1C44(1) DjVuLibre-3.5 C44(1)
2
6 c44 - DjVuPhoto encode.
7
9 c44 [options] inputfilename [outputfilename]
10
12 Produces a DjVuPhoto encoded image. The input image file inputfilename
13 can be either a portable gray-map (PGM) or a portable pix-map (PPM).
14 Input images compressed with JPEG are also accepted. It is however sug‐
15 gested to only use high quality JPEG files (low compression ratio,
16 large size) because the wavelet compression will increase the defects
17 already present in highly compressed JPEG files.
18 The program produces a DjVuPhoto file outputfilename. If the output
20 file name is not specified, a default file name will be generated by
21 replacing the input file name suffix by suffix djvu.
22 The main design objective for the DjVu wavelets consisted of allowing
24 progressive rendering and smooth scrolling of large images with limited
25 memory requirements. Decoding functions process the compressed data
26 and update a memory efficient representation of the wavelet coeffi‐
27 cients. Imaging function then can quickly render an arbitrary segment
28 of the image using the available data. Both process can be carried out
29 in two threads of execution. This design plays an important role in
30 the DjVu system. We investigated various state-of-the-art wavelet com‐
31 pression schemes. Although these schemes may achieve slightly smaller
32 file sizes, the decoding functions did not even approach our require‐
33 ments. The IW44 wavelets reach these requirements today and may in the
34 future implement more modern refinements if these refinements can be
35 implemented within our constraints.
36
38 DjVuPhoto files are logically composed of a sequence of "slices" con‐
39 taining successive image refinements. Slices are grouped in "chunks"
40 defining the progressive rendering sequence. The viewer is able to
41 display an intermediate image after processing each chunk. A typical
42 DjVuPhoto files contains 80 to 120 slices grouped into 1 to 4 chunks.
43 The quality selection options provide various ways to specify the num‐
45 ber of chunks and the number of slices per chunk. The c44 program adds
46 slices to the current chunk until exceeding a target number of slices,
47 a target file size, or a target quality specification. The following
48 options define targets for each chunk. The option argument contain
49 several numerical values (one per chunk) separated by either commas or
50 pluses.
51 -slice n+...+n
53 Specify the number of slices in each chunk. The option argument
54 contains plus-separated numerical values (one per chunk) indi‐
55 cating the number of slices per chunk. Option -slice 74+13+10,
56 for instance, would be appropriate for compressing a photo‐
57 graphic image with three progressive refinements. More quality
58 and more refinements can be obtained with option -slice
59 72+11+10+10.
60 -slice n,...,n
62 Specify the cumulative number of slices for each chunk. Since
63 the final quality is determined by the total number of slices,
64 it is often more convenient to use comma-separated values (one
65 per chunk) indicating the cumulative number of slices for each
66 chunk (i.e. including those encoded in all previous chunks).
67 The values suggested above can also be expressed as -slice
68 74,87,97 and -slice 72,83,93,103.
69 -size n,...,n
71 Specify size targets for each chunk expressed in bytes. The
72 option argument can be either a plus-separated list specifying a
73 size for each chunk, or a comma separated list specifying cumu‐
74 lative sizes for each chunk and all previous chunks. Size tar‐
75 gets are approximates. Slices will be added to each chunk until
76 exceeding the specified target.
77 -bpp n,...,n
79 Specify size targets for each chunk expressed in bits-per-pixel.
80 Both comma-separated and plus-separated specifications are
81 accepted. Option -bpp 0.25,0.5,1 usually provides good results.
82 -percent n,...,n
84 Specify size targets for each chunk expressed as a percentage of
85 the input file size. Both comma-separated and plus-separated
86 specifications are accepted. Results can be drastically differ‐
87 ent according to the format of the input image (raw or JPEG com‐
88 pressed).
89 -decibel n,...,n
91 Specify quality targets for each chunk expressed as a comma-sep‐
92 arated list of increasing decibel values. Decibel values range
93 from 16 (very low quality) to 48 (very high quality). This cri‐
94 terion should not be relied upon when re-encoding an image pre‐
95 viously compressed by another compression scheme. Selecting
96 this option significantly increases the compression time.
97 -dbfrac frac
99 Indicate that the decibel values specified in option -decibel
100 should be computed by averaging the mean squared errors of only
101 the fraction frac of the most mis-represented blocks of 32 x 32
102 pixels. This option is useful with composite images containing
103 solid color features (e.g. an image with a large white border).
104 Providing no quality specification options automatically selects a
106 default quality specification -slice 74,89,99. Multiple quality speci‐
107 fication options are allowed. The program outputs a file whose total
108 number of chunks is the largest number of chunks of all quality speci‐
109 fications. Slices are added to each chunk until reaching any of the
110 quality target for this chunk.
111
113 The following additional options are supported:
114 -dpi n Specify the resolution information encoded into the output file
116 expressed in dots per inch. The resolution information encoded
117 in DjVu files determine how the decoder scales the image on a
118 particular display. Meaningful resolutions range from 25 to
119 1200. The default value, 100 dpi, should be suitable for most
120 photographic images.
121 -gamma n
123 Specify the gamma correction information encoded into the output
124 file. The argument n specified the gamma value of the device
125 for which the input image was designed. The default value is
126 2.2. This is appropriate for images designed for a standard
127 computer monitor.
128 -mask pbmfilename
130 The design of the IW44 wavelets allows for compressing partially
131 masked images. This option can be used when certain pixels of a
132 background image are going to be covered by foreground objects
133 like text or drawings. File pbmfile must be a PBM file whose
134 size matches the size of the input file. Each black pixel in
135 pbmfile means that the value of the corresponding pixel in the
136 input file is irrelevant. The IW44 encoder will replace the
137 masked pixels by a color value whose coding cost is minimal (see
138 http://www.djvuzone.org/djvu/techpapers/mask/index.djvu for
139 technical details.)
140 -crcbnormal
142 Select normal chrominance encoding. Chrominance information is
143 encoded at the same resolution as the luminance. This is the
144 default.
145 -crcbhalf
147 Selects half resolution chrominance encoding. Chrominance
148 information is encoded at half the luminance resolution.
149 -crcbdelay n
151 This option can be used with -crcbnormal and -crcbhalf to modify
152 the quality of the chrominance information. The option argu‐
153 ments specifies a parameter n, expressed in slices, that reduces
154 the bit-rate associated with the chrominance. The default
155 chrominance encoding delay is 10 slices.
156 -crcbfull
158 Select the highest possible quality for encoding the chrominance
159 information. This is equivalent to specifying -crcbnormal and
160 -crcbdelay 0.
161 -crcbnone
163 Disable the encoding of the chrominance. Only the luminance
164 information will be encoded. The resulting image will show in
165 shades of gray.
166
168 The default quality setting of the DjVuLibre version of c44 has been
169 increased. It produces larger files with a better quality. Quality
170 can be lowered using the quality selection options!
171
173 The encoder requires more memory than necessary.
174 The rechunking capability is currently broken.
175
177 This program was written by Léon Bottou <leonb@users.sourceforge.net>
178 and was then improved by Andrei Erofeev <andrew_erofeev@yahoo.com>,
179 Bill Riemers <docbill@sourceforge.net> and many others.
180
182 djvu(1), pnm(5), cjpeg(1).
183DjVuLibre-3.5 10/11/2001 C44(1)