1 .TH CJPEG 1 "30 December 2009"

     2 .SH NAME

     3 cjpeg \- compress an image file to a JPEG file

     4 .SH SYNOPSIS

     5 .B cjpeg

     6 [

     7 .I options

     8 ]

     9 [

    10 .I filename

    11 ]

    12 .LP

    13 .SH DESCRIPTION

    14 .LP

    15 .B cjpeg

    16 compresses the named image file, or the standard input if no file is

    17 named, and produces a JPEG/JFIF file on the standard output.

    18 The currently supported input file formats are: PPM (PBMPLUS color

    19 format), PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster

    20 Toolkit format).  (RLE is supported only if the URT library is available.)

    21 .SH OPTIONS

    22 All switch names may be abbreviated; for example,

    23 .B \-grayscale

    24 may be written

    25 .B \-gray

    26 or

    27 .BR \-gr .

    28 Most of the "basic" switches can be abbreviated to as little as one letter.

    29 Upper and lower case are equivalent (thus

    30 .B \-BMP

    31 is the same as

    32 .BR \-bmp ).

    33 British spellings are also accepted (e.g.,

    34 .BR \-greyscale ),

    35 though for brevity these are not mentioned below.

    36 .PP

    37 The basic switches are:

    38 .TP

    39 .BI \-quality " N[,...]"

    40 Scale quantization tables to adjust image quality.  Quality is 0 (worst) to

    41 100 (best); default is 75.  (See below for more info.)

    42 .TP

    43 .B \-grayscale

    44 Create monochrome JPEG file from color input.  Be sure to use this switch when

    45 compressing a grayscale BMP file, because

    46 .B cjpeg

    47 isn't bright enough to notice whether a BMP file uses only shades of gray.

    48 By saying

    49 .BR \-grayscale ,

    50 you'll get a smaller JPEG file that takes less time to process.

    51 .TP

    52 .B \-optimize

    53 Perform optimization of entropy encoding parameters.  Without this, default

    54 encoding parameters are used.

    55 .B \-optimize

    56 usually makes the JPEG file a little smaller, but

    57 .B cjpeg

    58 runs somewhat slower and needs much more memory.  Image quality and speed of

    59 decompression are unaffected by

    60 .BR \-optimize .

    61 .TP

    62 .B \-progressive

    63 Create progressive JPEG file (see below).

    64 .TP

    65 .BI \-scale " M/N"

    66 Scale the output image by a factor M/N.  Currently supported scale factors are

    67 8/N with all N from 1 to 16.

    68 .TP

    69 .B \-targa

    70 Input file is Targa format.  Targa files that contain an "identification"

    71 field will not be automatically recognized by

    72 .BR cjpeg ;

    73 for such files you must specify

    74 .B \-targa

    75 to make

    76 .B cjpeg

    77 treat the input as Targa format.

    78 For most Targa files, you won't need this switch.

    79 .PP

    80 The

    81 .B \-quality

    82 switch lets you trade off compressed file size against quality of the

    83 reconstructed image: the higher the quality setting, the larger the JPEG file,

    84 and the closer the output image will be to the original input.  Normally you

    85 want to use the lowest quality setting (smallest file) that decompresses into

    86 something visually indistinguishable from the original image.  For this

    87 purpose the quality setting should be between 50 and 95; the default of 75 is

    88 often about right.  If you see defects at

    89 .B \-quality

    90 75, then go up 5 or 10 counts at a time until you are happy with the output

    91 image.  (The optimal setting will vary from one image to another.)

    92 .PP

    93 .B \-quality

    94 100 will generate a quantization table of all 1's, minimizing loss in the

    95 quantization step (but there is still information loss in subsampling, as well

    96 as roundoff error).  This setting is mainly of interest for experimental

    97 purposes.  Quality values above about 95 are

    98 .B not

    99 recommended for normal use; the compressed file size goes up dramatically for

   100 hardly any gain in output image quality.

   101 .PP

   102 In the other direction, quality values below 50 will produce very small files

   103 of low image quality.  Settings around 5 to 10 might be useful in preparing an

   104 index of a large image library, for example.  Try

   105 .B \-quality

   106 2 (or so) for some amusing Cubist effects.  (Note: quality

   107 values below about 25 generate 2-byte quantization tables, which are

   108 considered optional in the JPEG standard.

