Commodore Free 10/Commodore_Free_Issue_10_2007_Commodore_Computer_Club.d64/t.potrace

Text File | 2023-02-26 | 10KB | 341 lines

u               Potrace
   Open source tracing application
      Frequently Asked Questions

My first name for this program was
"pbm2eps", which was very lame and
also inaccurate, because both the
input and output file format might
evolve over time. So I started looking
for a cooler name.

Since this program does tracing, I
wanted a name with the word "trace" in
it. I did a Google seach for each
combination "atrace"..."ztrace" of a
single letter plus "trace". Believe it
or check for yourself: every single
one of them was already taken by some
other software!

Most of them were something boring,
like the name of a function in some
obscure statistics package written in
Fortran. If I remember correctly, the
only exception was "ytrace", which
brought up a foreign language porn
site called "sexo con perros"

I am not sure what "ytrace" might mean
in some foreign language, but I have a
pretty good idea what "sexo con
perros" means, and so I didn't want to
take any risks giving this name to my
program.

So I started looking for 2-letter
combinations. It so happens that
"potrace" didn't yield any hits on
Google, except a few sites that had
misspelled the word "podrace" (as in
pod-racing) from Star Wars Episode I.
This was okay, I thought, and would
not attract any trademark infringement
suits. So I settled for the name
Potrace. I particularly like this name
because it can also mean
"polygon-based tracer", which actually
describes quite accurately what
Potrace does.

By the way, it is pronounced
"po-trace", not "pot-race". If it
conjurs up mental images of people
running around a track with cooking
pots, then you are not pronouncing it
correctly.

Question: Is there a mailing list? How
can I be notified of new releases?

Answer: There is currently no Potrace
mailing list. However, Potrace has a
project page at Sourceforge.net. This
page provides a discussion forum, as
well as a mechanism for reporting
bugs. To be automatically notified of
new Potrace releases, go to the
Potrace project page on
Sourceforge.net, find the section on
"Latest File Releases" and click on
the little envelope icon under
"Notes/Monitor" Basic usage and errors

Question: Can you show me the options
used to generate the examples pictures
on the Potrace website? I tried to do
it myself, but failed.

Answer: All the examples were
generated with the default settings.
The EPS files were generated by the
following command: cat file.pbm \
potrace > file.eps but the following
also works: potrace file.pbm

The input to Potrace in each of these
examples was a PBM file. But since
most browsers can't display PBM, I
posted the images in PNG format on the
website, and I also scaled them. You
need to shift-click on the left-hand
side image to download each original
PBM file.

The greymaps were generated by this
command:

cat file.pbm \ potrace -gx2

This generates a PGM file. Again, for
posting on the web, I have further
translated this to the PNG format by
piping the output through pnmtopng.
For experts only: If you want to know
exactly how I processed these files,
you can look at the Makefile that I
used.

Question: I installed and ran Potrace,
and it did not work. I got the error
message: sh: line 1: compress: command
not found. What am I doing wrong?

Answer: This error occurs only with
Potrace version 1.3 or earlier. When
this happens, you should either
install the newest version of Potrace,
or else run your existing version with
the -c option, and everything should
work fine.

Question: How can I improve the
quality of the output of Potrace?

Answer: The most effective way to do
so is to improve the quality of the
input! This seems obvious, but not
everybody thinks of this. Scan your
image at a higher resolution. Scan
your image in greyscale, and use a
program such as mkbitmap to generate a
high-resolution bitmap to trace.

In some cases, it also helps to
decrease the resolution of the input
image. If the resolution of your input
file is incredibly high, the output
generated by Potrace might be larger
than necessary. This also sometimes
happens if the input is speckly or
noisy. Rather than trying to simplify
the vector image, it often helps to
just downscale the input bitmap.

Question: How can I reduce the number
of nodes in the output of Potrace?

Answer: Potrace already makes an
attempt to reduce the number of nodes,
and this behavior is controlled by the
--opttolerance parameter (or can be
turned off entirely with --longcurve).
However, I have decided not to
sacrifice quality for quantity, and
Potrace will not reduce the number of
nodes beyond what it considers
tolerable.

