The hand
The brush is your finger.
A painter sets the width of a mark by how they hold the brush, not by picking a number. Getting that onto a phone meant finding out what the glass actually knows about a fingertip, and then taking most of the controls away.
What the glass knows
A phone cannot feel you press
The obvious answer is pressure, and it is not available. Apple dropped the hardware that measured it after the iPhone XS, so on a current phone the force of a finger reads as nothing at all. React Native, which this app is built in, passes on a touch's position and little else.
So the first version used speed: move fast, draw thin. It works, and it is backwards from what a hand expects. A slow, careful stroke came out fat, and a quick flick came out fine.
Kept underneath, as a fallback. Never right on its own.
The contact patch
What it does know is how much of you is touching it
iOS measures the ellipse a fingertip presses into the glass. It is the right variable for brush width, and React Native never hands it over, so the app carries a small native module of its own to read it.
Raw, it disappoints in three ways, and each has an answer. It arrives in visible steps, so a smooth press climbs a staircase: the module passes on every sample the screen takes rather than one a frame, and averaging them recovers the detail between steps. Its usable range is narrow, and different for every hand: the app learns the band your finger actually produces and maps that onto the whole brush, so a light touch is a hairline and a flat pad is broad, whatever your numbers are. And iOS says when it is guessing, so when the reading is uncertain, speed comes back in underneath it.
On the phone now. Press lighter for a finer line.
The long version: Brush thickness
The rest of the width
Most of it is not a control at all
A mark's width is the size you chose, times what your hand is doing, times what is left in the brush. Only the first of those is a setting.
- ContactHow much of your fingertip is on the glass, with speed filling in where the sensor is unsure.
- DwellStop moving and the mark widens and pools, the way paint gathers under a brush that has stopped.
- The reservoirThe brush empties as it travels, so a long stroke thins and breaks up toward its end.
- The dipA colour stays on the brush until you choose another. Choosing one is the rinse.
All four are in every stroke, and none of them is a slider.
The one control left
Four ways to ask for a thin line
Four were drawn at true phone scale and painted in. A rack of four named brushes, the way a painter owns a few and knows each by its mark. A pinch on the paper, with nothing on screen the rest of the time. A slider down the edge of the sheet, idle until touched. And no control at all, leaving the contact patch to carry everything.
The last one cannot do it alone: after calibration a finger covers about three times its own finest mark, which is one brush's worth of range, not a set of brushes. What shipped is quieter than all four: a hairline above the colours with its stops marked along it, dragged with the thumb that is already down there.
On the phone now, taking up one line of the screen.
The long version: Four ways, drawable at phone scale
Everything else
What the good phone painting apps put on the screen
Procreate Pocket has a toolbar and gestures, and keeps its radial menu off by default. Sketchbook is built around a radial puck and carries the complaint that it has to be solved before it helps. Paper had a tray that tucked away and took its width from how you held the stylus. Zen Brush keeps a small quiet bar, and its reviewers still single out the sizing.
Nijimi's answer sits closest to Zen Brush: a card of colours, a line for size, a water slot, and three buttons at the top. The radial dial that was built here lost for the reason the survey predicted, that a control you have to summon tells you nothing until you already know it.
Kept: the paper is the screen, and the controls take one strip of it.
The long version: Painting on a phone · The dial prototype
What it comes to
Nothing to learn before you start
The whole instruction the app gives is one line on its start screen: drag a finger across the sheet, and tilt the phone to move the water. Everything above is there so that line is enough.