Terrain brushes

TerrainBrush answers one question — *for this world-space sample, what is the weight of this stamp?* — and it does not know whether the answer will scale a height, a layer weight or a scatter probability. It carries a shape, a falloff curve and fraction, a radius in metres, a strength, a stamp spacing and a rotation mode. BrushStamp is one landing of it; BrushStroke turns a path into evenly spaced stamps; BrushFootprint is the region a stamp can reach.

Three consumers, one implementation, one settings panel section, one set of tests. Unreal implements the sculpt brush, the paint brush and the foliage brush three times, so a soft edge sculpted at strength 0.3 and a soft edge painted at strength 0.3 are different shapes there. Here they are the same shape by construction.

You do not want it for anything whose falloff is not radial — a ramp is defined by two picked points and a width, which is why TerrainSculpt.Ramp does not take a brush at all.

⚠ Falloff is the fraction of the radius that falls off, not where the falloff starts. The two read almost the same on a slider and are inverses of each other. Zero is a hard disc; one is falloff all the way in, with no plateau at all.

⚠ A half-falloff brush has a plateau, and that surprises the tools that read slopes. Every sample within the inner half is at full weight, so a mound built with the default brush is a mesa — the 3×3 neighbourhood at its top is flat, a smooth changes nothing there and an erosion finds no slope to exceed the talus angle. That is correct behaviour for all three; a test of any of them has to build a cone.

⚠ Spacing is a fraction of the radius, not a distance. Making it a distance means that turning the radius up thins the stroke out until it is a row of discs. Unity and every paint application spell it this way.

BrushStroke accumulates a drag one pointer move at a time and hands back stamps:

⚠ The first call always stamps, because an artist who clicks without dragging expects one stamp rather than none — which is what makes the same type the Single tool as well as the Paint one.

⚠ Spacing is by distance travelled, with the leftover carried across pointer events. Stamping per event ties the density of a stroke to the frame rate and to how fast the pointer was moving, so the same gesture leaves a different mark on a fast machine.

⚠ A random rotation is a hash of the stamp index, not a draw from a generator. The angle of stamp N depends only on N, so a stroke can be undone and redone to the same result — the same property the scatter needs, for the same reason.

IBrushMask is a function from the unit square to a weight. It is an interface rather than a texture so that this assembly needs no image type; the editor hands it something backed by an alpha texture, and a test hands it a lambda.

BrushFalloff.Evaluate is arithmetic on one number, and the four curves are Unreal's:

The same brush, three consumers — which is the whole argument:

A soft edge sculpted at strength 0.3 and a soft edge painted at strength 0.3 are the same shape, because both went through WeightAt.

A hard-edged brush, for a cut that has to read as deliberate:

⚠ Falloff 0 is a disc with a step at its rim, which on a heightfield is a wall one sample thick. That is sometimes exactly what a road cut wants and is never what a landform does.

• The terrain heightfield — what the weights are applied to.
• Sculpting a heightfield — the kernels that consume a stamp.
• docs/plan/31 § D12 — why the brush is one service, and what having three of them costs.