Tifftopnm User Manual(0) Tifftopnm User Manual(0)
NAME
tifftopnm - convert a TIFF file into a PNM image
SYNOPSIS
tifftopnm
[-alphaout={alpha-filename,-}] [-headerdump] [-respectfillorder]
[-byrow] [tiff-filename]
You may abbreviate any option to its shortest unique prefix. You may
use two hyphens instead of one in options. You may separate an option
and its value either by an equals sign or white space.
DESCRIPTION
This program is part of Netpbm(1).
tifftopnm reads a TIFF file as input and produces a PNM image as out-
put. The type of the output file depends on the input file - if it’s
black & white, generates a PBM image; if it’s grayscale, generates a
PGM image; otherwise, a PPM image. The program tells you which type it
is writing.
If the TIFF file contains multiple images (multiple ’directories,’),
tifftopnm generates a multi-image PNM output stream. Before Netpbm
10.27 (March 2005), however, it would just ignore all but the first
input image.
This program cannot read every possible TIFF file -- there are myriad
variations of the TIFF format. However, it does understand monochrome
and gray scale, RGB, RGBA (red/green/blue with alpha channel), CMYK
(Cyan-Magenta-Yellow-Black ink color separation), and color palette
TIFF files. An RGB file can have either single plane (interleaved)
color or multiple plane format. The program reads 1-8 and 16 bit-per-
sample input, the latter in either bigendian or littlendian encoding.
Tiff directory information may also be either bigendian or littlendian.
There are many TIFF formats that tifftopnm can read only if the image
is small enough to fit in memory. tifftopnm uses the TIFF library’s
TIFFRGBAImageGet() function to process the TIFF image if it can get
enough memory for TIFFRGBAImageGet() to store the whole image in memory
at once (that’s what TIFFRGBAImageGet() does). If not, tifftopnm uses
a more primitive row-by-row conversion strategy using the raw data
returned by TIFFReadScanLine() and native intelligence. That native
intelligence does not know as many formats as TIFFRGBAImageGet() does.
And certain compressed formats simply cannot be read with TIFFReadScan-
Line().
Before Netpbm 10.11 (October 2002), tifftopnm never used TIFFRGBAIm-
ageGet(), so it could not interpret many of the formats it can inter-
pret today.
There is no fundamental reason that this program could not read other
kinds of TIFF files even when they don’t fit in memory all at once.
The existing limitations are mainly because no one has asked for more.
The PNM output has the same maxval as the Tiff input, except that if
the Tiff input is colormapped (which implies a maxval of 65535) the PNM
output has a maxval of 255. Though this may result in lost informa-
tion, such input images hardly ever actually have more color resolution
than a maxval of 255 provides and people often cannot deal with PNM
files that have maxval > 255. By contrast, a non-colormapped Tiff
image that doesn’t need a maxval > 255 doesn’t have a maxval > 255, so
when tifftopnm sees a non-colormapped maxval > 255, it takes it seri-
ously and produces a matching output maxval.
Another exception is where the TIFF maxval is greater than 65535, which
is the maximum allowed by the Netpbm formats. In that case, tifftopnm
uses a maxval of 65535, and you lose some information in the conver-
sion.
The tiff-filename argument names the regular file that contains the
Tiff image. If you specify ’-’ or don’t specify this argument,
tfftopnm uses Standard Input. In either case, the file must be seek-
able. That means no pipe, but any regular file is fine.
OPTIONS
-alphaout=alpha-filename
tifftopnm creates a PGM file containing the alpha channel values
in the input image. If the input image doesn’t contain an alpha
channel, the alpha-filename file contains all zero (transparent)
alpha values. If you don’t specify -alphaout,
tifftopnm does not generate an alpha file, and if the input
image has an alpha channel, tifftopnm simply discards it.
If you specify - as the filename, tifftopnm writes the alpha
output to Standard Output and discards the image.
See pamcomp(1)foronewaytouse the alpha output file.
-respectfillorder
By default, tifftopnm ignores the ’fillorder’ tag in the TIFF
input, which means it may incorrectly interpret the image. To
make it follow the spec, use this option. For a lengthy but
engaging discussion of why tifftopnm works this way and how to
use the -respectfillorder option, see the note on fillorder
below.
-byrow This option can make tifftopnm run faster.
tifftopnm has two different ways to do the conversion from Tiff
to PNM, using two different facilities of the TIFF library:
Whole Image
Decode the entire image into memory at once, using TIFFRGBAIm-
ageGet(), then convert to PNM and output row by row.
