A checker for the 250 box challenge
Draw a box freehand in the browser and every edge gets measured against the vanishing points your own lines imply. You get one number, one sentence about what went wrong, and the corrected box drawn over your own in red pencil.
What it checks
A box in three-point perspective has twelve edges in three groups of four. Within a group, all four edges should aim at the same vanishing point. That agreement — how tightly the four converge — is the whole measurement.
The checker never asks you to place a vanishing point. It reads your strokes, sorts the twelve edges into three families by direction, fits the vanishing point each family is actually pointing at, and then measures how far every individual edge misses it. The vanishing points come out of your drawing; they are not given to you and then marked against.
That is the same thing you do by hand when you extend your lines back and look at where they cross. It is quicker here, and it is measured in degrees rather than judged by eye — which makes it useful for the pass you do between real critiques, not a replacement for them.
How the score works
The number over the box is tightness, from 0 to 100. It is a restatement of one measurement. Each family of four edges gets an angular error in degrees — how far its edges miss the vanishing point they imply, combined as a root mean square, so one badly-off edge weighs more than a little wobble spread evenly — and the score comes from the average of the families. Zero error scores 100; the score falls as that error grows.
- 90 and up — every edge agrees on where it is going.
- 75–89 — solid, the convergence is close.
- 60–74 — getting there, a few edges wander.
- 40–59 — several edges miss their vanishing point.
- Under 40 — the edges do not share vanishing points yet.
Three dots under the score break the same measurement down per family, so you can see whether it is the verticals or one of the two receding directions doing the damage. Behind Details are the raw numbers: convergence in degrees, how far the corners drifted, and whether the individual lines were confident or wobbly.
A box that cannot be read as a box at all — too few edges, or edges that do not form a closed form — is not given a number. It says what is missing instead of inventing a grade.
The three drills
Freeform
Draw a box, any box. Graded on internal consistency: does it agree with itself. This is the one that matches the 250 box challenge.
Memory
A cube appears for a few seconds and then hides, leaving its near corner behind. You rebuild it. Scored on how close your box lands to the one you were shown, as well as on its own convergence.
Face
You are given one face of a cube and the vanishing point its depth runs toward. Extrude it into the full box. The free choice is how far back the far face sits, which is exactly the judgement the drill is for.
Practising the challenge
The 250 box challenge is a long drawing exercise: two hundred and fifty boxes in three-point perspective, drawn freehand, one at a time, checking convergence as you go. It is part of Drawabox, a free course, and the challenge and its instructions belong there — this is a tool for the checking half of it.
Things worth keeping in mind while you draw:
- Start from the corner nearest you and build outward, so the three directions are set before the box closes.
- Draw from the shoulder in one confident pass. A wobbly line that lands in the right place still beats a careful line that does not.
- Ghost the stroke over the page a couple of times before committing it.
- Vary the boxes. Rotate them, foreshorten them hard, put the vanishing points somewhere new. Two hundred and fifty of the same box teaches you one box.
- Check every one, and read the correction. Drawing a box without checking it is repetition, not practice.
What it doesn't do
It cannot look at a photograph of a box you drew on paper — there is no upload, and adding one is not on the table today. It has no account, no login and no server: the drawing, the grade and your history stay in the browser on the device you drew them on, and none of it is sent anywhere. Clear your browser data and it is gone. It also does not know anything about the course beyond the geometry, so it cannot tell you whether your line quality is improving in the ways a human critique would.
Questions
Is this a replacement for the manual line-extension check?
No. It is the same check done in degrees instead of by eye, so you can do it on every box without it costing you the evening. The judgement about your drawing — the part a person gives you — is still the part that matters.
What does the 0–100 score actually measure?
How far your twelve edges miss the three vanishing points they imply, in degrees: a root-mean-square error per family of four, averaged across the families. Zero error scores 100. It is one measurement re-dressed as a number, not a blend of several opinions.
Do I need an account, and does anything leave my device?
No account, and nothing leaves your device. Everything runs in the browser, your boxes are kept in the browser's own storage, and the app keeps working with no network at all once it has loaded.
Can I use it on an iPad, or with a drawing tablet?
Yes — it takes pen, touch and mouse input. If you are on a mouse there is a straight-line tool and a smoothing mode, because a mouse makes a fair test of freehand line quality impossible either way.
Can I draw a box that isn't in three-point perspective?
You can draw anything, but the measurement assumes a box with three groups of converging edges. A box with parallel edges in some direction has no finite vanishing point there, and that family is reported as not judgeable rather than scored.
What is the Y method?
Starting each box from the three edges of its nearest corner — a Y — so the three directions are committed before anything closes. The app can hand you a random one, or you can start from a blank sheet. There is a Y generator if you want one to draw from on paper.