Perspective Ruler Online
Place the vanishing points, then draw. Every stroke snaps to the point it aims at, like Clip Studio's ruler, in the browser.
Draw straight toward the vanishing point without a ruler
A perspective ruler is the oldest perspective tool there is: a pin in the vanishing point and a straightedge that pivots on it. This page is that ruler in a browser. Put the vanishing points where the scene needs them, then draw: every stroke is projected onto the line through its start point and the vanishing point it aims at, so a rough stroke becomes a ruled line that converges correctly. Verticals stay vertical in a level view or snap to the third point in a tilted one. It is the same behaviour as the perspective ruler in Clip Studio Paint and the assistants in Krita, without opening either.
Placing the points
Drag the VP1 and VP2 dots; when a point is off the canvas, an arrow at the edge shows where it went, and you can still drag it. Level VP1 and VP2 puts them on one horizon. Set from a camera places both points for a given lens and rotation, which is the reliable way to get a normal-looking view: for a 35 mm lens turned 32°, they land about a picture-width outside each edge. Tick Third point for a camera looking up or down, then click on the canvas to place VP3 far above or below.
Drawing
Block in with snapped strokes: the near corner of a box first, then its edges to each point, then the verticals. Switch to freehand for anything organic. The snap bar can also force a stroke to a particular point, which helps when the two directions are close together near the horizon. Undo, redo and clear are in the top bar; Ctrl/Cmd+Z works too.
Over a photo
Drop a reference photo onto the canvas and set its opacity. Place the points on the photo's own converging edges, or read them with the vanishing point finder first and copy them over; then everything you draw converges the way the photo does. This is the fastest way to add a building, a piece of furniture or a sign into a photographed scene.
Related
For a full grid rather than a ruler, the 2-point and 3-point grids; for the same snapping inside a painting app, Clip Studio's perspective ruler and Krita's assistants; for what to draw, how to draw perspective and how to draw a room in perspective.
Questions
What is a perspective ruler?
A ruler that pivots on a vanishing point, so every line you draw with it converges correctly. Physically it is a pin and a straightedge; in Clip Studio Paint it is the Perspective Ruler; here it is a canvas where each stroke is projected onto the line from its start point to the vanishing point it aims at.
How does the snapping decide which point to use?
From the direction of the first few pixels of the stroke. If you start drawing roughly toward VP1, the stroke locks to VP1; toward VP2, to VP2; up or down, to vertical (or VP3 when the third point is on). Start the stroke a little more clearly along the edge you mean if it picks the wrong one, or force a point in the snap bar.
How do I draw a freehand line?
Choose freehand in the snap bar. Curves, figures and lettering go on freehand; block-ins go on snapped strokes.
Where should the vanishing points be?
For a normal lens they belong outside the picture, roughly a picture-width out on each side for a 35–50 mm view. Place VP1 and VP2 from a lens and rotation puts them at the right spacing automatically; the vanishing point distance calculator gives the number.
Can I draw over a photo?
Yes. Drop, paste or open a photo; it sits under the sketch at the opacity you set. Drag the vanishing point dots onto the photo's converging edges, or run the photo through the vanishing point finder to get them exactly. The photo never leaves your device.
Can I export just the lines?
Download sketch (PNG) exports the strokes alone on a transparent background, without the guide fan or the horizon, ready to paste as a layer. PNG with reference bakes the photo in underneath.