Generating a mesh in Unity

Before writing about split mesh optimization I would like share a simple example that generates a mesh from scratch in Unity. The example is in C# and for Unity since I use it in my project 🙂

Generating a mesh allows manipulating of existing scene meshes or creation of new ones. For example, when splitting a mesh in half we have to modify the mesh of the original game object (since it is ”losing” a part) and create a new mesh for a new GameObject (the detached part).

In the following example I create a pyramid mesh and a GameObject to bring that mesh to the scene. We start by creating a new Mesh object:

 Mesh newMesh = new Mesh();

Once we have a mesh we have to define its geometry. A mesh consists of one or more triangles. A triangle is formed by joining three vertices. So we need a bunch of vertices to create triangles. To create a pyramid we need four vertices: three for the base and one for the tip:

Vector3[] vertices = new Vector3[4];

// Pyramid base vertices
vertices[0] = new Vector3(-1.0f, 0.0f, 0.0f);
vertices[1] = new Vector3(0.0f, 0.0f, -1.5f);
vertices[2] = new Vector3(1.0f, 0.0f, 0.0f);

// Pyramid tip vertex
vertices[3] = new Vector3(0.0f, 1.0f, -0.8f);

Now we have vertices, let’s create the triangles! A common practice to specify triangle vertices is with indices that point to the vertex array:

int numTriangles = 4;
int[] triangleIndices = new int[3 * numTriangles];

// Botton triangle
triangleIndices[0] = 0;
triangleIndices[1] = 1;
triangleIndices[2] = 2;

// Side triangles
triangleIndices[3] = 0;
triangleIndices[4] = 3;
triangleIndices[5] = 1;

triangleIndices[6] = 1;
triangleIndices[7] = 3;
triangleIndices[8] = 2;

triangleIndices[9] = 2;
triangleIndices[10] = 3;
triangleIndices[11] = 0;

Now we have one array that contains the vertex positions and one array that specifies the triangles. Next we have assign them to the mesh:

newMesh.vertices = vertices;
newMesh.triangles = triangleIndices;

Important: as far as I know the size of Mesh.triangles has to be divisible by three.

To bring the new mesh to the scene we need the following:

GameObject pyramidGameObject = new GameObject("PeteThePyramid");
pyramidGameObject.AddComponent<MeshRenderer>();
MeshFilter mf = pyramidGameObject.AddComponent<MeshFilter>();
mf.mesh = newMesh;

MeshRenderer component is needed so we actually see the mesh 🙂 (although without assigning a material to it we see the default magenta ”no material” color when pressing play). MeshFilter binds the newly created mesh to this GameObject.

The triangle that the code above creates should look something like this (wireframe mode in Scene view, paused):

Untitled3

I GIMPed the numbers on top of the triangle to show which vertices (of the array ”vertices”) are which.

When defining triangles with vertex indices the order of the vertices is VERY important. In Unity triangles are defined by listing the vertices in clockwise order (from the camera viewpoint). So the visible triangles in the picture above would be 0-3-1, 1-3-2 and 2-3-0.

If we define the last triangle in counter-clockwise order like this…

triangleIndices[9] = 0;
triangleIndices[10] = 3;
triangleIndices[11] = 2;

…the winding order of the triangle would be reversed and the triangle would not be visible:

Untitled

So be careful when defining triangles!

Now we have a simple pyramid mesh in the scene, cool! As you see generating a mesh is fairly simple but I wanted to post this since there cannot be too many examples 🙂

THE END

Special bonus to make thing more herp derp:

MeshCollider mc = pyramidGameObject.AddComponent<MeshCollider>();
mc.convex = true;
mc.sharedMesh = newMesh;
Rigidbody rb = pyramidGameObject.AddComponent<Rigidbody>();

As you probably guessed this makes the mesh a collider and adds a RigidBody component to the new GameObject. A view of the MeshCollider:

Untitled4

Problem: coplanar neighbor triangles

Splitting an already split mesh just by splitting all of the triangles that intersect the splitting plane and outputting a new set of triangles creates an bunch of new triangles that are adjacent to each other and are on the same plane.

problem
On the left: the original cube. Center: cube split in two – this is still ok. On the right: splitting the bottom part of the cube again creates triangles that could be joined to their neighbors.

These triangles are essentially useless since in most cases they can be joined to their neighboring triangles. This optimization is not mandatory but the more your split your meshes the more messier it gets. The picture below illustrates one clear candidate for optimization.

5
Meshes containing very thin triangles caused by non-optimized splitting algorithm.

In the next articles I will explain a simple optimization technique which will hopefully reduce the number of triangles in you scene significantly.

Triangle splitting and polygon triangulation

Triangle splitting

The first problem I needed to solve was how to split a mesh (at least a convex one) in two with a plane. This will hopefully be a helpful tool for severing arms, legs and heads 🙂

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A simple cube and a split plane

I started by splitting all the mesh triangles that intersect the splitting plane. The splitting algorithm I wrote is based on two very helpful articles written by Randy Gaul:

After the split we have a bunch of triangles that are on one side of the plane and a bunch of triangles that are on the other side of the plane. One could use these triangles to output two meshes but they would be missing a face (the intersection area).

Filling ”the gap”

Intersection polygon

So both of the meshes (or ”bunch of triangles”) created have an intersection area that has to be filled. This can be done by calculating a polygon from the intersection area and triangulating that into a face.

Triangles that are ”touching” the intersection plane have vertices that are on the intersection plane. From these vertices, I calculated the longest ”streak” of vertices that are connected to each other (with an edge) trying to ”get around” the intersection area. Hopefully this forms an edge loop (first and last vertices are connected) and we have the polygon that we can fill.

Triangulation

Filling a polygon means (in this context) triangulating the polygon. I used an simple polygon ear clipping method described in Ear-clipping Based Algorithms of Generating High-quality Polygon Triangulation.

Splitting a mesh and triangulating the intersection area were my first steps in this coding adventure. The first implementation was not very robust and needed many optimizations to reduce excess triangle creation. I will try to write about them in the following articles.

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Cube split in two (meshes)
4
Triangulated intersection area in a cylinder showing very ”thin” triangles caused by the ear clipping method

Dev blog!

Since The Incredible Machine, I’ve always been fascinated by games that simulate real life physics. For some twisted reason I also like games where you can cut, slice and crush your enemies in various ways (e.g. SOF and Carmageddon 2 to name a few). In Gang beasts you can control a ragdoll character and punch and throw other players around. However, it lacks the blood and gore 🙂

Last year I visited the Game Developers Conference in San Francisco. One of the most interesting presentations (for me) there was held by Dennis Gustafsson. He talked about on how they implemented glass breaking physics and dynamically created meshes in the mobile game Smash Hit. This motivated me to make start a new game project where you can break stuff.

This blog is a development diary for the game. I try to post progress reports but also talk about some of the problems I face when implementing the game and how to I try to solve them.

Herp. vs. Derp (working title, maybe…) is a 1 vs 1 3rd person fighting game where you can control your players limbs separately (like in Gang Beasts or Octodad). You can also use weapons to wound your opponent or, if lucky, cut their limbs off in a spectacle of blood and agony.