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Pamcomp User Manual
pamcomp
Updated: 17 July 2004
Table Of Contents
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]
[-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.
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 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 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 alpha 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
alpha mask described above. If you supply both an overlay image that
has transparency information and an alpha 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 alpha masks in order to
have a non-opaque overlay. Some Netpbm programs that convert from
image formats such as PNG 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.
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).
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 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 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 an alpha mask (the -alpha option),
pamcomp uses the product of the opacity indicated by the alpha
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 underlaying 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
underlaying image. See
this description 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.
- -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
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
ppmmix and pnmpaste are simpler, less general
versions of the same tool.
ppmcolormask and pbmmask, and pambackground can help with
generating an alpha mask.
pnmcomp is an older program that
runs faster, but has less function.
pnm
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).
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