Row By Row
Read, convert, and output one row at a time using TIFFReadScan-
line().
Whole Image is preferable because the Tiff library does more of
the work, which means it understands more of the Tiff format
possibilities now and in the future. Also, some compressed TIFF
formats don’t allow you to extract an individual row.
Row By Row uses far less memory, which means with large images,
it can run in environments where Whole Image cannot and may also
run faster. And because Netpbm code does more of the work, it’s
possible that it can be more flexible or at least give better
diagnostic information if there’s something wrong with the TIFF.
In Netpbm, we stress function over performance, so by default we
try Whole Image first, and if we can’t get enough memory for the
decoded image or TIFFRGBAImageGet() fails, we fall back to Row
By Row. But if you specify the -byrow option, tifftopnm will
not attempt Whole Image. If Row By Row does not work, it simply
fails.
See Color Separation (CMYK) TIFFs for a description of one way
Row By Row makes a significant difference in your results.
Whole Image costs you precision when your TIFF image uses more
than 8 bits per sample. TIFFRGBAImageGet() converts the samples
to 8 bits. tifftopnm then scales them back to maxval 65535, but
the lower 8 bits of information is gone.
Before Netpbm 10.11 (October 2002), tifftopnm always did Row By
Row. Netpbm 10.12 always tried Whole Image first. -byrow came
in with Netpbm 10.13 (January 2003).
-headerdump
Dump TIFF file information to stderr. This information may be
useful in debugging TIFF file conversion problems.
NOTES
Fillorder
There is a piece of information in the header of a TIFF image called
’fillorder.’ The TIFF specification quite clearly states that this
value tells the order in which bits are arranged in a byte in the
description of the image’s pixels. There are two options, assuming
that the image has a format where more than one pixel can be repre-
sented by a single byte: 1) the byte is filled from most significant
bit to least significant bit going left to right in the image; and 2)
the opposite.
However, there is confusion in the world as to the meaning of fil-
lorder. Evidence shows that some people believe it has to do with byte
order when a single value is represented by two bytes.
These people cause TIFF images to be created that, while they use a
MSB-to-LSB fillorder, have a fillorder tag that says they used LSB-to-
MSB. A program that properly interprets a TIFF image will not end up
with the image that the author intended in this case.
For a long time, tifftopnm
did not understand fillorder itself and assumed the fillorder was MSB-
to-LSB regardless of the fillorder tag in the TIFF header. And as far
as I know, there is no legitimate reason to use a fillorder other than
MSB-to-LSB. So users of tifftopnm
were happily using those TIFF images that had incorrect fillorder tags.
So that those users can continue to be happy, tifftopnm
today continues to ignore the fillorder tag unless you tell it not to.
(It does, however, warn you when the fillorder tag does not say MSB-to-
LSB that the tag is being ignored).
If for some reason you have a TIFF image that actually has LSB-to-MSB
fillorder, and its fillorder tag correctly indicates that, you must use
the -respectfillorder
option on tifftopnm
to get proper results.
Examples of incorrect TIFF images are at ftp://weather.noaa.gov.
They are apparently created by a program called faxtotiff.
This note was written on January 1, 2002.
Color Separation (CMYK) TIFFs
Some TIFF images contain color information in CMYK form, whereas PNM
images use RGB. There are various formulas for converting between
these two forms, and tifftopnm can use either of two.
The TIFF library (Version 3.5.4 from libtiff.org) uses Y=(1-K)*(1-B)
(similar for R and G) in its TIFFRGBAImageGet() service. When
tifftopnm works in Whole Image mode, it uses that service, so that’s
the conversion you get.
But when tifftopnm runs in Row By Row mode, it does not use TIFFRGBAIm-
ageGet(), and you get what appears to be more useful: Y=1-(B+K). This
is the inverse of what pnmtotiffcmyk does.
See the -byrow option for more information on Whole Image versus Row By
Row mode.
Before Netpbm 10.21 (March 2004), tifftopnm used the Y=(1-K)*(1-B) for-
mula always.
SEE ALSO
pnmtotiff(1), pnmtotiffcmyk(1), pamcomp(1), pnm(1)
AUTHOR
Derived by Jef Poskanzer from tif2ras.c, which is Copyright (c) 1990 by
Sun Microsystems, Inc. Author: Patrick J. Naughton
(naughton@wind.sun.com).
netpbm documentation 27 March 2005 Tifftopnm User Manual(0)
