Chop 3D objects to meshlets
Meshlete is a library and command line tool for Windows and Linux, which converts 3D models to meshlet-based 3D models. I developed the tool originally for my tiled software rasterizer and Arduino graphics library hobby project (video), but it can be useful for other applications as well, e.g. DX12-based modern renderers using mesh shaders.
Meshlets are small chunks of 3D geometry consisting some small number of vertices and triangles. Below is an image of a 3D model from Blender called “Suzanne” which is split to 666 meshlets of max 64 vertices in each (-mv 64) and visualized with different colors. Splitting the geometry processing to meshlets instead of processing the mesh as a whole has various benefits and has better fit with batch-based geometry processing (vs post-transform cache model) in modern GPU architectures as well.
The tool outputs the generated data in p3g file format that was developed to be Arduino-friendly and to enable rendering 3D models straight from read-only flash memory without need for run-time post-load data processing. The format is quite straight forward and documented in doc/file_format_p3g.xlsx if you want to parse it. The tool also supports debug output to Collada (dae) file format to visualize the generated meshlets e.g. in Blender. The input 3D model can be in dae, obj, fbx, lwo or 3ds file formats, but be aware that the format parsing can be a bit flaky so try different format if you have problems with one.
For the generated p3g file the tool supports custom vertex formats, which are defined in bin/vfmt.xml file, so you can define for example multiple UV’s, color channels, custom data layout and packing using expressions. There are some predefined formats in the file, but you can add more or define your own vertex format config file (use with -vc argument). When running the tool the used vertex format is defined with -vf argument followed by the name of the format in the config.
An example to split the Suzanne model to meshlets with bounding spheres and visibility cones and 10 bytes/vertex format (packed position+normal):
./meshlete -i ../test_data/suzanne.dae -o suzanne.p3g -vf p48n32 -mb -mc
Or to output debug 3D object to view in Blender with bounding spheres and visibility cones:
./meshlete -i ../test_data/suzanne.dae -do suzanne_debug.dae -vf p48n32 -db -dc
The tool can calculate bounding spheres (-mb and -db options) and visibility cones (-mc and -dc options) for the generated meshlets to help cull away geometry that doesn’t contribute to the final image for given camera view at run-time. The meshlet culling is more fine grained than classic object-level culling and can be done cheaply prior to any meshlet vertex processing thus improving the rendering performance. The storage requirements in p3g file for this culling data are quite small: 32 bits / meshlet for the bounding spheres and 32 bits / meshlet for the cones.
For example if the meshlet bounding sphere is outside given camera FOV, the meshlet geometry processing can be entirely skipped. The spheres can be also used for occlusion culling, i.e. if the sphere is further than previously rasterized depth values, the meshlet processing can be skipped. In my tiled software rasterizer the bounding spheres are tested against rasterized Hi-Z for fast occlusion culling (video). Furthermore, I’m also using the screen extents of the spheres to bin meshlets to tiles so having tight meshlet bounds reduces vertex processing and triangle setup cost.
The image below shows the yellow bounding spheres of meshlets for the Suzanne 3D model. For the tool you can use command line option -db to output the bounding spheres to the debug mesh file defined with -do.
Visibility cone culling is another cheap way to reduce run-time geometry processing of rigid geometry. The cone is stored for each meshlet, which defines a region in space where the meshlet is potentially visible. The check to cull the meshlet is very cheap operation of comparing dot product of cone and camera vectors against precalculated cone angle. A common method to calculate the visibility cone is by fitting a cone to meshlet triangle normals, but this enables meshlet culling only for regions where all meshlet triangles are back facing. The tool does also occlusion calculation for meshlets, which enables intra-object meshlet occlusion testing with visibility cones as well, e.g. meshlets in the inner parts of a cup can be conservatively culled due to the occlusion even if a meshlet has front facing triangles.
The image below visualizes the visibility cones for the Suzanne 3D model with the red and green cones. You can use command line option -dc to output visibility cones to the debug output file defined with -do. For the visualization red cones are used to show visibility reflex cone (i.e. region where the meshlet is invisible) when the cone apex angle is greater than 180 degree, and green cones the region where the meshlet is visible and the angle is <=180 degrees. Note how in model concavities (ears, mouth) the cones are green because of the occlusion.
Meshlete depends on mini_sxp core library, so first you need to clone the library (or copy the zip) to some directory. Then you need to create a "symlink" with the install file, which creates a "virtual" directory inside the project dir that points to the mini_sxp directory. The symlink should appear as regular directory called "mini_sxp" inside the project root dir. Below are install instructions for Windows and Linux. If you have problems with the install, you can try to create the symlink manually from command line (note on Windows you have to use regular command prompt "cmd" and not PowerShell), or if everything else fails, just copy the mini_sxp project in the project dir under "mini_sxp" name.
Run install.bat (as administrator) and type in the directory where you cloned the mini_sxp library (i.e. where mini_sxp README.md file resides). Now you should be able to open meshlete.sln in Visual Studio or use makefile for GCC to compile the library, the command-line tool and the sample. If the created symlink doesn't work, Visual Studio will fail to load mini_sxp library and GCC fails the compilation.
Run install.sh (after enabling execution rights for the script file) and type in the directory where you cloned the mini_sxp library (i.e. where mini_sxp README.md file resides). Now you can compile the project with GCC using the makefile with instructions. If the created symlink doesn't work GCC fails the compilation.
Some planned further improvements (excluding issues) of the library:
- Option to prune completely occluded meshlets [S0]
- Spatial data structure to optimize triangle search in case of unavailable adjacent triangles [S1]
- Reassignment passes to move triangles to more optimal meshlets [S2]
- Option for simplified visibility cone generation purely from normals [S1]
- Support for different heuristics for "the best triangle" to be included to a generated meshlet [S1]
- Sort meshlets by visibility cone angle to render object roughly from outside to inside [S0]
- Option to quantize vertex UVs with object UV bounds [S1]
Meshlete is licensed under MIT license. See LICENSE for details.