The short answer
Pressure is how hard you push. Tilt is how far the barrel is leaned over. Azimuth is which way the barrel is pointing. They are three completely separate measurements, they arrive with every single touch sample, and a drawing app can support any combination of them. Most of the confusion about "my Apple Pencil does not work properly" comes from expecting one of them to do another one's job.
Pressure
What it measures
Force at the tip, reported as a value that an app normalises against the maximum the hardware can report. It is continuous, not stepped, and it updates many times per stroke.
What it should change
Density and width, together. On a real tool, pressing harder both darkens the mark and spreads the tip, so an app that only changes opacity feels thin and one that only changes width feels like a marker. Flying Chalk clamps pressure to a sensible floor so that a barely-there touch still leaves a visible mark instead of nothing at all — a stroke that vanishes because you feathered it is a bug, not realism.
How to test it
Draw one slow line and gradually lean on the pencil. If the line changes as you press, pressure is live. If it is uniform, either the tool ignores pressure by design or the app does not read it.
Tilt
What it measures
The angle between the barrel and the glass, delivered as an altitude angle: upright is a right angle, flat is near zero. Apps normally invert and normalise it so that zero means upright and one means flat.
What it should change
The size and shape of the contact patch. Laying the pencil over should widen the mark and stretch it, and — depending on the material being simulated — make it darker or paler. Chalk laid on its side sheds more dust and goes denser; graphite on its side goes lighter and greyer.
How to test it
Hold the pencil still and draw the same short stroke twice, once upright and once almost flat. If the second stroke is not just fatter but a different shape, tilt is being used properly.
Azimuth
What it measures
The direction the barrel points, projected onto the screen. It is a compass bearing, not an amount.
What it should change
The orientation of the mark. Azimuth on its own does nothing visible — it only matters once tilt has stretched the contact patch into an ellipse, because then azimuth is what decides which way that ellipse lies. Without azimuth you get a big round blob; with it you get the broad edge of a stick of chalk.
The trap nobody warns you about
When the pencil is close to vertical, the azimuth reading becomes unstable — there is barely any horizontal projection left to measure, so the reported angle can swing wildly from sample to sample. An app that feeds that raw value straight into a rotation will make marks visibly jitter under an upright pencil. Reading azimuth as a direction vector rather than as a raw angle is one way to keep it steady, and interpolating it along the shortest arc stops a stroke from spinning a full turn when the bearing crosses from one side of the compass to the other.
What none of them are
None of these three is stroke smoothing, stabilisation or prediction. Those are separate features that reshape the path you drew. Pressure, tilt and azimuth do not move your line — they decide what gets stamped along it.
Which of them does your app read?
Three strokes will tell you. A slow line with rising pressure tests pressure. The same short stroke drawn upright and then flat tests tilt. A flat stroke swept sideways and then swept along the barrel direction tests azimuth: if both look identical, azimuth is being ignored. Do this with a chalk, pastel or charcoal tool — pens and markers legitimately ignore all three.
Frequently asked questions
Is azimuth the same as rotation?
No. Azimuth is the direction the barrel points across the screen. Barrel rotation around the pencil's own axis is a different thing, and Flying Chalk does not use it.
Why does my line jitter when I hold the pencil upright?
Because azimuth becomes ill-defined near vertical and can swing between samples. An app that rotates its marks straight from the raw angle will show that as visible shimmer.
Do fingers report pressure?
Not usefully. On a finger the force value is either zero or a constant, so a correctly written app substitutes a fixed value and lets speed carry the expression instead.
Related guides
- Apple Pencil tilt shading: how it works and how to use it
- Drawing on an iPad with your finger vs an Apple Pencil
- How to fix Apple Pencil lag and palm rejection problems
Try it on a chalkboard
Flying Chalk is a free chalkboard drawing app for iPad and iPhone. Pressure and tilt on Apple Pencil, eight chalk-family sticks, eleven surfaces. No ads, no account, no in-app purchases.