Attention: Here be dragons

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Using ImmediateMesh

The ImmediateMesh is a convenient tool to create dynamic geometry using an OpenGL 1.x-style API. Which makes it both approachable to use and efficient for meshes which need to be updated every frame.

Generating complex geometry (several thousand vertices) with this tool is inefficient, even if it's done only once. Instead, it is designed to generate simple geometry that changes every frame.

First, you need to create a MeshInstance3D and add an ImmediateMesh to it in the Inspector.

Next, add a script to the MeshInstance3D. The code for the ImmediateMesh should go in the _process() function if you want it to update each frame, or in the _ready() function if you want to create the mesh once and not update it. If you only generate a surface once, the ImmediateMesh is just as efficient as any other kind of mesh as the generated mesh is cached and reused.

To begin generating geometry you must call surface_begin(). surface_begin() takes a PrimitiveType as an argument. PrimitiveType instructs the GPU how to arrange the primitive based on the vertices given whether it is triangles, lines, points, etc. A complete list can be found under the Mesh class reference page.

Once you have called surface_begin() you are ready to start adding vertices. You add vertices one at a time. First you add vertex specific attributes such as normals or UVs using surface_set_****() (e.g. surface_set_normal()). Then you call surface_add_vertex() to add a vertex with those attributes. For example:

GDScript

# Add a vertex with normal and uv.
surface_set_normal(Vector3(0, 1, 0))
surface_set_uv(Vector2(1, 1))
surface_add_vertex(Vector3(0, 0, 1))

Only attributes added before the call to surface_add_vertex() will be included in that vertex. If you add an attribute twice before calling surface_add_vertex(), only the second call will be used.

Finally, once you have added all your vertices call surface_end() to signal that you have finished generating the surface. You can call surface_begin() and surface_end() multiple times to generate multiple surfaces for the mesh.

The example code below draws a single triangle in the _ready() function.

GDScript

extends MeshInstance3D

func _ready():
    # Begin draw.
    mesh.surface_begin(Mesh.PRIMITIVE_TRIANGLES)

    # Prepare attributes for add_vertex.
    mesh.surface_set_normal(Vector3(0, 0, 1))
    mesh.surface_set_uv(Vector2(0, 0))
    # Call last for each vertex, adds the above attributes.
    mesh.surface_add_vertex(Vector3(-1, -1, 0))

    mesh.surface_set_normal(Vector3(0, 0, 1))
    mesh.surface_set_uv(Vector2(0, 1))
    mesh.surface_add_vertex(Vector3(-1, 1, 0))

    mesh.surface_set_normal(Vector3(0, 0, 1))
    mesh.surface_set_uv(Vector2(1, 1))
    mesh.surface_add_vertex(Vector3(1, 1, 0))

    # End drawing.
    mesh.surface_end()

The ImmediateMesh can also be used across frames. Each time you call surface_begin() and surface_end(), you are adding a new surface to the ImmediateMesh. If you want to recreate the mesh from scratch each frame, call clear_surfaces() before calling surface_begin().

GDScript

extends MeshInstance3D

func _process(delta):

    # Clean up before drawing.
    mesh.clear_surfaces()

    # Begin draw.
    mesh.surface_begin(Mesh.PRIMITIVE_TRIANGLES)

    # Draw mesh.

    # End drawing.
    mesh.surface_end()

The above code will dynamically create and draw a single surface each frame.