Geometry

Geometry

core/buffer_geometry.mojo, core/buffer_attribute.mojo, core/geometry_store.mojo and geometries/. A BufferGeometry holds named vertex attributes and an optional index. Seven builders make a box, a sphere, a plane, a circle, a ring, a cylinder and a cone.

three.js: BufferGeometry, BufferAttribute, BoxGeometry, SphereGeometry, PlaneGeometry, CircleGeometry, RingGeometry, CylinderGeometry, ConeGeometry.

BufferAttribute

A flat List[Float32] with an item size. BufferAttribute(data, 3) holds vectors of three floats.

Member	Meaning
count() -> Int	The number of items.
component(index, offset) -> Float32	One float of one item.
vector3(index) -> Vector3	One item as a vector. Item size must be three.

BufferGeometry

Member	Meaning
set_attribute(name, attribute)	Add or replace an attribute.
attribute_view(name) -> ref BufferAttribute	Borrow an attribute. Bind it with ref.
clone_attribute(name) -> BufferAttribute	Copy an attribute.
has_attribute(name) -> Bool	Whether the attribute exists.
set_index(index)	Set the triangle index. An empty list clears it.
vertex_count() -> Int	The number of vertices in position.
triangle_count() -> Int	The number of triangles.
corner(triangle, corner) -> Vector3	A corner position.
corner_index(triangle, corner) -> Int	Which vertex that corner is.

Attribute names are the constants POSITION, NORMAL and UV. A geometry needs position. It needs normal for smooth shading and uv for a texture.

GeometryStore

assets.geometries.add(geometry) stores a geometry and returns a GeometryId. get(id) borrows it. Many meshes share one geometry. See Meshes and assets.

Box

var solid = cube(Length(1.0, METER))
var brick = box(Length(2.0, METER), Length(1.0, METER), Length(0.5, METER))

A box has twenty-four vertices, four per face. Each face carries its own normal and texture coordinates. The whole image covers each face once.

Sphere

var ball = sphere(Length(1.0, METER), 24, 16)   # segments around, rings down

A latitude and longitude sphere. Each normal points away from the center, so shading is smooth. u runs once around the equator. v runs from one at the north pole to zero at the south pole. It needs at least three segments and two rings.

Plane

var sheet = plane(Length(2.0, METER), Length(1.0, METER))        # one quad
var grid = plane(Length(20.0, METER), Length(20.0, METER), 4, 4)  # sixteen quads

A rectangle in the xy plane, facing +z, centered on the origin. The vertex order, winding and texture coordinates match three.js. Turn the node a quarter turn about x to make a floor.

The texture coordinates run once across the rectangle. To tile an image, rewrite the uv attribute, as examples/floor.mojo does.

Circle

var disk = circle(Length(1.0, METER), 32)                                       # a full disk
var slice = circle(Length(1.0, METER), 8, Angle(90.0, DEGREE), Angle(180.0, DEGREE))  # a pie slice

A disk in the xy plane, facing +z, centered on the origin. Vertex zero is the center. The rim follows it, one vertex per segment and one more to close the seam. Each triangle runs from a rim vertex to the next one and back to the center.

The third argument is where the rim starts, counter-clockwise from +x. The fourth is how far it sweeps. A full turn is the default. A shorter sweep makes a pie slice.

The texture coordinates map the square around the disk onto the image. The center is (0.5, 0.5). A pie slice shows its part of the image.

Ring

var washer = ring(Length(0.5, METER), Length(1.0, METER), 32, 1)   # segments around, then across
var arc = ring(Length(0.5, METER), Length(1.0, METER), 16, 1, Angle(0.0, DEGREE), Angle(90.0, DEGREE))

A flat ring in the xy plane, facing +z, centered on the origin. The first radius is the hole. The second is the outer edge. Vertices run in rows from the inner edge outwards, one per segment around and one more for the seam. The start angle and the sweep work as they do for the circle. A shorter sweep makes an arc.

The texture coordinates map the square around the outer edge onto the image, as the circle's do.

A ring with no hole is a disk. Use circle for it.

Cylinder

var can = cylinder(Length(1.0, METER), Length(1.0, METER), Length(2.0, METER), 32, 1)  # top radius, bottom radius, height
var bucket = cylinder(Length(0.8, METER), Length(1.2, METER), Length(2.0, METER))
var pipe = cylinder(Length(1.0, METER), Length(1.0, METER), Length(2.0, METER), 32, 1, open_ended=True)

A cylinder stands on the y axis, centered on the origin. The first radius is at the top and the second at the bottom. Different radii make a frustum. The side comes first, in rows from the top down, one vertex per segment around and one more for the seam. A cap follows at each end unless open_ended is true. Each cap has one center vertex per segment, as in three.js.

Side normals lean with the side, so a frustum shades smoothly. Cap normals point along the axis. u runs once around the side. v runs from one at the top to zero at the bottom. Each cap maps the square around it onto the image.

The sweep starts at +z and runs toward +x, as in three.js. The circle starts at +x instead. A shorter sweep makes a section with open sides.

Cone

var spike = cone(Length(1.0, METER), Length(2.0, METER), 32)

A cone is a cylinder with a top radius of zero. Its point is at +y. The point has one vertex per segment, each with its own normal. A cylinder with a bottom radius of zero is a cone the other way up. An end with no radius gets no cap.

Errors

• A negative or zero extent raises.
• A sphere with too few segments or rings raises.
• A plane with fewer than one segment raises.
• A circle needs a positive radius and at least three segments.
• A ring needs a positive inner radius, a larger outer radius, three segments around and one across.
• A cylinder needs a positive height, radii that are not negative and not both zero, three segments around and one down the side.
• A cone needs a positive radius and a positive height.
• A sweep must be positive and at most one turn.
• An index entry beyond the last vertex raises.