Vanishing Point Finder
Drag two lines along parallel edges. Get the vanishing points, the horizon, the lens the photo was taken with, and a grid you can draw over.
Finding vanishing points in a photo
Every set of parallel edges in a scene meets at one point in the picture. This tool asks you to mark two edges from the same set; it extends them, finds the intersection, and draws a fan of guide lines from it. Do that for two sets of horizontal edges and you have the horizon. Add the verticals and you have full three-point construction over the photo.
Choose edges that are long and clearly parallel: the top and bottom of a wall, the lines of a roof and a foundation, window sills on the same face. Avoid edges that only look parallel (a road that curves, a fence that turns).
What you get
Vanishing points in pixel coordinates, even when they fall outside the picture. The horizon line, dashed, with its height as a percentage of the frame and its tilt. A camera estimate: lens in 35 mm-equivalent millimetres, field of view, and how much the camera was tilted and turned. A grid overlay you can download on the photo or as a transparent layer to trace over. And a button that opens the 3-point grid at the same lens, tilt and rotation, so you can draw the scene from scratch with the photo as a reference.
Reading the camera estimate
The lens estimate assumes the picture centre is where the camera pointed (no cropping) and that the two sets of edges are at right angles to each other. Photos straight from a camera or phone satisfy the first; two faces of one building satisfy the second. A large "roll" means the horizon is tilted; straighten the photo first if you need a precise match.
Why bother
Matching the vanishing points of a reference photo is the single most useful thing a perspective tool can do for an illustrator working from photos: you draw with the photo's own geometry instead of guessing, and you can add objects that sit convincingly in the scene. It is also the fastest way to learn to see perspective; after a dozen photos you will start finding the horizon by eye.
Related
Perspective correction straightens one plane of the photo once you know where it is. How to draw perspective explains horizon and vanishing points from the drawing side.
Questions
How do I find the vanishing point in a photo?
Pick two edges that you know are parallel in the real scene, such as the top and bottom edges of a wall, a row of windows, or the two sides of a road. Extend them until they cross; that crossing is the vanishing point for that direction. This tool does the extending for you and marks the point.
How do I find the horizon line?
Find two vanishing points for two horizontal directions (two faces of a building). The straight line through them is the horizon, which is also the camera's eye level. In a one-point photo, the horizon is the horizontal line through the single vanishing point.
How does it know the lens?
If the two directions are perpendicular (the two faces of a rectangular building), their vanishing points constrain the camera's focal length: f² = −(v₁ − c)·(v₂ − c), where c is the image centre. From f the field of view and the 35 mm-equivalent lens follow. It assumes the photo has not been cropped off-centre.
Why does it say the points cannot be perpendicular?
Both vanishing points sit on the same side of the picture centre, which cannot happen for edges at right angles with a centred lens. Usually one pair of lines is following edges that are not parallel to each other, or the photo was cropped heavily. Adjust the lines.
Does it upload my photo?
No. Everything is computed in your browser.