   109 .B cjpeg

   110 emits a warning message when you give such a quality value, because some

   111 other JPEG programs may be unable to decode the resulting file.  Use

   112 .B \-baseline

   113 if you need to ensure compatibility at low quality values.)

   114 .PP

   115 The

   116 .B \-quality

   117 option has been extended in IJG version 7 for support of separate quality

   118 settings for luminance and chrominance (or in general, for every provided

   119 quantization table slot).  This feature is useful for high-quality

   120 applications which cannot accept the damage of color data by coarse

   121 subsampling settings.  You can now easily reduce the color data amount more

   122 smoothly with finer control without separate subsampling.  The resulting file

   123 is fully compliant with standard JPEG decoders.

   124 Note that the

   125 .B \-quality

   126 ratings refer to the quantization table slots, and that the last value is

   127 replicated if there are more q-table slots than parameters.  The default

   128 q-table slots are 0 for luminance and 1 for chrominance with default tables as

   129 given in the JPEG standard.  This is compatible with the old behaviour in case

   130 that only one parameter is given, which is then used for both luminance and

   131 chrominance (slots 0 and 1).  More or custom quantization tables can be set

   132 with

   133 .B \-qtables

   134 and assigned to components with

   135 .B \-qslots

   136 parameter (see the "wizard" switches below).

   137 .B Caution:

   138 You must explicitly add

   139 .BI \-sample " 1x1"

   140 for efficient separate color

   141 quality selection, since the default value used by library is 2x2!

   142 .PP

   143 The

   144 .B \-progressive

   145 switch creates a "progressive JPEG" file.  In this type of JPEG file, the data

   146 is stored in multiple scans of increasing quality.  If the file is being

   147 transmitted over a slow communications link, the decoder can use the first

   148 scan to display a low-quality image very quickly, and can then improve the

   149 display with each subsequent scan.  The final image is exactly equivalent to a

   150 standard JPEG file of the same quality setting, and the total file size is

   151 about the same --- often a little smaller.

   152 .PP

   153 Switches for advanced users:

   154 .TP

   155 .B \-dct int

   156 Use integer DCT method (default).

   157 .TP

   158 .B \-dct fast

   159 Use fast integer DCT (less accurate).

   160 .TP

   161 .B \-dct float

   162 Use floating-point DCT method.

   163 The float method is very slightly more accurate than the int method, but is

   164 much slower unless your machine has very fast floating-point hardware.  Also

   165 note that results of the floating-point method may vary slightly across

   166 machines, while the integer methods should give the same results everywhere.

   167 The fast integer method is much less accurate than the other two.

   168 .TP

   169 .B \-nosmooth

   170 Don't use high-quality downsampling.

   171 .TP

   172 .BI \-restart " N"

   173 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is

   174 attached to the number.

   175 .B \-restart 0

   176 (the default) means no restart markers.

   177 .TP

   178 .BI \-smooth " N"

   179 Smooth the input image to eliminate dithering noise.  N, ranging from 1 to

   180 100, indicates the strength of smoothing.  0 (the default) means no smoothing.

   181 .TP

   182 .BI \-maxmemory " N"

   183 Set limit for amount of memory to use in processing large images.  Value is

   184 in thousands of bytes, or millions of bytes if "M" is attached to the

   185 number.  For example,

   186 .B \-max 4m

   187 selects 4000000 bytes.  If more space is needed, temporary files will be used.

   188 .TP

   189 .BI \-outfile " name"

   190 Send output image to the named file, not to standard output.

   191 .TP

   192 .B \-verbose

   193 Enable debug printout.  More

   194 .BR \-v 's

   195 give more output.  Also, version information is printed at startup.

   196 .TP

   197 .B \-debug

   198 Same as

   199 .BR \-verbose .

   200 .PP

   201 The

   202 .B \-restart

   203 option inserts extra markers that allow a JPEG decoder to resynchronize after

   204 a transmission error.  Without restart markers, any damage to a compressed

   205 file will usually ruin the image from the point of the error to the end of the

   206 image; with restart markers, the damage is usually confined to the portion of

   207 the image up to the next restart marker.  Of course, the restart markers

   208 occupy extra space.  We recommend

   209 .B \-restart 1

   210 for images that will be transmitted across unreliable networks such as Usenet.

