# Procedural geometry¶

There are many ways to procedurally generate geometry in Godot. In this tutorial series we will explore a few of them. Each technique has its own benefits and drawbacks, so it is best to understand each one and how it can be useful in a given situation.

## What is geometry?¶

Geometry is a fancy way of saying shape. In computer graphics, geometry is typically represented by an array of positions called "vertices". In Godot, geometry is represented by Meshes.

## What is a Mesh?¶

Many things in Godot have mesh in their name: the Mesh, the ArrayMesh, the MeshInstance, the MultiMesh, and the MultiMeshInstance. While they are all related, they have slightly different uses.

Meshes and ArrayMeshes are resources that are drawn using a MeshInstance node. Resources like Meshes and ArrayMeshes cannot be added to the scene directly. A MeshInstance represents one instance of a mesh in your scene. You can reuse a single mesh in multiple MeshInstances to draw it in different parts of your scene with different materials or transformations (scale, rotation, position etc.).

If you are going to draw the same object many times, it can be helpful to use a MultiMesh with a MultiMeshInstance. MultiMeshInstances draw meshes thousands of times very cheaply by taking advantage of hardware instancing. The drawback with using a MultiMeshInstance is that each of your mesh's surfaces are limited to one material for all instances. It uses an instance array to store different colors and transformations for each instance, but all the instances of each surface use the same material.

## What a Mesh is¶

A Mesh is composed of one or more surfaces. A surface is an array composed of multiple sub-arrays containing vertices, normals, UVs, etc. Normally the process of constructing surfaces and meshes is hidden from the user in the VisualServer, but with ArrayMeshes, the user can construct a Mesh manually by passing in an array containing the surface information.

### Superfici¶

Each surface has its own material. Alternatively, you can override the material for all surfaces in the Mesh when you use a MeshInstance using the material_override property.

### Surface array¶

The surface array is an array of length `ArrayMesh.ARRAY_MAX`. Each position in the array is filled with a sub-array containing per-vertex information. For example, the array located at `ArrayMesh.ARRAY_NORMAL` is a PoolVector3Array of vertex normals. See Mesh.ArrayType for more information.

The surface array can be indexed or non-indexed. Creating a non-indexed array is as easy as not assigning an array at the index `ArrayMesh.ARRAY_INDEX`. A non-indexed array stores unique vertex information for every triangle, meaning that when two triangles share a vertex, the vertex is duplicated in the array. An indexed surface array only stores vertex information for each unique vertex and then also stores an array of indices which maps out how to construct the triangles from the vertex array. In general, using an indexed array is faster, but it means you have to share vertex data between triangles, which is not always desired (e.g. when you want per-face normals).

## Strumenti¶

Godot provides different ways of accessing and working with geometry. More information on each will be provided in the following tutorials.

### ArrayMesh¶

The ArrayMesh resource extends Mesh to add a few different quality of life functions and, most importantly, the ability to construct a Mesh surface through scripting.

Per ulteriori informazioni sull'ArrayMesh, consulta il tutorial ArrayMesh.

### MeshDataTool¶

The MeshDataTool is a resource that converts Mesh data into arrays of vertices, faces, and edges that can be modified at runtime.

Per maggiori informazioni su MeshDataTool, consulta il tutorial MeshDataTool.

### SurfaceTool¶

The SurfaceTool allows the creation of Meshes using an OpenGL 1.x immediate mode style interface.

Per maggiori informazioni su SurfaceTool, consulta il tutorial: ref:SurfaceTool <doc_surfacetool>.

### ImmediateGeometry¶

ImmediateGeometry is a node that uses an immediate mode style interface (like SurfaceTool) to draw objects. The difference between ImmediateGeometry and the SurfaceTool is that ImmediateGeometry is a node itself that can be added to the scene tree and is drawn directly from the code, while the SurfaceTool generates a Mesh that needs to be added to a MeshInstance to be seen.

ImmediateGeometry is useful for prototyping because of its straightforward API, but it is slow because the geometry is rebuilt every frame. It is most useful for adding simple geometry for visual debugging (e.g. by drawing lines to visualize physics raycasts etc.).

Per ulteriori informazioni su ImmediateGeometry, consultare il Tutorial ImmediateGeometry.

## Quale dovrei usare?¶

Which approach you use depends on what you are trying to do and what kind of procedure you are comfortable with.

Both SurfaceTool and ArrayMesh are excellent for generating static geometry (meshes) that don't change over time.

Using an ArrayMesh is slightly faster than using a SurfaceTool, but the API is a little more challenging. Additionally, SurfaceTool has a few quality of life methods such as `generate_normals()` and `index()`.

ImmediateGeometry regenerates the mesh every frame, so it is much slower than ArrayMesh or SurfaceTool. However, if you need the geometry to change every frame anyway, it provides a much easier interface that may even be a little faster than generating an ArrayMesh every frame.

The MeshDataTool is not fast, but it gives you access to all kinds of properties of the mesh that you don't get with the others (edges, faces, etc.). It is incredibly useful when you need that sort of data to transform the mesh, but it is not a good idea to use it if that extra information is not needed. The MeshDataTool is best used if you are going to be using an algorithm that requires access to the face or edge array.