In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common
Options ), pngtopam recognizes the following command line options:
-verbose
Display various information about the input PNG image and the conversion process.
If you want even more information about the PNG image, use pngcheck (not part of Netpbm).
-alphapam
Produce a single output image containing the main image (foreground) and the transparency channel
or transparency mask. This image is in the PAM format with tuple type of either GRAYSCALE_ALPHA
(which has a depth of 2 channels) or RGB_ALPHA (which has a depth of 4 channels).
You can specify only one of -alphapam, -alpha, and -mix. With none of them, pngtopam produces an
image of the foreground of the input image and discards transparency information.
This option was new in Netpbm 10.44 (September 2008).
-alpha Output the transparency channel or transparency mask of the image. The result is either a PBM file
or a PGM file, depending on whether different levels of transparency appear.
pngtopam discards the main image (the foreground).
You can specify only one of -alphapam, -alpha, and -mix. With none of them, pngtopam produces an
image of the foreground of the input image and discards transparency information.
-mix Compose the image with the transparency or transparency mask against a background. The background
color is determined by the bKGD chunk in the PNG, except that you can override it with
-background. If the PNG has no bKGD chunk and you don't specify -background, the background color
is white.
You can specify only one of -alphapam, -alpha, and -mix. With none of them, pngtopam produces an
image of the foreground of the input image and discards transparency information.
-background=color
This option specifies the background color with which to mix the image when you specify -mix.
color is as described for the argument of the pnm_parsecolor() library routine .
Examples:
• -background=rgb:01/ff/80
• -background=rgbi:1/255/128
If you don't specify -background, the background color is what is specified in the PNG image, and
if the PNG doesn't specify anything, white.
You cannot specify -background unless you also specify -mix. Before Netpbm 10.27 (March 2005),
you could specify -background without -mix and it was just ignored. (This caused a usability
problem).
-gamma=value
This option causes pngtopam to respect the image gamma information in the PNG file (from the gAMA
chunk). Probably by historical accident, pngtopam ignores that information by default, assuming
the image uses the same gamma transformation as a Netpbm image, so the output image has different
colors than the PNG file actually represents if the PNG doesn't actually do that. (However, it is
rare for a PNG file to use a gamma transformation different from what the Netpbm formats specify,
or if it does, to specify with a gAMA chuck what that is).
But when you do specify -gamma, you get a rather strange additional function, probably a
historical mistake: pngtopam incorporates the specified screen gamma value into the output pixels,
so that the samples in the Netpbm output deviate from the Netpbm format specifications and are
appropriate raw intensity values to send to the display. This function essentially just exercises
the ability of the PNG library to make gamma corrections to the pixels as it reads them from the
PNG file to produce values appropriate for sending to a certain display in certain viewing
conditions. It's a strange function because it has nothing to do with PNG and because in Netpbm,
the normal way to make gamma corrections appropriate for sending to a ceratin display in certain
viewing conditions is with the program pngtopam, applied to the normal output of pngtopam.
If you specify -gamma, but the PNG image does not specify what gamma transformation it uses (there
is no gAMA chunk), pngtopam assumes a simple power transformation with an image gamma of 1.0.
That is probably not not the actual image gamma; it is much more likely to be .45.
Because the gammas of uncompensated monitors are around 2.6, which results in an image-gamma of
0.45, some typical situations are: when the image-gamma is 0.45 (use -verbose to check) and the
picture is too light, your system is gamma-corrected, so convert with "-gamma 1.0". When no gAMA
chunk is present or the image-gamma is 1.0, use 2.2 to make the picture lighter and 0.45 to make
the picture darker.
One oddity to be aware of when using -gamma on an image with transparency: The PNG image specifies
that a certain color is transparent, i.e. every pixel in the image of that color is transparent.
But pngtopam interprets this as applying to the gamma-corrected space, and there may be less
precision in that space than in the original, which means multiple uncorrected colors map to the
same corrected color. So imagine that the image contains 3 shades of white (gray) and specifies
that one of them is transparent. After gamma correction, those three shades are
indistinguishable, so pngtopam considers pixels of all three shades to be transparent.
-text=file
Writes the tEXt and zTXt chunks to a file, in a format as described in the pnmtopng user manual.
These chunks contain text comments or annotations.
-time Prints the tIME chunk to stderr.
-byrow This option can make pngtopam run faster or in environments where it would otherwise fail.
pngtopam has two ways to do the conversion from PNG to PAM, using respectively two facilities of
the PNG library:
Whole Image
Decode the entire image into memory at once, using png_read_image(), then convert to PAM and
output row by row.
Row By Row
Read, convert, and output one row at a time using png_read_row().
Whole Image is generally preferable because the PNG library does more of the work, which means it
understands more of the PNG format possibilities now and in the future. Also, if the PNG is
interlaced, pngtopam does not know how to assemble the rows in the right order.
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 PNG.
The Netpbm native code may do something correctly that the PNG library does incorrectly, or vice
versa.
In Netpbm, we stress function over performance, so by default pngtopam uses Whole Image. You can
select Row By Row with -byrow if you want the speed or resource requirement improvement.
-byrow was new in Netpbm 10.54 (March 2011).
-orientraw
A TIFF stream contains raster data which can be arranged in the stream various ways. Most
commonly, it is arranged by rows, with the top row first, and the pixels left to right within each
row, but many other orientations are possible.
The common orientation is the same on the Netpbm formats use, so tifftopnm can do its jobs quite
efficiently when the TIFF raster is oriented that way.
But if the TIFF raster is oriented any other way, it can take a considerable amount of processing
for tifftopnm to convert it to Netpbm format.
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