The png2mesh library can convert .png images into adaptive meshes.
In order to do so, it uses the t8code library to build a mesh and refine it at the positions of dark pixels in the original image.
The generated output are 2 .pvtu files that can be visualized with the paraview software. The first .pvtu file stores the original mesh, the second a 2:1 balanced version of that mesh.
You can choose between a quad mesh, triangle mesh or a hybrid mesh containing both shapes.
First of all, using png2mesh makes a lot of fun and offers a non-standard way to look at adaptive mesh refinement.
Additionally, as researchers working with adaptive meshes we often have to present talks about this topic. More than once we were in a situation where we wanted to generate a pictures of a specific adaptive mesh. Usually, this requires hardcoding a refinement rule and comes with a lot of fine-tuning effort to get the desired result.
With png2mesh this task is now as simple as using a drawing program: Open gimp or your editor of choice, draw dark pixels where you want your mesh to be refined, parse the image through png2mesh and you have your example mesh :)
optional: Paraview to visualize the resulting meshes.
Before installing ensure that t8code, p4est, sc and libpng are installed. If t8code, p4est and sc are installed to non-standard directories, the following environment variables must be set:
T8_INCLUDE -- Include directory of t8code, p4est and sc.
T8_LIB -- Library directory containing the t8code, p4est and sc libraries.
If libpng is installed in a non-standard directory (or cannot be found), the following environment variables must be set:
PNG_INCLUDE -- Include directory containing png.h.
PNG_LIB -- Library directory containing the png library.
png2mesh uses CMake.
To build it on a linux environment, perform the following steps:
mkdir buildcd buildcmake ..make
You find some example .png files in the examples/ folder.
To build the png2mesh logo from the file examples/png2mesh_logo.png call
./png2mesh_demo -f examples/png2mesh_logo.png -m9
and look at the created file t8_png_balance_png2mesh_logo.png_Quad.pvtu in Paraview.
To build a reasonable mesh from the heart.png you must set the threshold to >= 255:
png2mesh_demo -t 255 -f examples/heart.png
Use the program as png2mesh_demo [OPTIONS], where OPTIONS can be any of the following command line parameters:
| Option | Parameter | Effect |
|---|---|---|
| -h (--help) | NONE | Display a short help message. |
| -f (--file) | FILENAME | The input png file. |
| -i (--invert) | NONE | Invert the refinement (refine bright areas, not dark). |
| -l (--level) | INT >= 0 | The initial refinement level of the mesh. Default 0. |
| -m (--maxlevel) | INT >= 0 | The maximum allowed refinement level of the mesh. Default 10. |
| -t (--threshold) | INT >= 0 and <= 3 * 255 | How sensitive the refinement reacts to RGB values. The mesh is refined in areas with red + green + blue < threshold. |
If you use png2mesh or pictures generated from it, please cite this github page with Johannes Holke as the author. We would apprechiate a citation of t8code as well.
