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16 JPEG (pronounced "jay-peg") is a standardized compression method for
29 The programs read the specified input file, or standard input if none is
39 style is a little more foolproof, and it loses no functionality if you don't
47 This syntax works on all systems, so it is useful for scripts.
51 format). (RLE is supported only if the URT library is available, which it
61 one letter. Upper and lower case are equivalent (-BMP is the same as -bmp).
71 Quality is 0 (worst) to 100 (best); default is 75.
99 16, where M is the destination DCT size, which is 8 by
102 -targa Input file is Targa format. Targa files that contain
113 purpose the quality setting should be between 50 and 95; the default of 75 is
119 in the quantization step (but there is still information loss in subsampling,
120 as well as roundoff error). This setting is mainly of interest for
136 provided quantization table slot). This feature is useful for high-quality
140 is fully compliant with standard JPEG decoders.
142 the last value is replicated if there are more q-table slots than parameters.
144 default tables as given in the JPEG standard. This is compatible with the old
145 behaviour in case that only one parameter is given, which is then used for
150 quality selection, since the default value used by library is 2x2!
153 JPEG file, the data is stored in multiple scans of increasing quality. If the
154 file is being transmitted over a slow communications link, the decoder can use
156 improve the display with each subsequent scan. The final image is exactly
158 file size is about the same --- often a little smaller.
163 CAUTION: arithmetic coded JPEG is not yet widely
168 Default is 8 (baseline format).
173 the DCT+Quantization stage is lossless for N=1).
175 extension is required for this feature. SmartScale
176 enabled JPEG is not yet widely implemented, so many
187 support is required for this feature. Reversible
188 color transform support is not yet widely implemented,
204 gamut YCC support (JFIF version 2) is required.
214 The float method is very slightly more accurate than
215 the int method, but is much slower unless your machine
220 is much less accurate than the other two.
225 N MCU blocks if "B" is attached to the number.
233 large images. Value is in thousands of bytes, or
234 millions of bytes if "M" is attached to the number.
236 space is needed, temporary files will be used.
239 or -debug Also, version information is printed at startup.
244 to the end of the image; with restart markers, the damage is usually confined
249 The -smooth option filters the input to eliminate fine-scale noise. This is
259 even at low quality settings. (This switch is poorly
260 named, since it does not ensure that the output is
288 colors. (-colors is the recommended name, -quantize
289 is provided only for backwards compatibility.)
293 highest quality output.) Currently, this is equivalent
296 -grayscale Force grayscale output even if JPEG file is color.
300 -rgb Force RGB output even if JPEG file is grayscale.
301 This is provided to support applications that don't
306 16, where N is the source DCT size, which is 8 for
307 baseline JPEG. If the /N part is omitted, then M
309 given input. For baseline JPEG this is equivalent to
311 JPEG is 8. Scaling is handy if the image is larger
316 colormapped format is emitted if -colors or -grayscale
317 is specified, or if the JPEG file is grayscale;
318 otherwise, 24-bit full-color format is emitted.
321 more than 256 colors, -colors 256 is assumed (unless
323 specify -fast, the default number of colors is 216.
326 colormapped format is emitted if -colors or -grayscale
327 is specified, or if the JPEG file is grayscale;
328 otherwise, 24-bit full-color format is emitted.
330 -pnm Select PBMPLUS (PPM/PGM) output format (this is the
331 default format). PGM is emitted if the JPEG file is
332 grayscale or if -grayscale is specified; otherwise
333 PPM is emitted.
337 -targa Select Targa output format. Grayscale format is
338 emitted if the JPEG file is grayscale or if
339 -grayscale is specified; otherwise, colormapped format
340 is emitted if -colors is specified; otherwise, 24-bit
341 full-color format is emitted.
348 The float method is very slightly more accurate than
349 the int method, but is much slower unless your machine
354 is much less accurate than the other two.
359 By default, Floyd-Steinberg dithering is applied when
360 quantizing colors; this is slow but usually produces
361 the best results. Ordered dither is a compromise
362 between speed and quality; no dithering is fast but
364 no effect unless color quantization is being done.
365 Ordered dither is only available in -onepass mode.
368 file. This is useful for producing multiple files
377 The one-pass method is faster and needs less memory,
378 but it produces a lower-quality image. -onepass is
380 the one-pass method is always used for grayscale
381 output (the two-pass method is no improvement then).
384 large images. Value is in thousands of bytes, or
385 millions of bytes if "M" is attached to the number.
387 space is needed, temporary files will be used.
390 or -debug Also, version information is printed at startup.
395 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
400 to get a satisfactory conversion. -smooth 10 or so is often helpful.
408 The -optimize option to cjpeg is worth using when you are making a "final"
411 is often a lot more than it is on larger files. (At present, -optimize
412 mode is always selected when generating progressive JPEG files.)
416 (Conversion of GIF files to JPEG is usually a bad idea anyway.)
422 "-grayscale -scale 1/8" is the fastest case.
428 When producing a color-quantized image, "-onepass -dither ordered" is fast but
430 acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
434 "-dct float" is slower than "-dct int"; in this case it is not worth using,
435 because its theoretical accuracy advantage is too small to be significant
450 If more space is needed than will fit in the available main memory (as
464 The default memory usage limit (-maxmemory) is set when the software is
470 variable JPEGMEM to set the default memory limit. The value is specified as
472 specified when the program was compiled, and itself is overridden by an
475 On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
476 use. (Extended or expanded memory is also used if available.) Most
492 there is no image degradation at all, which would not be true if you used
495 quality. However, while the image data is losslessly transformed, metadata
511 N MCU blocks if "B" is attached to the number.
531 jpegtran's default behavior when transforming an odd-size image is designed
533 transformation set. As stated, transpose is able to flip the entire image
535 untouched, but is able to flip all rows of the image. Similarly, vertical
536 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
546 Obviously, a transformation with -trim is not reversible, so strictly speaking
547 jpegtran with this switch is not lossless. Also, the expected mathematical
553 -perfect Fails with an error if the transformation is not
560 image region but losslessly preserves what is inside. Like the rotate and
561 flip transforms, lossless crop is restricted by the current JPEG format: the
574 A complementary lossless-wipe option is provided to discard (gray out) data
575 inside a given image region while losslessly preserving what is outside:
582 This option discards the chrominance channels if the input image is YCbCr
584 luminance channel is preserved exactly, so this is a better method of reducing
586 is particularly handy for fixing a monochrome picture that was mistakenly
589 a grayscale JPEG is substantially less than that for a color JPEG.)
592 Currently supported scale factors are M/N with all M from 1 to 16, where N is
593 the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted,
595 baseline JPEG this is equivalent to M/8 scaling, since the source DCT size
596 for baseline JPEG is 8. CAUTION: An implementation of the JPEG SmartScale
597 extension is required for this feature. SmartScale enabled JPEG is not yet
613 Note that this option will copy thumbnails as-is;
615 The default behavior is -copy comments. (Note: in IJG releases v6 and v6a,
633 file. The maximum size of a COM block is 64K, but you can have as many of
640 standard output. The command line syntax is
646 the JPEG file is read from standard input. (This may not work on some
650 Ordinarily, the COM block is added after any existing COM blocks, but you
656 The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like
657 systems, it is
659 The output file is written to standard output. The input file comes from
660 the named file, or from standard input if no input file is named.
662 On most non-Unix systems, the syntax is
672 text must be surrounded with quotes so that it is treated as a single
681 wrjpgcom will not add a COM block if the provided comment string is empty.
686 particular, the source code for rdjpgcom is intended as an illustration of