3D mesh transparency

material3d.alphaMode controls how a mesh uses its base color and texture alpha:

Mode	Color pass	Shadow pass
opaque (default)	Ignores alpha and writes depth	Opaque shadow
mask	Discards pixels with alpha below alphaCutoff	Uses the same cutoff
blend	Blends back to front without writing depth	Uses stable coverage dithering for an approximate translucent shadow

For textured meshes, effective alpha is the base color alpha multiplied by the map alpha. alphaCutoff defaults to 0.5. The renderer draws opaque and masked meshes first, then sorts blended meshes by camera-space center depth. Intersecting blended meshes and overlapping triangles within one mesh may still have ordering artifacts; use mask for crisp cutouts.

Mask cutoff: 0.50Glass opacity: 0.45Cast shadows

The green cutout uses texture alpha for both its visible shape and shadow. The red and blue panes overlap: the closer red pane should blend over the blue one.

api.updateNodes([
    {
        id: 'cutout',
        type: 'mesh3d',
        geometry: { type: 'plane' },
        x: 36,
        y: 77,
        width: 150,
        height: 150,
        z: 25,
        scale3d: [150, 150, 1],
        rotation3d: [0, Math.PI, 0],
        material3d: {
            map: '/mesh3d-alpha-mask.svg',
            alphaMode: 'mask',
            alphaCutoff: 0.5,
            castShadow: true,
        },
    },
    {
        id: 'glass',
        type: 'mesh3d',
        geometry: { type: 'plane' },
        x: 235,
        y: 95,
        width: 150,
        height: 170,
        z: 0,
        scale3d: [150, 170, 1],
        rotation3d: [0, Math.PI, 0],
        material3d: {
            baseColor: [1, 0.2, 0.16, 0.45],
            alphaMode: 'blend',
            castShadow: true,
        },
    },
]);

The scene also needs a Camera3D, a light, and a receiving floor; see the Lighting example. Blended shadows are a single-layer approximation, so multiple overlapping glass casters do not accumulate physical transmittance.

For a glTF mesh with one material, the loader inherits its alphaMode (OPAQUE, MASK, or BLEND) and alphaCutoff; an explicit material3d value overrides either property. The current loader flattens multiple primitives into one mesh and uses the first material for all of them, so different alpha modes within one imported model are not supported yet.

GLB export preserves an untextured mesh's explicit alpha mode and mask cutoff. The current exporter does not embed base-color textures, so a texture-shaped cutout cannot round-trip through GLB yet.