Provided by: netpbm_11.05.02-1.1build1_amd64 
NAME
pamcomp - composite (overlay) two Netpbm images together
SYNOPSIS
pamcomp
[-align={left | center | right | beyondleft | beyondright}] [-valign={top | middle | bottom| above |
below}] [-xoff=X] [-yoff=Y] [-alpha=alpha-pgmfile] [-invert] [-opacity=opacity] [-mixtransparency]
[-linear] overlay_file [underlying_file [output_file]]
Minimum unique abbreviation of option is acceptable. You may use double hyphens instead of single hyphen
to denote options. You may use white space in place of the equals sign to separate an option name from
its value.
DESCRIPTION
This program is part of Netpbm(1).
pamcomp reads two images and produces a composite image with one of the images overlayed on top of the
other, possible translucently. The images need not be the same size. The input and outputs are Netpbm
format image files.
In its simplest use, pamcomp simply places the image in the file overlay_file on top of the image in the
file underlying_file, blocking out the part of underlying_file beneath it.
If you add the -alpha option, then pamcomp uses the image in file alpha-pgmfile as a transparency mask,
which means it determines the level of transparency of each point in the overlay image. The transparency
mask must have the same dimensions as the overlay image. In places where the transparency mask defines
the overlay image to be opaque, the composite output contains only the contents of the overlay image; the
underlying image is totally blocked out. In places where the transparency mask defines the overlay image
to be transparent, the composite output contains none of the overlay image; the underlying image shows
through completely. In places where the transparency mask shows a value in between opaque and
transparent (translucence), the composite image contains a mixture of the overlay image and the
underlying image and the level of translucence determines how much of each.
The transparency mask is a PGM file in which a white pixel represents opaqueness and a black pixel
transparency. Anything in between is translucent. (Like any Netpbm program, pamcomp will see a PBM file
as if it is PGM).
If the overlay image is a PAM image of tuple type RGB_ALPHA or GRAYSCALE_ALPHA, then the overlay image
contains transparency information itself and pamcomp uses it the same way as the transparency mask
described above. If you supply both an overlay image that has transparency information and a
transparency mask, pamcomp multiplies the two opacities to get the opacity of the overlay pixel.
Before Netpbm 10.25 (October 2004), pamcomp did not recognize the transparency information in a PAM image
-- it just ignored it. So people had to make appropriate transparency masks in order to have a non-
opaque overlay. Some Netpbm programs that convert from image formats that contain transparency
information are not able to create RGB_ALPHA or GRAYSCALE_ALPHA PAM output, so you have to use the old
method -- extract the transparency information from the original into a separate transparency mask and
use that as input to pamcomp.
The output image is always of the same dimensions as the underlying image. pamcomp uses only parts of
the overlay image that fit within the underlying image.
The output image is a PAM image. Its tuples are color, grayscale, or black and white, whichever is the
"highest" format between the two input images. The maxval of the output is the least common multiple of
the maxvals of the input, up to the maximum possible PAM maxval, 65535.
The output has an opacity channel if and only if the underlying image does, and then the opacities are as
described under the -mixtransparency option. Before Netpbm 10.56 (September 2011), the output never has
an opacity channel.
To specify where on the underlying image to place the overlay image, use the -align, -valign, -xoff, and
-yoff options. Without these options, the default horizontal position is flush left and the default
vertical position is flush top.
The overlay image, in the position you specify, need not fit entirely within the underlying image.
pamcomp uses only the parts of the overlay image that appear above the underlying image. It is possible
to specify positioning such that none of the overlay image is over the underlying image -- i.e. the
overlay is out of frame. If you do that, pamcomp issues a warning.
The overlay and underlying images may be of different formats (e.g. overlaying a PBM text image over a
full color PPM image) and have different maxvals. The output image has the more general of the two input
formats and a maxval that is the least common multiple the two maxvals (or the maximum maxval allowable
by the format, if the LCM is more than that).
ARGUMENTS
The overlay_file argument is the name of the file containing the
overly image, while underlying_file is the name of the file
containing the underlying image. For either, you may specify '-'
to indicate Standard Input, and underlying file defaults to Standard
Input. Make sure you aren't specifying (or defaulting) Standard Input as
both.
Note that there may be a third input file, identified by an -alphafile option.
The output_file argument is the name of the file to which
pamcomp writes the output, creating or truncating it first. You may
specify '-' to indicate Standard Output, in which
case pamcomp does not truncate it. Note that pamcomp is
unusual among Netpbm programs, as a historical accident, in having an output
file argument; Netpbm programs normally write to Standard Output only.
OPTIONS
In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common
Options ), pamcomp recognizes the following command line options:
-align=alignment
This option selects the basic horizontal position of the overlay image with respect to the
underlying image, in syntax reminiscent of HTML. left means flush left, center means centered,
and right means flush right.
The -xoff option modifies this position.
beyondleft means just out of frame to the left -- the right edge of the overlay is flush with the
left edge of the underlying image. beyondright means just out of frame to the right. These
alignments are useful only if you add a -xoff option. These two values were added in Netpbm
10.10 (October 2002).
The default is left.
-valign=alignment
This option selects the basic vertical position of the overlay image with respect to the
underlying image, in syntax reminiscent of HTML. top means flush top, middle means centered, and
bottom means flush bottom.
The -yoff option modifies this position.
above means just out of frame to the top -- the bottom edge of the overlay is flush with the top
edge of the underlying image. below means just out of frame to the bottom. These alignments are
useful only if you add a -yoff option. These two values were added in Netpbm 10.10 (October
2002).
The default is top.
-xoff=x
This option modifies the horizontal positioning of the overlay image with respect to the
underlying image as selected by the -align option. pamcomp shifts the overlay image from that
basic position x pixels to the right. x can be negative to indicate shifting to the left.
