Why chalk marks look the way they do

The short answer

A stick of chalk is compressed powder. When you drag it across a board, the board abrades it, and what stays behind is loose particles wedged into the high points and pits of a rough surface. It is not a film of colour lying on top of the board the way ink or paint is. Almost everything distinctive about how chalk looks follows from that one sentence.

The texture belongs to the board, not to the stick

This is the part that surprises people. The speckle you see in a chalk stroke is not a property of the chalk — it is a picture of the surface. The stick is uniform. What varies is where the surface offers a high point for powder to catch on and where it offers a pit that the stick bridges over without touching.

You can prove it to yourself in ten seconds. Draw the same stroke, with the same stick and the same pressure, on a slate board and on smooth card. The slate gives you a coarse, broken, granular mark. The card gives you something much flatter and more even. Same chalk, different picture, because the surface is doing the work.

The corollary matters: choosing the surface is a bigger creative decision than choosing the stick. If the marks look wrong, look at what you are drawing on before you blame what you are drawing with.

Two scales of roughness at once

Real surfaces are not evenly rough. There is a fine roughness — individual grains, the tooth of the material — and a coarser, slower undulation across the surface, gentle rises and hollows. The fine roughness decides whether powder catches at a given point. The coarse undulation decides whether whole regions of the stroke are heavy or light. It is the second one that makes chalk clump into patches instead of looking like evenly scattered salt, and it is the one that gets forgotten.

Why the mark looks luminous but never glossy

A film of glossy paint reflects light in one direction — that is what a highlight is. A layer of loose powder has an enormous number of tiny facets pointing every which way, so it scatters incoming light diffusely in all directions. From any viewing angle you are receiving light from the mark, but you never receive a mirror-like reflection from it.

That combination reads to the eye as matte and, at the same time, oddly bright — the mark seems to glow slightly against a dark board. It also means chalk looks much the same from every seat in a room, which is not true of a glossy whiteboard marker under overhead lights, where students at certain angles see a sheet of glare instead of writing.

Why the mark is never pure white

Two separate reasons, and both of them are worth knowing.

The physical one: a diffuse powder scattered over a dark ground is not an opaque layer. Some of the dark ground still shows through between and around the particles, so the perceived brightness of the mark is a mixture of powder and board. Even a heavily packed chalk stroke sits well below the brightness of a sheet of white paper in the same light.

The perceptual one: even if you could produce a pure white mark on a dark board, you would not want it. Maximum contrast at close range is harsh, and it is one of the things people describe as a board being tiring to look at. Real chalk runs slightly warm — a bone or ivory off-white rather than a screen's brightest value — and that is part of why a real board looks comfortable and a naive digital imitation looks like a black rectangle with fluorescent lines on it.

Ghosts, wear and why a clean board looks fake

Erasing removes the loose powder. It does not remove the powder that has been pressed into the pits, and it does not undo the polishing and scratching that years of use inflict on the surface. So an old board carries a permanent record of itself: faint ghost marks where something was written repeatedly, brighter zones where the surface has been worn smooth and no longer bites, scratches, and a general haze in the middle where hands and erasers have been.

These are not defects for the purposes of how a board looks. They are the reason a used board looks like a used board. A perfectly uniform, perfectly clean dark rectangle does not read as a chalkboard at all — it reads as a coloured rectangle. Anyone reproducing a board has to put the wear back in.

Humidity and the state of the stick

How a stick bites is not constant. A damp board takes chalk differently from a dry one: moisture in the surface makes the mark denser, stickier and harder to erase cleanly, which is why a board wiped with a wet cloth and used immediately behaves badly and why people are told to let it dry. Very dry conditions make the mark powdery and easy to smear.

The stick's own condition matters too. A fresh cylinder presents a small rounded tip; a worn one presents a flat face at whatever angle you habitually hold it; a broken one presents a chisel. The same stick draws differently over its lifetime, which is an ordinary fact of using it and a nuisance for anyone trying to model it.

What all this implies for a simulation

Take the physics seriously and the requirements for a convincing digital chalk follow directly. Three of them do most of the work.

The grain must be anchored to the surface

If the texture belongs to the board, then in a simulation the texture has to belong to the canvas, not to the stroke. Flying Chalk keeps a tileable surface-roughness texture fixed in canvas coordinates and samples it at each point the stroke touches, so two passes over the same place hit the same texture and reinforce the same speckle — the way real powder accumulates in the same pits. Two layers are used, fine noise for the grains and a slower undulation over the top so the grains clump rather than scatter evenly, which is the two-scale roughness described above.

The alternative — baking random noise into the stroke itself — fails for a reason that is not obvious until you try it. A stroke is drawn as many overlapping stamps along its path, so if each stamp carries its own independent randomness, the overlaps average the randomness out. You get a stroke that is noisy at the thin ends and flat grey everywhere the stamps pile up, which is precisely backwards.

A dry mark is a deposit with holes in it

The tempting model is to build the mark up out of semi-transparent dots until it becomes solid. That produces smoke. The physical picture is the opposite: a solid deposit of pigment, with holes bitten out of it wherever the surface did not support it. Flying Chalk models it that way, with a threshold that decides whether a given point receives a deposit at all. A brush that stays too transparent never crosses that threshold and looks like fog rather than like chalk.

The side of the stick has to behave differently

Tilting a stick over changes the contact patch from a point to a long face aligned with the barrel, and different dry materials respond differently: charcoal stretches furthest and darkens as it flattens, graphite stretches but goes paler and greyer, wax-based sticks read the surface bluntly because the wax fills the pits rather than sitting in them, and wet media ignore the roughness altogether because they flood the tooth. In Flying Chalk those differences are just numbers attached to each tool — how far the patch stretches, how much it broadens, whether tilt darkens or lightens, how strongly it reads the roughness — which is why switching tool changes behaviour rather than only changing colour.

The thing to take away

Chalk is not a colour, it is a process: abrasion, deposition, and a surface that decides where the powder lands. Once you see a mark that way, the ragged edges stop looking like imperfections and start looking like information about the board you are drawing on.

Related guides

• Why digital chalk and pastel look different from a brush
• Why simulating chalk on a tablet is hard
• Are green chalkboards easier on the eyes? An honest answer