Pamcomp User Manual(1) General Commands Manual Pamcomp User Manual(1)
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 dou-
ble 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 im-
ages need not be the same size. The input and outputs are Netpbm for-
mat image files.
In its simplest use, pamcomp simply places the image in the file over-
lay_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 al-
pha-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 transpar-
ency 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 informa-
tion itself and pamcomp uses it the same way as the transparency mask
described above. If you supply both an overlay image that has trans-
parency 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 peo-
ple 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 -- ex-
tract 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 im-
age. 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 in-
put 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 -mixtrans-
parency 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 op-
tions, 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 over-
lay image that appear above the underlying image. It is possible to
specify positioning such that none of the overlay image is over the un-
derlying 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 dif-
ferent 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 In-
put 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 reminis-
cent 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 im-
age. 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 reminis-
cent 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 im-
age with respect to the underlying image as selected by the
-valign option. pamcomp shifts the overlay image from that ba-
sic 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 transpar-
ency 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 im-
age, 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 opac-
ity 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 sub-
tracted 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 tech-
nique.
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 out-
put is a combination of the transparency of the inputs and the
transparency of the underlying image affects the underlying im-
age'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 num-
bers that are not linearly proportional to intensity. So pam-
comp, 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-pro-
portional numbers.
Sometimes you are not working with true PNM or PAM images, but
rather a variation in which the sample values are in fact di-
rectly 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 inten-
sity-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 reason-
able 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 gen-
erating 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, pnm-
comp, 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)
Generated by dwww version 1.15 on Sat Jun 29 02:18:23 CEST 2024.