c44 - DjVuPhoto encode.
c44 [options] inputfilename
Produces a DjVuPhoto encoded image. The input image file inputfilename
can be either a portable gray-map (PGM
) or a portable pix-map
). Input images compressed with JPEG
also accepted. It is however suggested to only use high quality
files (low compression ratio, large size) because the
wavelet compression will increase the defects already present in highly
The program produces a DjVuPhoto file outputfilename
. If the output file
name is not specified, a default file name will be generated by replacing the
input file name suffix by suffix djvu
The main design objective for the DjVu wavelets consisted of allowing
progressive rendering and smooth scrolling of large images with limited memory
requirements. Decoding functions process the compressed data and update a
memory efficient representation of the wavelet coefficients. Imaging function
then can quickly render an arbitrary segment of the image using the available
data. Both process can be carried out in two threads of execution. This design
plays an important role in the DjVu system. We investigated various
state-of-the-art wavelet compression schemes. Although these schemes may
achieve slightly smaller file sizes, the decoding functions did not even
approach our requirements. The IW44
wavelets reach these
requirements today and may in the future implement more modern refinements if
these refinements can be implemented within our constraints.
DjVuPhoto files are logically composed of a sequence of "slices"
containing successive image refinements. Slices are grouped in
"chunks" defining the progressive rendering sequence. The viewer is
able to display an intermediate image after processing each chunk. A typical
DjVuPhoto files contains 80 to 120 slices grouped into 1 to 4 chunks.
The quality selection options provide various ways to specify the number of
chunks and the number of slices per chunk. The c44
program adds slices
to the current chunk until exceeding a target number of slices, a target file
size, or a target quality specification. The following options define targets
for each chunk. The option argument contain several numerical values (one per
chunk) separated by either commas or pluses.
- -slice n+...+n
- Specify the number of slices in each chunk. The option
argument contains plus-separated numerical values (one per chunk)
indicating the number of slices per chunk. Option -slice 74+13+10,
for instance, would be appropriate for compressing a photographic image
with three progressive refinements. More quality and more refinements can
be obtained with option -slice 72+11+10+10.
- -slice n,...,n
- Specify the cumulative number of slices for each chunk.
Since the final quality is determined by the total number of slices, it is
often more convenient to use comma-separated values (one per chunk)
indicating the cumulative number of slices for each chunk (i.e. including
those encoded in all previous chunks). The values suggested above can also
be expressed as -slice 74,87,97 and -slice
- -size n,...,n
- Specify size targets for each chunk expressed in bytes. The
option argument can be either a plus-separated list specifying a size for
each chunk, or a comma separated list specifying cumulative sizes for each
chunk and all previous chunks. Size targets are approximates. Slices will
be added to each chunk until exceeding the specified target.
- -bpp n,...,n
- Specify size targets for each chunk expressed in
bits-per-pixel. Both comma-separated and plus-separated specifications are
accepted. Option -bpp 0.25,0.5,1 usually provides good
- -percent n,...,n
- Specify size targets for each chunk expressed as a
percentage of the input file size. Both comma-separated and plus-separated
specifications are accepted. Results can be drastically different
according to the format of the input image (raw or JPEG
- -decibel n,...,n
- Specify quality targets for each chunk expressed as a
comma-separated list of increasing decibel values. Decibel values range
from 16 (very low quality) to 48 (very high quality). This criterion
should not be relied upon when re-encoding an image previously compressed
by another compression scheme. Selecting this option significantly
increases the compression time.
- -dbfrac frac
- Indicate that the decibel values specified in option
-decibel should be computed by averaging the mean squared errors of
only the fraction frac of the most mis-represented blocks of 32 x
32 pixels. This option is useful with composite images containing solid
color features (e.g. an image with a large white border).
Providing no quality specification options automatically selects a default
quality specification -slice 74,89,99
. Multiple quality specification
options are allowed. The program outputs a file whose total number of chunks
is the largest number of chunks of all quality specifications. Slices are
added to each chunk until reaching any of the quality target for this chunk.
The following additional options are supported:
- -dpi n
- Specify the resolution information encoded into the output
file expressed in dots per inch. The resolution information encoded in
DjVu files determine how the decoder scales the image on a particular
display. Meaningful resolutions range from 25 to 1200. The default value,
100 dpi, should be suitable for most photographic images.
- -gamma n
- Specify the gamma correction information encoded into the
output file. The argument n specified the gamma value of the device
for which the input image was designed. The default value is 2.2. This is
appropriate for images designed for a standard computer monitor.
- -mask pbmfilename
- The design of the IW44 wavelets allows for
compressing partially masked images. This option can be used when certain
pixels of a background image are going to be covered by foreground objects
like text or drawings. File pbmfile must be a PBM
file whose size matches the size of the input file. Each black pixel in
pbmfile means that the value of the corresponding pixel in the
input file is irrelevant. The IW44 encoder will replace the
masked pixels by a color value whose coding cost is minimal (see
- Select normal chrominance encoding. Chrominance information
is encoded at the same resolution as the luminance. This is the
- Selects half resolution chrominance encoding. Chrominance
information is encoded at half the luminance resolution.
- -crcbdelay n
- This option can be used with -crcbnormal and
-crcbhalf to modify the quality of the chrominance information. The
option arguments specifies a parameter n, expressed in slices, that
reduces the bit-rate associated with the chrominance. The default
chrominance encoding delay is 10 slices.
- Select the highest possible quality for encoding the
chrominance information. This is equivalent to specifying
-crcbnormal and -crcbdelay 0.
- Disable the encoding of the chrominance. Only the luminance
information will be encoded. The resulting image will show in shades of
The default quality setting of the DjVuLibre version of c44
increased. It produces larger files with a better quality. Quality can be
lowered using the quality selection options!
The encoder requires more memory than necessary.
The rechunking capability is currently broken.
This program was written by Léon Bottou
<firstname.lastname@example.org> and was then improved by Andrei Erofeev
<email@example.com>, Bill Riemers <firstname.lastname@example.org>
and many others.