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Pamcomp User Manual(0) Pamcomp User Manual(0)
NAME
pamcomp - composite (overlay) two Netpbm images together
SYNOPSIS
pamcomp
[-align={left|center|right| beyondleft|beyondright}] [-valign={top|mid-
dle|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
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 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
alpha-pgmfile as an alpha mask, which means it determines the level of
transparency of each point in the overlay image. The alpha mask must
have the same dimensions as the overlay image. In places where the
alpha 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 alpha 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 alpha mask shows a value in between opaque and trans-
parent (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 alpha mask is a PGM file in which a white pixel represents opaque-
ness and a black pixel transparency. Anything in between is translu-
cent. (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 alpha mask
described above. If you supply both an overlay image that has trans-
parency information and an alpha mask, pamcomp multiplies the two opac-
ities 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 alpha 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
alpha 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 -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
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 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
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 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).
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
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 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
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 alpha mask. If you
don't specify this option, there is no alpha mask, which is
equivalent to having an alpha mask specify total opaqueness
everywhere.
You can specify - as the value of this option and the alpha 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 alpha 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 float-
ing 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 an alpha mask (the -alpha option), pamcomp uses
the product of the opacity indicated by the alpha mask (as modi-
fied 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 underlaying 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 underlaying image. See this description (5)
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 out-
put 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 non-gamma-adjusted variation. This is relevant
only when you mix pixels, using the -opacity option or an alpha
mask (the -alpha option).
The alpha 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 conver-
sions.
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
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 an alpha mask.
pnmcomp(1)
is an older program that runs faster, but has less function.
pnm(5)
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).
netpbm documentation 13 August 2011 Pamcomp User Manual(0)
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