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<H1>pamcomp</H1>
Updated: 13 August 2011
<p>
<A HREF="#index">Table Of Contents</A>
<H2>NAME</H2>
pamcomp - composite (overlay) two Netpbm images together
<H2 id="synopsis">SYNOPSIS</H2>
<B>pamcomp</B>
[<B>-align=</B>{<B>left</B>|<B>center</B>|<B>right</B>|
<B>beyondleft</b>|<b>beyondright</b>}]
<BR>
[<B>-valign=</B>{<B>top</B>|<B>middle</B>|<B>bottom</B>|
<b>above</b>|<b>below</b>}]
<BR>
[<B>-xoff=</B><I>X</I>]
[<B>-yoff=</B><I>Y</I>]
<BR>
[<B>-alpha=</B><I>alpha-pgmfile</I>]
[<B>-invert</B>]
[<B>-opacity=</b><i>opacity</i>]
[<b>-mixtransparency</b>]
[<b>-linear</b>]
<BR>
<I>overlay_file</I>
[<I>underlying_file</I> [<I>output_file</I>]]
<p>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.
<H2 id="description">DESCRIPTION</H2>
<p>This program is part of <a href="index.html">Netpbm</a>.
<B>pamcomp</B> 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.
<P>In its simplest use, <B>pamcomp</B> simply places the image in the
file <I>overlay_file</I> on top of the image in the file
<I>underlying_file</I>, blocking out the part of <I>underlying_file</I>
beneath it.
<P>If you add the <B>-alpha</B> option, then <B>pamcomp</B> uses the
image in file <I>alpha-pgmfile</I> 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.
<P>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, <B>pamcomp</B> will see a PBM
file as if it is PGM).
<p>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 <b>pamcomp</b> 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, <b>pamcomp</b>
multiplies the two opacities to get the opacity of the overlay pixel.
<p>Before Netpbm 10.25 (October 2004), <b>pamcomp</b> 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
<b>pamcomp</b>.
<P>The output image is always of the same dimensions as the underlying
image. <B>pamcomp</B> uses only parts of the overlay image that fit
within the underlying image.
<p>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.
<p>The output has an opacity channel if and only if the underlying image does,
and then the opacities are as described under the <b>-mixtransparency</b>
option. Before Netpbm 10.56 (September 2011), the output never has an opacity
channel.
<P>To specify where on the underlying image to place the overlay
image, use the <B>-align</B>, <B>-valign</B>, <B>-xoff</B>, and
<B>-yoff</B> options. Without these options, the default horizontal
position is flush left and the default vertical position is flush top.
<p>The overlay image, in the position you specify, need not fit entirely
within the underlying image. <b>pamcomp</b> uses only the parts of the
overlay image that appear above the underlying image. It is possible to
specify positioning such that <em>none</em> of the overlay image is
over the underlying image -- i.e. the overlay is out of frame. If you
do that, <b>pamcomp</b> issues a warning.
<P> 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).
<H2 id="options">OPTIONS</H2>
<DL COMPACT>
<DT><B>-align=</b><i>alignment</i>
<DD>
This option selects the basic horizontal position of the overlay image
with respect to the underlying image, in syntax reminiscent of HTML.
<b>left</b> means flush left, <b>center</b> means centered, and <b>right</b>
means flush right.
<p>The <b>-xoff</b> option modifies this position.
<b>beyondleft</b> means just out of frame to the left -- the right edge of
the overlay is flush with the left edge of the underlying image.
<b>beyondright</b> means just out of frame to the right. These alignments
are useful only if you add a <b>-xoff</b> option. These two values were
added in Netpbm 10.10 (October 2002).
<p>The default is <b>left</b>.
<DT><B>-valign=</b><i>alignment</i>
<DD>
This option selects the basic vertical position of the overlay image
with respect to the underlying image, in syntax reminiscent of HTML.
<b>top</b> means flush top, <b>middle</b> means centered, and <b>bottom</b>
means flush bottom.
<p>The <b>-yoff</b> option modifies this position.
<b>above</b> means just out of frame to the top -- the bottom edge of
the overlay is flush with the top edge of the underlying image.
<b>below</b> means just out of frame to the bottom. These alignments
are useful only if you add a <b>-yoff</b> option. These two values were
added in Netpbm 10.10 (October 2002).
<p>The default is <b>top</b>.
<DT><B>-xoff=</B><I>x</I>
<DD>This option modifies the horizontal positioning of the overlay
image with respect to the underlying image as selected by the
<b>-align</b> option. <b>pamcomp</b> shifts the overlay image from
that basic position <i>x</i> pixels to the right. <i>x</i> can be
negative to indicate shifting to the left.
<p>The overlay need not fit entirely (or at all) on the underlying image.
<B>pamcomp</B> uses only the parts that lie over the underlying image.
<P>Before Netpbm 10.10 (October 2002), <b>-xoff</b> was mutually
exclusive with <b>-align</b> and always measured from the left edge.