The overlay need not fit entirely (or at all) on the underlying image. pamcomp uses only the
parts that lie over the underlying image.
Before Netpbm 10.10 (October 2002), -xoff was mutually exclusive with -align and always measured
from the left edge.
-yoff=y
This option modifies the vertical positioning of the overlay image with respect to the underlying
image as selected by the -valign option. pamcomp shifts the overlay image from that basic
position y pixels downward. y can be negative to indicate shifting upward.
The overlay need not fit entirely (or at all) on the underlying image. pamcomp uses only the
parts that lie over the underlying image.
Before Netpbm 10.10 (October 2002), -xoff was mutually exclusive with -valign and always measured
from the top edge.
-alpha=alpha-pgmfile
This option names a file that contains the transparency mask. If you don't specify this option,
there is no transparency mask, which is equivalent to having a transparency mask specify total
opaqueness everywhere.
You can specify - as the value of this option and the transparency mask will come from Standard
Input. If you do this, don't specify Standard Input as the source of any other input image.
-invert
This option inverts the sense of the values in the transparency mask, which effectively switches
the roles of the overlay image and the underlying image in places where the two intersect.
-opacity=opacity
This option tells how opaque the overlay image is to be, i.e. how much of the composite image
should be from the overlay image, as opposed to the underlying image. opacity is a floating point
number, with 1.0 meaning the overlay image is totally opaque and 0.0 meaning it is totally
transparent. The default is 1.0.
If you specify a transparency mask (the -alpha option), pamcomp uses the product of the opacity
indicated by the transparency mask (as modified by the -invert option, as a fraction, and this
opacity value. The -invert option does not apply to this opacity value.
As a simple opacity value, the value makes sense only if it is between 0 and 1, inclusive.
However, pamcomp accepts all values and performs the same arithmetic computation using whatever
value you provide. An opacity value less than zero means the underlay image is intensified and
then the overlay image is "subtracted" from it. An opacity value greater than unity means the
overlay image is intensified and the underlay image subtracted from it. In either case, pamcomp
clips the resulting color component intensities so they are nonnegative and don't exceed the
output image's maxval.
This may seem like a strange thing to do, but it has uses. You can use it to brighten or darken
or saturate or desaturate areas of the underlay image. See this description(1) of the technique.
This option was added in Netpbm 10.6 (July 2002). Before Netpbm 10.15 (April 2003), values less
than zero or greater than unity were not allowed.
-mixtransparency
This option controls what pamcomp does where both the underlying and overlay image are non-opaque.
By default, the output image has the same transparency as the underlying image and the
transparency of the underlying image has no effect on the composition of color.
But with this option, pamcomp composes the image according to a plastic transparency metaphor: the
underlying and overlay images are plastic slides. The output image is the slide you get when you
stack up those two slides. So the transparency of the output is a combination of the transparency
of the inputs and the transparency of the underlying image affects the underlying image's
contribution to the output image's color.
Unlike the metaphorical slide, a PAM pixel has a color even where it is completely transparent, so
pamcomp departs from the metaphor in that case and makes the output color identical to the
underlying image.
This option was new in Netpbm 10.56 (September 2011). Before that, the output is always opaque
and the pamcomp ignores the transparency of the underlying image.
-linear
This option indicates that the inputs are not true Netpbm images but rather a light-intesity-
proportional variation. This is relevant only when you mix pixels, using the -opacity option or a
transparency mask (the -alpha option).
The transparency mask and -opacity values indicate a fraction of the light intensity of a pixel.
But the PNM and PNM-equivalent PAM image formats represent intensities with gamma-adjusted numbers
that are not linearly proportional to intensity. So pamcomp, by default, performs a calculation
on each sample read from its input and each sample written to its output to convert between these
gamma-adjusted numbers and internal intensity-proportional numbers.
Sometimes you are not working with true PNM or PAM images, but rather a variation in which the
sample values are in fact directly proportional to intensity. If so, use the -linear option to
tell pamcomp this. pamcomp then will skip the conversions.
The conversion takes time. And the difference between intensity-proportional values and gamma-
adjusted values may be small enough that you would barely see a difference in the result if you
just pretended that the gamma-adjusted values were in fact intensity-proportional. So just to
save time, at the expense of some image quality, you can specify -linear even when you have true
PPM input and expect true PPM output.
For the first 13 years of Netpbm's life, until Netpbm 10.20 (January 2004), pamcomp's predecessor
pnmcomp always treated the PPM samples as intensity-proportional even though they were not, and
drew few complaints. So using -linear as a lie is a reasonable thing to do if speed is important
to you.
Another technique to consider is to convert your PNM image to the linear variation with pnmgamma,
run pamcomp on it and other transformations that like linear PNM, and then convert it back to true
PNM with pnmgamma -ungamma. pnmgamma is often faster than pamcomp in doing the conversion.
SEE ALSO
pammixmulti.html(1) mixes together two or more images of the same size, in various ways.
ppmmix(1) and pnmpaste(1) are simpler, less general versions of the same tool.
ppmcolormask(1) and pbmmask(1), and pambackground(1) can help with generating a transparency mask.
pnmcomp(1) is an older program that runs faster, but has less function.
pnm(1)
HISTORY
pamcomp was new in Netpbm 10.21 (March 2004). Its predecessor, pnmcomp, was one of the first programs
added to Netpbm when the project went global in 1993.
AUTHOR
Copyright (C) 1992 by David Koblas (koblas@mips.com).
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation
is at
http://netpbm.sourceforge.net/doc/pamcomp.html
netpbm documentation 13 August 2011 Pamcomp User Manual(1)