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Tux Paint
version 0.9.16
Advanced Staps HOWTO
Copyright 2006 by Albert Cahalan for the Tux Paint project
New Breed Software
albert@users.sf.net
http://www.newbreedsoftware .com/tuxpaint/
March 8, 2006 - March 8, 2006
About this HOWTO
This HOWTO assumes that you want to make an excellent Tux Paint stamp
from a JPEG image. There are easier and faster methods that produce
lower quality.
This HOWTO assumes you are dealing with normal opaque objects. Dealing
with semi-transparent objects (fire, moving fan blade, kid's baloon) or
light-giving objects (fire, lightbulb, sun) is best done with custom
software. Images with perfect solid-color backgrounds are also best done
with custom software, but are not troublesome to do as follows.
Image choice is crucial
License
If you wish to submit artwork to the Tux Paint developers for
consideration for inclusion in the official project, or if you wish to
release your own copy of Tux Paint, bundled with your own graphics,
you need an image that is compatible with the GNU General Public
License used by Tux Paint.
Images produced by the US government are Public Domain, but be aware
that the US government sometimes uses other images on the web. Google
image queries including either site:gov or site:mil will supply many
suitable images. (Note: the *.mil sites include non-military content,
too!)
Your own images can be placed in the Public Domain by declaring it so.
(Hire a lawyer if you feel the need for legal advice.)
For personal use, any image you can legitimately modify and use for
your own personal use should be fine.
Image Size and Orientation:
You need an image that has a useful orientation. Perspective is an
enemy. Images that show an object from the corner are difficult to fit
into a nice drawing. As a general rule, telephoto side views are the
best. The impossible ideal is that, for example, two wheels of a car
are perfectly hidden behind the other two.
Rotating an image can make it blurry, especially if you only rotate by
a few degrees. Images that don't need rotation are best, images that
need lots of rotation (30 to 60 degrees) are next best, and images
that need just a few degrees are worst. Rotation will also make an
image darker because most image editing software is very bad about
gamma handling. (Rotation is only legitimate for gamma=1.0 images.)
Very large images are more forgiving of mistakes, and thus easier to
work with. Choose an image with an object that is over 1000 pixels
across if you can. You can shrink this later to hide your mistakes.
Be sure that the image is not too grainy, dim, or washed out.
Pay attention to feet and wheels. If they are buried in something, you
will need to draw new ones. If only one is buried, you might be able
to copy the other one as a replacement.
Prepare the image:
First of all, be sure to avoid re-saving the image as a JPEG. This
causes quality loss. There is a special tool called jpegtran that lets
you crop an image without the normal quality loss. If you want a GUI for
it, use ljcrop. Otherwise, use it like this:
jpegtran -trim -copy none -crop 512x1728+160+128 < src.jpg >
cropped.jpg
Bring that image up in your image editor. If you didn't crop it yet, you
may find that your image editor is very slow. Rotate and crop the image
as needed. Save the image -- choose whatever native format supports
layers, masks, alpha, etc. GIMP users should choose "XCF", and Adobe
Photoshop users should choose "PSD", for example.
If you have rotated or cropped the image in your image editor, flatten
it now. You need to have just one RGB layer without mask or alpha.
Open the layers dialog box. Replicate the one layer several times. From
top to bottom you will need something like this:
1. unmodified image (write-protect this if you can)
2. an image you will modify -- the "WIP" layer
3. solid green (write-protect this if you can)
4. solid magenta (write-protect this if you can)
5. unmodified image (write-protect this if you can)
Give the WIP layer a rough initial mask. You might start with a
selection, or by using the grayscale value of the WIP layer. You might
invert the mask.
Warning: once you have the mask, you may not rotate or scale the image
normally. This would cause data loss. You will be given special scaling
instructions later.
Prepare the mask:
Get used to doing Ctrl-click and Alt-click on the thumbnail images in
the layers dialog. You will need this to control what you are looking at
and what you are editing. Sometimes you will be editing things you can't
see. For example, you might edit the mask of the WIP layer while looking
at the unmodified image. Pay attention so you don't screw up. Always
verify that you are editing the right thing.
Set an unmodified image as what you will view (the top one is easiest).
Set the WIP mask as what you will edit. At some point, perhaps not
immediately, you should magnify the image to about 400% (each pixel of
the image is seen and edited as a 4x4 block of pixels on your screen).