   211 .PP

   212 The

   213 .B \-smooth

   214 option filters the input to eliminate fine-scale noise.  This is often useful

   215 when converting dithered images to JPEG: a moderate smoothing factor of 10 to

   216 50 gets rid of dithering patterns in the input file, resulting in a smaller

   217 JPEG file and a better-looking image.  Too large a smoothing factor will

   218 visibly blur the image, however.

   219 .PP

   220 Switches for wizards:

   221 .TP

   222 .B \-arithmetic

   223 Use arithmetic coding.

   224 .B Caution:

   225 arithmetic coded JPEG is not yet widely implemented, so many decoders will be

   226 unable to view an arithmetic coded JPEG file at all.

   227 .TP

   228 .B \-baseline

   229 Force baseline-compatible quantization tables to be generated.  This clamps

   230 quantization values to 8 bits even at low quality settings.  (This switch is

   231 poorly named, since it does not ensure that the output is actually baseline

   232 JPEG.  For example, you can use

   233 .B \-baseline

   234 and

   235 .B \-progressive

   236 together.)

   237 .TP

   238 .BI \-qtables " file"

   239 Use the quantization tables given in the specified text file.

   240 .TP

   241 .BI \-qslots " N[,...]"

   242 Select which quantization table to use for each color component.

   243 .TP

   244 .BI \-sample " HxV[,...]"

   245 Set JPEG sampling factors for each color component.

   246 .TP

   247 .BI \-scans " file"

   248 Use the scan script given in the specified text file.

   249 .PP

   250 The "wizard" switches are intended for experimentation with JPEG.  If you

   251 don't know what you are doing, \fBdon't use them\fR.  These switches are

   252 documented further in the file wizard.txt.

   253 .SH EXAMPLES

   254 .LP

   255 This example compresses the PPM file foo.ppm with a quality factor of

   256 60 and saves the output as foo.jpg:

   257 .IP

   258 .B cjpeg \-quality

   259 .I 60 foo.ppm

   260 .B >

   261 .I foo.jpg

   262 .SH HINTS

   263 Color GIF files are not the ideal input for JPEG; JPEG is really intended for

   264 compressing full-color (24-bit) images.  In particular, don't try to convert

   265 cartoons, line drawings, and other images that have only a few distinct

   266 colors.  GIF works great on these, JPEG does not.  If you want to convert a

   267 GIF to JPEG, you should experiment with

   268 .BR cjpeg 's

   269 .B \-quality

   270 and

   271 .B \-smooth

   272 options to get a satisfactory conversion.

   273 .B \-smooth 10

   274 or so is often helpful.

   275 .PP

   276 Avoid running an image through a series of JPEG compression/decompression

   277 cycles.  Image quality loss will accumulate; after ten or so cycles the image

   278 may be noticeably worse than it was after one cycle.  It's best to use a

   279 lossless format while manipulating an image, then convert to JPEG format when

   280 you are ready to file the image away.

   281 .PP

   282 The

   283 .B \-optimize

   284 option to

   285 .B cjpeg

   286 is worth using when you are making a "final" version for posting or archiving.

   287 It's also a win when you are using low quality settings to make very small

   288 JPEG files; the percentage improvement is often a lot more than it is on

   289 larger files.  (At present,

   290 .B \-optimize

   291 mode is always selected when generating progressive JPEG files.)

   292 .SH ENVIRONMENT

   293 .TP

   294 .B JPEGMEM

   295 If this environment variable is set, its value is the default memory limit.

   296 The value is specified as described for the

   297 .B \-maxmemory

   298 switch.

   299 .B JPEGMEM

   300 overrides the default value specified when the program was compiled, and

   301 itself is overridden by an explicit

   302 .BR \-maxmemory .

   303 .SH SEE ALSO

   304 .BR djpeg (1),

   305 .BR jpegtran (1),

   306 .BR rdjpgcom (1),

   307 .BR wrjpgcom (1)

   308 .br

   309 .BR ppm (5),

   310 .BR pgm (5)

   311 .br

   312 Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",

   313 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.

   314 .SH AUTHOR

   315 Independent JPEG Group

   316 .SH BUGS

   317 GIF input files are no longer supported, to avoid the Unisys LZW patent.

   318 (Conversion of GIF files to JPEG is usually a bad idea anyway.)

   319 .PP

   320 Not all variants of BMP and Targa file formats are supported.

   321 .PP

   322 The

   323 .B \-targa

   324 switch is not a bug, it's a feature.  (It would be a bug if the Targa format

   325 designers had not been clueless.)