen; also, djpeg runs |
| much faster when scaling down the output. |
| |
| -bmp Select BMP output format (Windows flavor). 8-bit |
| colormapped format is emitted if -colors or -grayscale |
| is specified, or if the JPEG file is grayscale; |
| otherwise, 24-bit full-color format is emitted. |
| |
| -gif Select GIF output format (LZW-compressed). Since GIF |
| does not support more than 256 colors, -colors 256 is |
| assumed (unless you specify a smaller number of |
| colors). If you specify -fast, the default number of |
| colors is 216. |
| |
| -gif0 Select GIF output format (uncompressed). Since GIF |
| does not support more than 256 colors, -colors 256 is |
| assumed (unless you specify a smaller number of |
| colors). If you specify -fast, the default number of |
| colors is 216. |
| |
| -os2 Select BMP output format (OS/2 1.x flavor). 8-bit |
| colormapped format is emitted if -colors or -grayscale |
| is specified, or if the JPEG file is grayscale; |
| otherwise, 24-bit full-color format is emitted. |
| |
| -pnm Select PBMPLUS (PPM/PGM) output format (this is the |
| default format). PGM is emitted if the JPEG file is |
| grayscale or if -grayscale is specified; otherwise |
| PPM is emitted. |
| |
| -targa Select Targa output format. Grayscale format is |
| emitted if the JPEG file is grayscale or if |
| -grayscale is specified; otherwise, colormapped format |
| is emitted if -colors is specified; otherwise, 24-bit |
| full-color format is emitted. |
| |
| Switches for advanced users: |
| |
| -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 decompression 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 decompression |
| 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. |
| |
| If the JPEG image was compressed using a quality level |
| of 85 or below, then there should be little or no |
| perceptible quality difference between the two |
| algorithms. When decompressing images that were |
| compressed using quality levels above 85, however, the |
| difference between the fast and int methods becomes |
| more pronounced. With images compressed using |
| quality=97, for instance, the fast method incurs |
| generally about a 4-6 dB loss in PSNR relative to the |
| int method, but this can be larger for some images. If |
| you can avoid it, do not use the fast method when |
| decompressing images that were compressed using quality |
| levels above 97. The algorithm often degenerates for |
| such images and can actually produce a more lossy |
| output image than if the JPEG image had been compressed |
| using lower quality levels. |
| -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. |
| |
| -dither fs Use Floyd-Steinberg dithering in color quantization. |
| -dither ordered Use ordered dithering in color quantization. |
| -dither none Do not use dithering in color quantization. |
| By default, Floyd-Steinberg dithering is applied when |
| quantizing colors; this is slow but usually produces |
| the best results. Ordered dither is a compromise |
| between speed and quality; no dithering is fast but |
| usually looks awful. Note that these switches have |
| no effect unless color quantization is being done. |
| Ordered dither is only available in -onepass mode. |
| |
| -map FILE Quantize to the colors used in the specified image |
| file. This is useful for producing multiple files |
| with identical color maps, or for forcing a predefined |
| set of colors to be used. The FILE must be a GIF |
| or PPM file. This option overrides -colors and |
| -onepass. |
| |
| -nosmooth Use a faster, lower-quality upsampling routine. |
| |
| -onepass Use one-pass instead of two-pass color quantization. |
| The one-pass method is faster and needs less memory, |
| but it produces a lower-quality image. -onepass is |
| ignored unless you also say -colors N. Also, |
| the one-pass method is always used for grayscale |
| output (the two-pass method is no improvement then). |
| |
| -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. |
| |
| -verbose Enable debug printout. More -v's give more printout. |
| or -debug Also, version information is printed at startup. |
| |
| |
| HINTS FOR CJPEG |
| |
| Color GIF files are not the ideal input for JPEG; JPEG is really intended for |
| compressing full-color (24-bit) images. In particular, don't try to convert |
| cartoons, line drawings, and other images that have only a few distinct |
| colors. GIF works great on these, JPEG does not. If you want to convert a |
| GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options |
| to get a satisfactory conversion. -smooth 10 or so is often helpful. |
| |
| 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.) |
