C44(1) DjVuLibre-3.5 C44(1)

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 L'eon 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).

DjVuLibre-3.5 10/11/2001 C44(1)