For many applications, the problem of
reducing output file size can be
solved by reducing the resolution of
the input image. This sounds too
simple to be true, but can give
surprisingly good results. One of the
most common "beginner's mistakes" when
using Potrace is to start with an
image that contains too much detail or
noise. Blurring the input image can
also help, particularly with grey-
scale images.

3. File formats Question: Why can
Potrace not read/write PNG files? It
is such a popular file format.

Answer: In keeping with the old unix
philosophy "write programs that do
only one thing and do it well", I am
trying to avoid writing a front end
for every possible graphics file
format known to mankind. Instead, I
provide support for asmall number of
generic formats which are easy to
convert to and from.

The PNG format is relatively complex
to implement, and there already exist
many excellent programs for converting
between PNG and the PNM format which
Potrace can read. See the next
question.

Question: What programs can read/write
PBM files on Linux? Gimp does not seem
to help me... How can I convert PNG to
the formats that Potrace can read?

Answer: Gimp, unfortunately, cannot
write PBM files (although it can read
them) .However, Gimp can write PNM
files, as well as BMP files. Both file
formats are understood by Potrace
version 1.1 or later. Note that the
files have to be bitmaps, i.e., they
should only use two colors black and
white. All other colors or grey values
will be converted to black and white
before Potrace begins its processing.

A good program for converting anything
to PBM is the convert command line
utility of ImageMagick. It is invoked
simply as "convert inputfile
outputfile" . It is pre-installed on
many systems, and can otherwise be
obtained from www.imagemagick.org. The
output format is determined by the
filename extension, so for converting
to pbm, you simply type:

convert image.png image.pbm

Another very useful such set of
command line utilities for converting
different image formats are the
"netpbm" utilities, see
netpbm.sourceforge.net. Again, these
tools are preinstalled on many popular
systems. They allow you to convert
between file formats on the command
line. Particularly useful are pngtopnm
and pnmtopng, but you can also convert
to and from other formats using e.g.
giftopnm, anytopnm and so forth. You
can use these commands in a pipeline,
for instance

cat file.png \ pngtopnm \ potrace >
file.eps

Question: I am working to add Potrace
support to another program, but the
output of Potrace leaves me a little
stumped. How can I parse the output of
Potrace into some format which my
program can use?

Answer: To get parseable output from
Potrace, I suggest using the -q and -
c options. The default output is
optimized for size, not readability.
With 'potrace -qc', you should get
exactly what you need.

If you use the output from Potrace for
processing by another program, I also
recommend increasing the output
resolution, e.g., -u=100 or -u=1000,
to reduce the effect of roundoff
errors. Note that the output
coordinates correspond to input
(bitmap) coordinates, multiplied by
the unit length, then rounded to
integers.

4. Color Question: Can Potrace handle
color images?

Answer: The short answer is "no".
Potrace can only handle 2-valued
images at the moment. It does not
matter whether the two colors are
called "black" and "white" or "on" and
"off" - however, there can be only two
of them.

Question: Will color support be added
to Potrace in the future?

Answer: Maybe.

Question: How can I work around the
lack of color support?

Answer: There are many ways in which
Potrace can be useful in processing
color images, with some extra work.
For example, you can trace an image to
SVG format using the --svg and
--opaque options, and then use e.g.
Sodipodi or Inkscape to color it
manually.

Or you can extract individual color
components from your image using the
Gimp or ppmcolormask (part of the
netpbm package), trace them
separately, and then overlay the
pieces to get a multicolored image.
You can get pretty good results for
posterized images. I have used a
command line similar to this:

cat img.gif \ giftopnm \ ppmcolormask
#641b1b \ potrace

Recent versions of Inkscape have a
built-in Potrace engine that can
handle color images via color
quantization or multiple scanning,
thanks to the great work of Bob
Jamison and the Inkscape team.

Another interesting application of
Potrace to color images is described
in the fascinating article Automatic
Generation of Stained Glass from
Scanned Photos by C. Scott Ananian.
Graphical user interface

 Question: I don't like command line
utilities. Is there a graphical user
interface for Potrace?

Answer: Yes, there are now several
graphical user interfaces (GUI's) for
Potrace, contributed by various other
people. There are GUI's for Linux and
for

Mac OSX. I have not tried them myself,
so I cannot say if they work or if
they have all the features you want. I
suggest to try them out. Please look
under GUI's and related software on
the main Potrace web site.



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