Select parts of the image that need to be 100% opaque or 0% opaque. If
you can select the object or background somewhat accurately by color, do
so. As needed to avoid selecting any pixels that should be partially
opaque (generally at the edge of the object) you should grow, shrink,
and invert the selection.
Fill the 100% opaque areas with white, and the 0% opaque areas with
black. This is most easily done by drag-and-drop from the
foreground/background color indicator. You should not see anything
happen, because you are viewing the unmodified image layer while editing
the mask of the WIP layer. Large changes might be noticable in the
thumbnail.
Now you must be zoomed in.
Check your work. Hide the top unmodified image layer. Display just the
mask, which should be a white object on a black background (probably
with unedited grey at the edge). Now display the WIP layer normally, so
that the mask is active. This should show your object over top of the
next highest enabled layer, which should be green or magenta as needed
for maximum contrast. You might wish to flip back and forth between
those backgrounds by repeatedly clicking to enable/disable the green
layer. Fix any obvious and easy problems by editing the mask while
viewing the mask.
Go back to viewing the top unmodified layer while editing the WIP mask.
Set your drawing tool the paintbrush. For the brush, choose a small
fuzzy circle. The 5x5 size is good for most uses.
With a steady hand, trace around the image. Use black around the
outside, and white around the inside. Avoid making more than one pass
without switching colors (and thus sides).
Flip views a bit, checking to see that the mask is working well. When
the WIP layer is composited over the green or magenta, you should see a
tiny bit of the original background as an ugly fringe around the edge.
If this fringe is missing, then you made the object mask too small. The
fringe consists of pixels that are neither 100% object nor 0% object.
For them, the mask should be neither 100% nor 0%. The fringe gets
removed soon.
View and edit the mask. Select by color, choosing either black or white.
Most likely you will see unselected specks that are not quite the
expected color. Invert the selection, then paint these away using the
pencil tool. Do this operation for both white and black.
Replace the fringe and junk pixels:
Still viewing the mask, select by color. Choose black. Shrink the
selection by several pixels, being sure to NOT shrink from the edges of
the mask (the shrink helps you avoid and recover from mistakes).
Now disable the mask. View and edit the unmasked WIP layer. Using the
color picker tool, choose a color that is average for the object.
Drag-and-drop this color into the selection, thus removing most of the
non-object pixels.
This solid color will compress well and will help prevent ugly color
fringes when Tux Paint scales the image down. If the edge of the object
has multiple colors that are very different, you should split up your
selection so that you can color the nearby background to be similar.
Now you will paint away the existing edge fringe. Be sure that you are
editing and viewing the WIP image. Frequent layer visibility changes
will help you to see what you are doing. You are likely to use all of:
* composited over green (mask enabled)
* composited over magenta (mask enabled)
* original (the top or bottom layer)
* composited over the original (mask enabled)
* raw WIP layer (mask DISABLED)
To reduce accidents, you may wish to select only those pixels that are
not grey in the mask. (Select by color from the mask, choose black, add
mode, choose white, invert. Alternately: Select all, select by color
from the mask, subtract mode, choose black, choose white.) If you do
this, you'll probably want to expand the selection a bit and/or hide the
"crawling ants" line that marks the selection.
Use the clone tool and the brush tool. Vary the opacity as needed. Use
small round brushes mostly, perhaps 3x3 or 5x5, fuzzy or not. (It is
generally nice to pair up fuzzy brushes with 100% opacity and non-fuzzy
brushes with about 70% opacity.) Unusual drawing modes can be helpful
with semi-transparent objects.
The goal is to remove the edge fringe, both inside and outside of the
object. The inside fringe, visible when the object is composited over
magenta or green, must be removed for obvious reasons. The outside
fringe must also be removed because it will become visible when the
image is scaled down. As an example, consider a 2x2 region of pixels at
the edge of a sharp-edged object. The left half is black and 0% opaque.
The right half is white and 100% opaque. That is, we have a white object
on a black background. When Tux Paint scales this to 50% (a 1x1 pixel
area), the result will be a grey 50% opaque pixel. The correct result
would be a white 50% opaque pixel. To get this result, we would paint
away the black pixels. They matter, despite being 0% opaque.
