view release on metacpan or  search on metacpan

src/Source/LibJPEG/usage.txt  view on Meta::CPAN

	-scale M/N	Scale the output image by a factor M/N.  Currently
			supported scale factors are M/N with all M from 1 to
			16, where N is the source DCT size, which is 8 for
			baseline JPEG.  If the /N part is omitted, then M
			specifies the DCT scaled size to be applied on the
			given input.  For baseline JPEG this is equivalent to
			M/8 scaling, since the source DCT size for baseline
			JPEG is 8.  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 gray-scale;
			otherwise, 24-bit full-color format is emitted.

	-gif		Select GIF output format.  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 gray-scale;
			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
			gray-scale or if -grayscale is specified; otherwise
			PPM is emitted.

	-rle		Select RLE output format.  (Requires URT library.)

	-targa		Select Targa output format.  Gray-scale format is
			emitted if the JPEG file is gray-scale 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 integer DCT method (default).
	-dct fast	Use fast integer DCT (less accurate).
	-dct float	Use floating-point DCT method.
			The float method is very slightly more accurate than
			the int method, but is much slower unless your machine
			has very fast floating-point hardware.  Also note that
			results of the floating-point method may vary slightly
			across machines, while the integer methods should give
			the same results everywhere.  The fast integer method
			is much less accurate than the other two.

	-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	Don't use high-quality upsampling.

	-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 gray-scale
			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, temporary files will be used.

	-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.)

GIF input files are no longer supported, to avoid the Unisys LZW patent
(now expired).
(Conversion of GIF files to JPEG is usually a bad idea anyway.)


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.