| |
| |
| HINTS FOR DJPEG |
| |
| To get a quick preview of an image, use the -grayscale and/or -scale switches. |
| "-grayscale -scale 1/8" is the fastest case. |
| |
| Several options are available that trade off image quality to gain speed. |
| "-fast" turns on the recommended settings. |
| |
| "-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality. |
| When producing a color-quantized image, "-onepass -dither ordered" is fast but |
| much lower quality than the default behavior. "-dither none" may give |
| acceptable results in two-pass mode, but is seldom tolerable in one-pass mode. |
| |
| |
| HINTS FOR BOTH PROGRAMS |
| |
| If the memory needed by cjpeg or djpeg exceeds the limit specified by |
| -maxmemory, an error will occur. You can leave out -progressive and -optimize |
| (for cjpeg) or specify -onepass (for djpeg) to reduce memory usage. |
| |
| On machines that have "environment" variables, you can define the environment |
| variable JPEGMEM to set 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 switch. |
| |
| |
| JPEGTRAN |
| |
| jpegtran performs various useful transformations of JPEG files. |
| It can translate the coded representation from one variant of JPEG to another, |
| for example from baseline JPEG to progressive JPEG or vice versa. It can also |
| perform some rearrangements of the image data, for example turning an image |
| from landscape to portrait format by rotation. For EXIF files and JPEG files |
| containing Exif data, you may prefer to use exiftran instead. |
| |
| jpegtran works by rearranging the compressed data (DCT coefficients), without |
| ever fully decoding the image. Therefore, its transformations are lossless: |
| there is no image degradation at all, which would not be true if you used |
| djpeg followed by cjpeg to accomplish the same conversion. But by the same |
| token, jpegtran cannot perform lossy operations such as changing the image |
| quality. However, while the image data is losslessly transformed, metadata |
| can be removed. See the -copy option for specifics. |
| |
| jpegtran uses a command line syntax similar to cjpeg or djpeg. |
| On Unix-like systems, you say: |
| jpegtran [switches] [inputfile] >outputfile |
| On most non-Unix systems, you say: |
| jpegtran [switches] inputfile outputfile |
| where both the input and output files are JPEG files. |
| |
| To specify the coded JPEG representation used in the output file, |
| jpegtran accepts a subset of the switches recognized by cjpeg: |
| -optimize Perform optimization of entropy encoding parameters. |
| -progressive Create progressive JPEG file. |
| -arithmetic Use arithmetic coding. |
| -restart N Emit a JPEG restart marker every N MCU rows, or every |
| N MCU blocks if "B" is attached to the number. |
| -scans file Use the scan script given in the specified text file. |
| See the previous discussion of cjpeg for more details about these switches. |
| If you specify none of these switches, you get a plain baseline-JPEG output |
| file. The quality setting and so forth are determined by the input file. |
| |
| The image can be losslessly transformed by giving one of these switches: |
| -flip horizontal Mirror image horizontally (left-right). |
| -flip vertical Mirror image vertically (top-bottom). |
| -rotate 90 Rotate image 90 degrees clockwise. |
| -rotate 180 Rotate image 180 degrees. |
| -rotate 270 Rotate image 270 degrees clockwise (or 90 ccw). |
| -transpose Transpose image (across UL-to-LR axis). |
| -transverse Transverse transpose (across UR-to-LL axis). |
| |
| The transpose transformation has no restrictions regarding image dimensions. |
| The other transformations operate rather oddly if the image dimensions are not |
| a multiple of the iMCU size (usually 8 or 16 pixels), because they can only |
| transform complete blocks of DCT coefficient data in the desired way. |
| |
| jpegtran's default behavior when transforming an odd-size image is designed |
| to preserve exact reversibility and mathematical consistency of the |
| transformation set. As stated, transpose is able to flip the entire image |
| area. Horizontal mirroring leaves any partial iMCU column at the right edge |
| untouched, but is able to flip all rows of the image. Similarly, vertical |
| mirroring leaves any partial iMCU row at the bottom edge untouched, but is |
| able to flip all columns. The other transforms can be built up as sequences |
| of transpose and flip operations; for consistency, their actions on edge |
| pixels are defined to be the same as the end result of the corresponding |
| transpose-and-flip sequence. |
| |
