NAME
c44 - DjVuPhoto encode.
SYNOPSIS
c44 [options] inputfilename [outputfilename]
DESCRIPTION
Produces a DjVuPhoto encoded image. The input image file inputfilename
can be either a portable gray-map (PGM) or a portable pix-map (PPM).
Input images compressed with JPEG are also accepted. It is however
suggested to only use high quality JPEG files (low compression ratio,
large size) because the wavelet compression will increase the defects
already present in highly compressed JPEG files.
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.
QUALITY SELECTION OPTIONS
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 72,83,93,103.
-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 results.
-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 compressed).
-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.
OTHER OPTIONS
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
http://www.djvuzone.org/djvu/techpapers/mask/index.djvu for
technical details.)
-crcbnormal
Select normal chrominance encoding. Chrominance information is
encoded at the same resolution as the luminance. This is the
default.
-crcbhalf
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.
-crcbfull
Select the highest possible quality for encoding the chrominance
information. This is equivalent to specifying -crcbnormal and
-crcbdelay 0.
-crcbnone
Disable the encoding of the chrominance. Only the luminance
information will be encoded. The resulting image will show in
shades of gray.
REMARKS
The default quality setting of the DjVuLibre version of c44 has been
increased. It produces larger files with a better quality. Quality
can be lowered using the quality selection options!
BUGS
The encoder requires more memory than necessary.
The rechunking capability is currently broken.
CREDITS
This program was written by Leon Bottou <leonb@users.sourceforge.net>
and was then improved by Andrei Erofeev <andrew_erofeev@yahoo.com>,
Bill Riemers <docbill@sourceforge.net> and many others.
SEE ALSO
djvu(1), pnm(5), cjpeg(1).