Tux Paint can scale images down by a very large factor, so it is
important to extend the edge of your object outward by a great deal.
Right at the edge of your object, you should be very accurate about
this. As you go outward away from the object, you can get a bit sloppy.
It is reasonable to paint outward by a dozen pixels or more. The farther
you go, the more Tux Paint can scale down without creating ugly color
fringes. For areas that are more than a few pixels away from the object
edge, you should use the pencil tool (or sloppy select with
drag-and-drop color) to ensure that the result will compress well.
Save the image for Tux Paint
It is very easy to ruin your hard work. Image editors can silently
destroy pixels in 0% opaque areas. The conditions under which this
happens may vary from version to version. If you are very trusting, you
can try saving your image directly as a PNG. Be sure to read it back in
again to verify that the 0% opaque areas didn't turn black or white,
which would create fringes when Tux Paint scales the image down. If you
need to scale your image to save space (and hide your mistakes), you are
almost certain to destroy all the 0% opaque areas. So here is a better
way...
A Safer Way to Save:
Drag the mask from the layers dialog to the unused portion of the
toolbar (right after the last drawing tool). This will create a new
image consisting of one layer that contains the mask data. Scale this
as desired, remembering the settings you use. Often you should start
with an image that is about 700 to 1500 pixels across, and end up with
one that is 300 to 400.
Save the mask image as a NetPBM portable greymap (".pgm") file. (If
you are using an old release of The GIMP, you might need to convert
the image to greyscale before you can save it.) Choose the more
compact "RAW PGM" format. (The second character of the file should be
the ASCII digit "5", hex byte 0x35.)
You may close the mask image.
Going back to the multi-layer image, now select the WIP layer. As you
did with the mask, drag this from the layers dialog to the toolbar.
You should get a single-layer image of your WIP data. If the mask came
along too, get rid of it. You should be seeing the object and the
painted-away surroundings, without any mask thumbnail in the layers
dialog. If you scaled the mask, then scale this image in exactly the
same way. Save this image as a NetPBM portable pixmap (".ppm") file.
(Note: ppm, not pgm.) (If you choose the RAW PPM format, the second
byte of the file should be the ASCII digit "6", hex byte 0x36.)
Now you need to merge the two files into one. Do that with the
pnmtopng command, like this:
pnmtopng -force -compression 9 -alpha mask.pgm fg.ppm >
final-stamp.png

4
docs/CHANGES.txt
View file

@ -86,6 +86,10 @@ $Id$
---------------------------
* Moved "Extending Tux Paint" into its own separate document.
* Created an Advanced Stamps HOWTO (documenting how to create
stamp PNGs that scale well without edge fringing).
Albert Cahalan <albert@users.sf.net>
* Compiling, porting and packaging updates:
-----------------------------------------
* Split parts of "tuxpaint.c" into separate source files:

7
docs/Makefile
View file

@ -12,13 +12,14 @@
LINKS=links -dump -no-numbering -no-references
all: README.txt OPTIONS.txt FAQ.txt EXTENDING.txt
all: README.txt OPTIONS.txt FAQ.txt EXTENDING.txt ADVANCED-STAMPS-HOWTO.txt
clean:
-rm README.txt
-rm OPTIONS.txt
-rm FAQ.txt
-rm EXTENDING.txt
-rm ADVANCED-STAMPS-HOWTO.txt
README.txt: html/README.html
$(LINKS) $< > $@
@ -32,3 +33,7 @@ FAQ.txt: html/FAQ.html
EXTENDING.txt: html/EXTENDING.html
$(LINKS) $< > $@
ADVANCED-STAMPS-HOWTO.txt: html/ADVANCED-STAMPS-HOWTO.html
$(LINKS) $< > $@

331
docs/html/ADVANCED-STAMPS-HOWTO.html Normal file
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@ -0,0 +1,331 @@
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html><head><title>Tux Paint Advanced Stamps HOWTO</title>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
</head>
<body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#FF0000"
alink="#FF00FF">
<center>
<h1><img src="images/tuxpaint-title.png" width=220 height=219
alt="Tux&nbsp;Paint"><br>
version
0.9.16
<br>
Advanced Staps HOWTO</h1>
<p>Copyright 2006 by Albert Cahalan for the Tux Paint project<br>
New Breed Software</p>