| For practical use, you may prefer to discard any untransformable edge pixels |
| rather than having a strange-looking strip along the right and/or bottom edges |
| of a transformed image. To do this, add the -trim switch: |
| -trim Drop non-transformable edge blocks. |
| Obviously, a transformation with -trim is not reversible, so strictly speaking |
| jpegtran with this switch is not lossless. Also, the expected mathematical |
| equivalences between the transformations no longer hold. For example, |
| "-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by |
| "-rot 180 -trim" trims both edges. |
| |
| If you are only interested in perfect transformations, add the -perfect switch: |
| -perfect Fail with an error if the transformation is not |
| perfect. |
| For example, you may want to do |
| jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg |
| to do a perfect rotation, if available, or an approximated one if not. |
| |
| This version of jpegtran also offers a lossless crop option, which discards |
| data outside of a given image region but losslessly preserves what is inside. |
| Like the rotate and flip transforms, lossless crop is restricted by the current |
| JPEG format; the upper left corner of the selected region must fall on an iMCU |
| boundary. If it doesn't, then it is silently moved up and/or left to the |
| nearest iMCU boundary (the lower right corner is unchanged.) Thus, the output |
| image covers at least the requested region, but it may cover more. The |
| adjustment of the region dimensions may be optionally disabled by attaching an |
| 'f' character ("force") to the width or height number. |
| |
| The image can be losslessly cropped by giving the switch: |
| -crop WxH+X+Y Crop to a rectangular region of width W and height H, |
| starting at point X,Y. |
| |
| If W or H is larger than the width/height of the input image, then the output |
| image is expanded in size, and the expanded region is filled in with zeros |
| (neutral gray). Attaching an 'f' character ("flatten") to the width number |
| will cause each block in the expanded region to be filled in with the DC |
| coefficient of the nearest block in the input image rather than grayed out. |
| Attaching an 'r' character ("reflect") to the width number will cause the |
| expanded region to be filled in with repeated reflections of the input image |
| rather than grayed out. |
| |
| A complementary lossless wipe option is provided to discard (gray out) data |
| inside a given image region while losslessly preserving what is outside: |
| -wipe WxH+X+Y Wipe (gray out) a rectangular region of width W and |
| height H from the input image, starting at point X,Y. |
| |
| Attaching an 'f' character ("flatten") to the width number will cause the |
| region to be filled with the average of adjacent blocks rather than grayed out. |
| If the wipe region and the region outside the wipe region, when adjusted to the |
| nearest iMCU boundary, form two horizontally adjacent rectangles, then |
| attaching an 'r' character ("reflect") to the width number will cause the wipe |
| region to be filled with repeated reflections of the outside region rather than |
| grayed out. |
| |
| A lossless drop option is also provided, which allows another JPEG image to be |
| inserted ("dropped") into the input image data at a given position, replacing |
| the existing image data at that position: |
| -drop +X+Y filename Drop (insert) another image at point X,Y |
| |
| Both the input image and the drop image must have the same subsampling level. |
| It is best if they also have the same quantization (quality.) Otherwise, the |
| quantization of the output image will be adapted to accommodate the higher of |
| the input image quality and the drop image quality. The trim option can be |
| used with the drop option to requantize the drop image to match the input |
| image. Note that a grayscale image can be dropped into a full-color image or |
| vice versa, as long as the full-color image has no vertical subsampling. If |
| the input image is grayscale and the drop image is full-color, then the |
| chrominance channels from the drop image will be discarded. |
| |
| Other not-strictly-lossless transformation switches are: |
| |
| -grayscale Force grayscale output. |
| This option discards the chrominance channels if the input image is YCbCr |
| (ie, a standard color JPEG), resulting in a grayscale JPEG file. The |
| luminance channel is preserved exactly, so this is a better method of reducing |
| to grayscale than decompression, conversion, and recompression. This switch |
| is particularly handy for fixing a monochrome picture that was mistakenly |