<DT><B>-yoff=</B><I>y</I>
<DD>This option modifies the vertical positioning of the overlay
image with respect to the underlying image as selected by the
<b>-valign</b> option. <b>pamcomp</b> shifts the overlay image from
that basic position <i>y</i> pixels downward. <i>y</i> can be
negative to indicate shifting upward.
<p>The overlay need not fit entirely (or at all) on the underlying image.
<B>pamcomp</B> uses only the parts that lie over the underlying image.
<P>Before Netpbm 10.10 (October 2002), <b>-xoff</b> was mutually
exclusive with <b>-valign</b> and always measured from the top edge.
<DT><B>-alpha=</B><i>alpha-pgmfile</i>
<DD>
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.
<p>
You can specify <b>-</b> 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.
<DT><B>-invert</B>
<DD>
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.
<DT><B>-opacity=</B><i>opacity</i>
<DD>
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. <i>opacity</i> 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.
<p>If you specify a transparency mask (the <b>-alpha</b> option),
<b>pamcomp</b> uses the product of the opacity indicated by the transparency
mask (as modified by the <b>-invert</b> option, as a fraction, and
this opacity value. The <b>-invert</b> option does not apply to this
opacity value.
<p>As a simple opacity value, the value makes sense only if it is
between 0 and 1, inclusive. However, <b>pamcomp</b> 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, <b>pamcomp</b> clips the resulting color component intensities
so they are nonnegative and don't exceed the output image's maxval.
<p>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 <a href="extendedopacity.html"> this description</a> of the technique.
<p>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.
<dt><b>-mixtransparency</b>
<dd>
This option controls what <b>pamcomp</b> does where both the underlying and
overlay image are non-opaque.
<p>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.
<p>But with this option, <b>pamcomp</b> 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.
<p>Unlike the metaphorical slide, a PAM pixel has a color even where it is
completely transparent, so <b>pamcomp</b> departs from the metaphor in that
case and makes the output color identical to the underlying image.
<p>This option was new in Netpbm 10.56 (September 2011). Before that, the
output is always opaque and the <b>pamcomp</b> ignores the transparency of the
underlying image.
<dt><b>-linear</b>
<dd>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 <b>-opacity</b> option or a transparency mask
(the <b>-alpha</b> option).
<p>The transparency mask and <b>-opacity</b> 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 <b>pamcomp</b>, 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.
<p>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 <b>-linear</b> option to
tell <b>pamcomp</b> this. <b>pamcomp</B> then will skip the
conversions.
<p>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 <b>-linear</b> even when you have true
PPM input and expect true PPM output.
<p>For the first 13 years of Netpbm's life, until Netpbm 10.20
(January 2004), <b>pamcomp</b>'s predecessor <b>pnmcomp</b> always
treated the PPM samples as intensity-proportional even though they
were not, and drew few complaints. So using <b>-linear</b> as a lie
is a reasonable thing to do if speed is important to you.
<p>Another technique to consider is to convert your PNM image to the
linear variation with <b>pnmgamma</b>, run <b>pamcomp</b> on it and
other transformations that like linear PNM, and then convert it back
to true PNM with <b>pnmgamma -ungamma</b>. <b>pnmgamma</b> is often
faster than <b>pamcomp</b> in doing the conversion.
</DL>
<H2 id="seealso">SEE ALSO</H2>
<p><B><A HREF="ppmmix.html">ppmmix</A></B> and <B><A
HREF="pnmpaste.html">pnmpaste</A></B> are simpler, less general
versions of the same tool.
<p><B><A HREF="ppmcolormask.html">ppmcolormask</A></B> and <B><A
HREF="pbmmask.html">pbmmask</A></B>, and <a
href="pambackground.html"><b>pambackground</b></a> can help with
generating a transparency mask.
<p><B><A HREF="pnmcomp.html">pnmcomp</A></B> is an older program that
runs faster, but has less function.
<P><B><A HREF="pnm.html">pnm</A></B>
<H2 id="history">HISTORY</H2>
<p><b>pamcomp</b> was new in Netpbm 10.21 (March 2004). Its predecessor,
<b>pnmcomp</b>, was one of the first programs added to Netpbm when the
project went global in 1993.
<H2 id="author">AUTHOR</H2>
Copyright (C) 1992 by David Koblas (<A
HREF="mailto:koblas@mips.com">koblas@mips.com</A>).
<HR>
<H2 id="index">Table Of Contents</H2>
<UL>
<LI><A HREF="#synopsis">SYNOPSIS</A>
<LI><A HREF="#description">DESCRIPTION</A>
<LI><A HREF="#options">OPTIONS</A>
<LI><A HREF="#seealso">SEE ALSO</A>
<LI><A HREF="#history">HISTORY</A>
<LI><A HREF="#author">AUTHOR</A>
</UL>
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