<p><a href="mailto:albert@users.sf.net">albert@users.sf.net</a><br>
<a href="http://www.newbreedsoftware.com/tuxpaint/">http://www.newbreedsoftware
.com/tuxpaint/</a></p>
<p>March 8, 2006 - March 8, 2006</p>
</center>
<h2>About this HOWTO</h2>
<blockquote>
<p>This HOWTO assumes that you want to make an excellent Tux&nbsp;Paint
stamp from a JPEG image. There are easier and faster methods that
produce lower quality.</p>
<p>This HOWTO assumes you are dealing with normal opaque objects. Dealing
with semi-transparent objects (fire, moving fan blade, kid's baloon)
or light-giving objects (fire, lightbulb, sun) is best done with custom
software. Images with perfect solid-color backgrounds are also best done
with custom software, but are not troublesome to do as follows.</p>
</blockquote>
<h2>Image choice is crucial</h2>
<blockquote>
<h3>License</h3>
<blockquote>
<p>If you wish to submit artwork to the Tux&nbsp;Paint developers for
consideration for inclusion in the official project, or if you wish to
release your own copy of Tux&nbsp;Paint, bundled with your own graphics,
you need an image that is compatible with the GNU
<a href="../COPYING.txt">General Public License</a> used by
Tux&nbsp;Paint.</p>
<p>Images produced by the US government are Public Domain, but be aware
that the US government sometimes uses other images on the web.
<a href="http://images.google.com/">Google image</a> queries including
either <code>site:gov</code> or <code>site:mil</code> will supply many
suitable images. (Note: the *.mil sites include non-military content,
too!)</p>
Your own images can be placed in the Public&nbsp;Domain by declaring it
so. (Hire a lawyer if you feel the need for legal advice.)</p>
<p>For personal use, any image you can legitimately modify and use
for your own personal use should be fine.</p>
</blockquote>
<h3>Image Size and Orientation:</h3>
<blockquote>
<p>You need an image that has a useful orientation. Perspective is
an enemy. Images that show an object from the corner are difficult to
fit into a nice drawing. As a general rule, telephoto side views are
the best. The impossible ideal is that, for example, two wheels of a
car are perfectly hidden behind the other two.</p>
<p>Rotating an image can make it blurry, especially if you only rotate by
a few degrees. Images that don't need rotation are best, images that need
lots of rotation (30 to 60 degrees) are next best, and images that need
just a few degrees are worst. Rotation will also make an image darker
because most image editing software is very bad about gamma handling.
(Rotation is only legitimate for gamma=1.0 images.)</p>
<p>Very large images are more forgiving of mistakes, and thus easier to
work with. Choose an image with an object that is over 1000 pixels
across if you can. You can shrink this later to hide your mistakes.</p>
<p>Be sure that the image is not too grainy, dim, or washed out.</p>
<p>Pay attention to feet and wheels. If they are buried in something,
you will need to draw new ones. If only one is buried, you might be
able to copy the other one as a replacement.</p>
</blockquote>
</blockquote>
<h2>Prepare the image:</h2>
<blockquote>
<p>First of all, be sure to avoid re-saving the image as a JPEG. This causes
quality loss. There is a special tool called
<a href="http://gnuwin32.sourceforge.net/packages/jpeg.htm">jpegtran</a>
that lets you crop an image without the normal quality loss. If you want a
GUI for it, use
<a href="http://astron.berkeley.edu/~mperrin/software/ljcrop/">ljcrop</a>.
Otherwise, use it like this:</p>
<blockquote>
<p><code>jpegtran -trim -copy none -crop 512x1728+160+128 &lt; src.jpg
&gt; cropped.jpg</code></p>
</blockquote>
<p>Bring that image up in your image editor. If you didn't crop it yet,
you may find that your image editor is very slow. Rotate and crop the
image as needed. Save the image &mdash; choose whatever native format
supports layers, masks, alpha, etc. <a href="http://www.gimp.org/">GIMP</a>
users should choose "XCF", and Adobe Photoshop users should choose "PSD",
for example.</p>
<p>If you have rotated or cropped the image in your image editor, flatten
it now. You need to have just one RGB layer <i>without mask or
alpha</i>.</p>
<p>Open the layers dialog box. Replicate the one layer several times.