| encoded as a color JPEG. (In such a case, the space savings from getting rid |
| of the near-empty chroma channels won't be large; but the decoding time for |
| a grayscale JPEG is substantially less than that for a color JPEG.) |
| |
| jpegtran also recognizes these switches that control what to do with "extra" |
| markers, such as comment blocks: |
| -copy none Copy no extra markers from source file. This setting |
| suppresses all comments and other metadata in the |
| source file. |
| -copy comments Copy only comment markers. This setting copies |
| comments from the source file but discards any other |
| metadata. |
| -copy all Copy all extra markers. This setting preserves |
| miscellaneous markers found in the source file, such |
| as JFIF thumbnails, Exif data, and Photoshop settings. |
| In some files, these extra markers can be sizable. |
| Note that this option will copy thumbnails as-is; |
| they will not be transformed. |
| The default behavior is -copy comments. (Note: in IJG releases v6 and v6a, |
| jpegtran always did the equivalent of -copy none.) |
| |
| Additional switches recognized by jpegtran are: |
| -outfile filename |
| -maxmemory N |
| -verbose |
| -debug |
| These work the same as in cjpeg or djpeg. |
| |
| |
| THE COMMENT UTILITIES |
| |
| The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file. |
| Although the standard doesn't actually define what COM blocks are for, they |
| are widely used to hold user-supplied text strings. This lets you add |
| annotations, titles, index terms, etc to your JPEG files, and later retrieve |
| them as text. COM blocks do not interfere with the image stored in the JPEG |
| file. The maximum size of a COM block is 64K, but you can have as many of |
| them as you like in one JPEG file. |
| |
| We provide two utility programs to display COM block contents and add COM |
| blocks to a JPEG file. |
| |
| rdjpgcom searches a JPEG file and prints the contents of any COM blocks on |
| standard output. The command line syntax is |
| rdjpgcom [-raw] [-verbose] [inputfilename] |
| The switch "-raw" (or just "-r") causes rdjpgcom to output non-printable |
| characters in JPEG comments. These characters are normally escaped for |
| security reasons. |
| The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG |
| image dimensions. If you omit the input file name from the command line, |
| the JPEG file is read from standard input. (This may not work on some |
| operating systems, if binary data can't be read from stdin.) |
| |
| wrjpgcom adds a COM block, containing text you provide, to a JPEG file. |
| Ordinarily, the COM block is added after any existing COM blocks, but you |
| can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG |
| file; it does not modify the input file. DO NOT try to overwrite the input |
| file by directing wrjpgcom's output back into it; on most systems this will |
| just destroy your file. |
| |
| The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like |
| systems, it is |
| wrjpgcom [switches] [inputfilename] |
| The output file is written to standard output. The input file comes from |
| the named file, or from standard input if no input file is named. |
| |
| On most non-Unix systems, the syntax is |
| wrjpgcom [switches] inputfilename outputfilename |
| where both input and output file names must be given explicitly. |
| |
| wrjpgcom understands three switches: |
| -replace Delete any existing COM blocks from the file. |
| -comment "Comment text" Supply new COM text on command line. |
| -cfile name Read text for new COM block from named file. |
| (Switch names can be abbreviated.) If you have only one line of comment text |
| to add, you can provide it on the command line with -comment. The comment |
| text must be surrounded with quotes so that it is treated as a single |
| argument. Longer comments can be read from a text file. |
| |
| If you give neither -comment nor -cfile, then wrjpgcom will read the comment |
| text from standard input. (In this case an input image file name MUST be |
| supplied, so that the source JPEG file comes from somewhere else.) You can |
| enter multiple lines, up to 64KB worth. Type an end-of-file indicator |
| (usually control-D or control-Z) to terminate the comment text entry. |
| |
| wrjpgcom will not add a COM block if the provided comment string is empty. |
| Therefore -replace -comment "" can be used to delete all COM blocks from a |
| file. |
| |
| These utility programs do not depend on the IJG JPEG library. In |
| particular, the source code for rdjpgcom is intended as an illustration of |
| the minimum amount of code required to parse a JPEG file header correctly. |
