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.
png2mesh will automatically search for installations of t8code, p4est and sc. If they are not found or you want to use a non-standard directory, you can provide the paths manually by setting -DT8CODE_DIR=PATH, -DP4EST_DIR=PATH, -DSC_DIR=PATH,
Note the PATH must point to a folder containing T8CODEConfig.cmake, P4ESTConfig,cmake or SCConfig.cmake respectively.
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.