From top to bottom you will need something like this:</p>
<ol>
<li>unmodified image (write-protect this if you can)
<li>an image you will modify &mdash; the "WIP" layer
<li>solid green (write-protect this if you can)
<li>solid magenta (write-protect this if you can)
<li>unmodified image (write-protect this if you can)
</ol>
<p>Give the WIP layer a rough initial mask. You might start with a
selection, or by using the grayscale value of the WIP layer. You might
invert the mask.</p>
<p><b>Warning:</b> once you have the mask, you may not rotate or
scale the image normally. This would cause data loss. You will be
given special scaling instructions later.</p>
</blockquote>
<h2>Prepare the mask:</h2>
<blockquote>
<p>Get used to doing Ctrl-click and Alt-click on the thumbnail images in the
layers dialog. You will need this to control what you are looking at and
what you are editing. Sometimes you will be editing things you can't see.
For example, you might edit the mask of the WIP layer while looking at the
unmodified image. Pay attention so you don't screw up. Always verify that
you are editing the right thing.</p>
<p>Set an unmodified image as what you will view (the top one is easiest).
Set the WIP mask as what you will edit. At some point, perhaps not
immediately, you should magnify the image to about 400% (each pixel of
the image is seen and edited as a 4x4 block of pixels on your screen).</p>
<p>Select parts of the image that need to be 100% opaque or 0% opaque.
If you can select the object or background somewhat accurately by
color, do so. As needed to avoid selecting any pixels that should be
partially opaque (generally at the edge of the object) you should
grow, shrink, and invert the selection.</p>
<p>Fill the 100% opaque areas with white, and the 0% opaque areas with
black. This is most easily done by drag-and-drop from the
foreground/background color indicator. You should not see anything happen,
because you are viewing the unmodified image layer while editing the mask
of the WIP layer. Large changes might be noticable in the thumbnail.</p>
<p>Now you must be zoomed in.</p>
<p>Check your work. Hide the top unmodified image layer. Display just the
mask, which should be a white object on a black background (probably
with unedited grey at the edge). Now display the WIP layer normally, so
that the mask is active. This should show your object over top of the
next highest enabled layer, which should be green or magenta as needed
for maximum contrast. You might wish to flip back and forth between
those backgrounds by repeatedly clicking to enable/disable the green
layer. Fix any obvious and easy problems by editing the mask while
viewing the mask.</p>
<p>Go back to viewing the top unmodified layer while editing the WIP mask.
Set your drawing tool the paintbrush. For the brush, choose a small fuzzy
circle. The 5x5 size is good for most uses.</p>
<p>With a steady hand, trace around the image. Use black around the outside,
and white around the inside. Avoid making more than one pass without
switching colors (and thus sides).</p>
<p>Flip views a bit, checking to see that the mask is working well. When
the WIP layer is composited over the green or magenta, you should see a
tiny bit of the original background as an ugly fringe around the edge.
If this fringe is missing, then you made the object mask too small.
The fringe consists of pixels that are neither 100% object nor 0% object.
For them, the mask should be neither 100% nor 0%. The fringe gets removed
soon.</p>
<p>View and edit the mask. Select by color, choosing either black or white.
Most likely you will see unselected specks that are not quite the expected
color. Invert the selection, then paint these away using the pencil tool.
Do this operation for both white and black.</p>
</blockquote>
<h2>Replace the fringe and junk pixels:</h2>
<blockquote>
<p>Still viewing the mask, select by color. Choose black. Shrink the
selection by several pixels, being sure to NOT shrink from the edges of
the mask (the shrink helps you avoid and recover from mistakes).</p>
<p>Now disable the mask. View and edit the unmasked WIP layer. Using the
color picker tool, choose a color that is average for the object.
Drag-and-drop this color into the selection, thus removing most of the
non-object pixels.</p>
<p>This solid color will compress well and will help prevent ugly color
fringes when Tux&nbsp;Paint scales the image down. If the edge of the
object has multiple colors that are very different, you should split up
your selection so that you can color the nearby background to be
similar.</p>
<p>Now you will paint away the existing edge fringe. Be sure that you are
editing and viewing the WIP image. Frequent layer visibility changes will
help you to see what you are doing. You are likely to use all of:</p>
<ul>
<li>composited over green (mask enabled)
<li>composited over magenta (mask enabled)
<li>original (the top or bottom layer)
<li>composited over the original (mask enabled)
<li>raw WIP layer (mask DISABLED)
</ul>
<p>To reduce accidents, you may wish to select only those pixels that are
not grey in the mask. (Select by color from the mask, choose black, add
mode, choose white, invert. Alternately: Select all, select by color from
the mask, subtract mode, choose black, choose white.) If you do this,
you'll probably want to expand the selection a bit and/or hide the
"crawling ants" line that marks the selection.</p>
<p>Use the clone tool and the brush tool. Vary the opacity as needed.
