Mesh and Tet Volume Items

Status: Complete Last Updated: 2026-10-06

This page tells how the viewport colors and lights a tri mesh item and a tet volume item. It also tells what the viewport does where two items cover the same surface.

The two tones

Each item type has its own tone. A tone comes from the primary color of the palette, which is the highlight color of the edition you run.

The turn happens in CIE LCh, so the lightness that you see does not change with the hue. Each tone also gets its own lightness, between L* 35 and 80. The palette chooses the two lightness values that keep the primary and the two tones far apart. The test is a CIE76 difference of at least 25 between each pair, in normal vision and under protanopia, deuteranopia and tritanopia. Each edition's seed passes it.

The tones are not stored. When the palette changes, the tones change with it, in the viewport and in the scene tree.

What colors an item

The first rule that applies wins:

1. A field. The item shows a scalar field of its asset through a colormap. See Fields.
2. A material. If you give the item a material that differs from the default, its albedo colors the item.
3. The tone of the item type. The default gray material never shows on a mesh or a tet volume.

Edges and light

Both types draw the edges of their triangles. A tet volume draws the triangles of its boundary and of its cut plane section. The edge color is the surface color, lighter by 15 in CIELAB L*. Both types use one shading function, so a tet volume and a mesh baked from it look the same.

The edge is less diffuse and smoother than the surface, so it looks shinier. A tri mesh shades smooth across edges under 45 degrees. A tet volume shades flat. A mesh item can store its own angle as creaseAngle in its type data. A boundary bake of a tet volume stores 0, so it shades flat and looks the same as the volume.

A highlight does not change these colors. A hovered or selected mesh or tet volume shows its outline only.

Fields

A field is a named value for each element or each vertex of an item's asset. An element field colors each triangle flat. A vertex field blends the color across each triangle. A vector field shows as its length.

The other fields come from the asset file, for example the result of a simulation. A field with one value only shows the middle of its colormap.

To color a tet volume, use the Coloring control of its row. To color either type, use the OPAL task construct.passenger.set_color. The task also takes a colormap and a range of your choice. A new field resets a range that you set for the old field.

Colormaps

A colormap is a JSON file in src/libs/libconstruct/construct/colormaps/. The file name is the colormap's id.

To add a colormap, add a file. No code changes. The build ships every file in the folder.

{
	"name": "Blue, green, red",
	"space": "linear",
	"stops": [
		{ "at": 0.0, "color": [0.1425, 0.1425, 0.95] },
		{ "at": 1.0, "color": [0.95, 0.1425, 0.1425] }
	]
}

• at is a position from 0 to 1. Between two stops the color blends in a straight line.
• space is linear or srgb. It tells how to read the color values. The viewport keeps linear values.
• A colormap needs at least two stops.

Two items in one place

Where two items cover the same surface, each pass moves its depth toward the camera by a small amount from a key. The key comes from the item. This gives three modes:

The move is under a pixel, and a pick takes it back out, so a pick returns the true surface point. For how this looks in the viewport, see Two items in one place.