Research

How brushes, knives and fingers really handle paint

Research for the paint engine, prompted by the first test on a phone: "I can't add 3D paint, I can only smear." Each finding ends with what the engine does about it.

1. A brush holds paint in two places

A brush head has a toe (the tip), a belly (the thickest part) and a heel (where the hair enters the ferrule). The belly is the reservoir. It feeds paint out through the toe according to how hard you press. Hog bristle splits at the tip ("flags") and carries more paint than synthetic, which is why painters use it for oils. Painters load "almost to the ferrule". Paint only on the tip runs out at once; paint packed at the heel floods.

A loaded brush therefore carries paint on the outside of the bristles, which comes off first, generously and thick, and inside the belly, which feeds a long, thinner stroke under pressure.

→ Each bristle now has two stores: surface paint, released readily, and belly paint, fed out slowly and more with pressure or when you slow down. Tapping a tube again piles on more surface paint.

2. Oil paint holds its shape: it's a yield-stress fluid

Oil paint behaves like a Bingham plastic: below a yield stress it doesn't flow, so "the surface can hold peaks when it is still". Impasto mediums are designed for exactly that: high yield stress and strong shear thinning, so paint flows under the brush and stops the moment the brush leaves.

→ Deposited paint keeps its height. A loaded brush rides on the paint it carries and picks up little, so strokes build up instead of scraping back. Only real pressure, or a thin oily medium, flattens it.

3. Impasto is laid on, not rubbed in

Impasto is painted with a stiff brush or knife, with paint straight from the tube, in broad, generous strokes, "as if you were spreading peanut butter on a piece of toast". Short strokes, light pressure, no blending on the canvas. A loaded flat squeezes paint out at its sides, and where the brush lifts, the paint pulls up into a lip.

→ Surface paint comes off heavier at the brush's outer edges, forming side ridges. Lifting the brush drops a raised lip at the end of the stroke.

4. The brush shape decides the mark

Shape	The mark
Flat	Rectangular, strong square edges, broad strokes
Bright	A short flat: stiff, short controlled strokes, distinct square edges
Filbert	A flat with an oval tip: broad with a soft top line that melts into its neighbours; on its side a thin line
Round	Pointed: fine lines and details that taper with pressure
Fan	Splayed: broken, textured, stippled marks such as grass and fur
Rigger	Long, thin, holds a lot: long continuous lines

→ The brush comes in Flat, Filbert and Round. Filbert's outer bristles trail behind and lay less, which rounds the stroke's end and softens its edges. Round's bristles fill a small disc, so its width follows pressure and it tapers.

5. The knife: flat, edge and tip

The flat of the blade, pressed and dragged, lays thick layers or blends. The edge gives sharp straight lines. The tip gives dots and curved accents. Paint is loaded along one edge, which is set against the canvas and pulled.

→ A light knife now lays a thick slab (about 1.5 mm), and pressing harder spreads it thinner. It used to top out near 0.4 mm.

6. What digital research warns about

Chu, Baxter, Wei & Govindaraju (Detail-Preserving Paint Modeling for 3D Brushes, NPAR 2010) showed that brush models built on "simple repeated exchange of paint between the 3D brush and 2D canvas" lose colour detail quickly. That's the smeary look. WetBrush (SIGGRAPH Asia 2015) goes further, simulating individual bristles and the paint as fluid on the GPU.

→ Two stores and low pickup from a loaded brush are the cheap version of that fix. A true bristle-level simulation belongs in the native Metal engine.

Sources

Seven brushes (2026-09-26), then nine (2026-09-27)

The owner asked for a wider set of brushes (and dropped the squeegee). Each is a setting of the same bristle model (position across the head, lag, lead at the tip, how much paint it lays) plus a width, so they all pick up, mix and lay paint the same way:

Brush	What it's for	How it's modelled
Flat	broad strokes, clean edges	a straight row; paint squeezes out at the sides
Filbert	soft-edged strokes, blending	the row's ends trail, so the tip is rounded
Round	lines and detail	a disc of bristles, 0.45 of the width
Bright	short, crisp, textured marks	a stiff flat, 0.8 wide, bristles held in line
Angle	edges, corners, tapering strokes	a flat whose tip leads on one side
Fan	grass, hair, foliage, soft blends	1.6 wide, bristles in clumps with gaps, tips on an arc
Rigger	long thin lines, branches, lashes	0.16 wide, lets paint go at under half the rate, so a line runs long
Mop	softening and blending wet paint, large soft passages	a wide soft round head, 1.5 wide, lays paint at about a third of the rate (its blending is untuned)
Dagger	sweeping lines that swell and taper, leaves, ribbons	a narrow slanted cut, 0.32 wide, one side leading far