Use small round brushes mostly, perhaps 3x3 or 5x5, fuzzy or not.
(It is generally nice to pair up fuzzy brushes with 100% opacity and
non-fuzzy brushes with about 70% opacity.) Unusual drawing modes can be
helpful with semi-transparent objects.</p>
<p>The goal is to remove the edge fringe, both inside and outside of
the object. The inside fringe, visible when the object is composited
over magenta or green, must be removed for obvious reasons. The
outside fringe must also be removed because it will become visible
when the image is scaled down. As an example, consider a 2x2 region of
pixels at the edge of a sharp-edged object. The left half is black
and 0% opaque. The right half is white and 100% opaque. That is, we
have a white object on a black background. When Tux&nbsp;Paint scales this
to 50% (a 1x1 pixel area), the result will be a grey 50% opaque pixel.
The correct result would be a white 50% opaque pixel. To get this
result, we would paint away the black pixels. They matter, despite
being 0% opaque.</p>
<p>Tux&nbsp;Paint can scale images down by a very large factor, so it is
important to extend the edge of your object outward by a great deal.
Right at the edge of your object, you should be very accurate about this.
As you go outward away from the object, you can get a bit sloppy. It is
reasonable to paint outward by a dozen pixels or more. The farther you go,
the more Tux&nbsp;Paint can scale down without creating ugly color fringes.
For areas that are more than a few pixels away from the object edge, you
should use the pencil tool (or sloppy select with drag-and-drop color) to
ensure that the result will compress well.</p>
</blockquote>
<h2>Save the image for Tux Paint</h2>
<blockquote>
<p>It is very easy to ruin your hard work. Image editors can silently
destroy pixels in 0% opaque areas. The conditions under which this
happens may vary from version to version. If you are very trusting,
you can try saving your image directly as a PNG. Be sure to read it
back in again to verify that the 0% opaque areas didn't turn black or
white, which would create fringes when Tux&nbsp;Paint scales the image down.
If you need to scale your image to save space (and hide your mistakes), you
are almost certain to destroy all the 0% opaque areas. So here is a better
way...</p>
<h3>A Safer Way to Save:</h3>
<blockquote>
<p>Drag the mask from the layers dialog to the unused portion of
the toolbar (right after the last drawing tool). This will create a
new image consisting of one layer that contains the mask data. Scale
this as desired, remembering the settings you use. Often you should
start with an image that is about 700 to 1500 pixels across, and end
up with one that is 300 to 400.</p>
<p>Save the mask image as a NetPBM portable greymap ("<code>.pgm</code>")
file. (If you are using an old release of The&nbsp;GIMP, you might need
to convert the image to greyscale before you can save it.) Choose the
more compact "RAW&nbsp;PGM" format. (The second character of the file
should be the ASCII digit "5", hex byte 0x35.)</p>
<p>You may close the mask image.</p>
<p>Going back to the multi-layer image, now select the WIP layer. As you
did with the mask, drag this from the layers dialog to the toolbar. You
should get a single-layer image of your WIP data. If the mask came along
too, get rid of it. You should be seeing the object and the painted-away
surroundings, without any mask thumbnail in the layers dialog. If you
scaled the mask, then scale this image in exactly the same way. Save
this image as a NetPBM portable pixmap ("<code>.ppm</code>") file.
(Note: ppm, not pgm.) (If you choose the RAW&nbsp;PPM format, the
second byte of the file should be the ASCII digit "6", hex byte 0x36.)</p>
<p>Now you need to merge the two files into one. Do that with the
<a href="http://netpbm.sourceforge.net/">pnmtopng</a> command, like
this:</p>
<blockquote><p><code>
pnmtopng -force -compression 9 -alpha mask.pgm fg.ppm &gt;
final-stamp.png
</code></p></blockquote>
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