Provided by: netpbm_11.01.00-2build1_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 ⟨index.html#commonoptions⟩ ), 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