How to fix perspective and straighten buildings in a photo

Straighten a building in five steps

1. 1

   Open the photo

   Open the photo with File› then Open… (Ctrl+O or ⌘O); to follow along, open the sample from this page. Look for lines that are truly vertical, such as window columns, and lines that should be level, such as floors.

2. 2

   Choose the Perspective Crop tool

   Right-click (or press and hold) the Crop tool in the toolbar and choose Perspective Crop. A frame with corner and edge handles appears around the whole photo.

   

3. 3

   Drag the corners onto the building

   Drag each corner handle so the sides of the frame follow lines that are truly vertical in the building. On the sample, put them on the outer edges of the leftmost and rightmost window columns: the top corners at the highest row with separate small balconies, just below the long balcony, and the bottom corners at the balconies of the lowest row of windows. The grid inside the frame shows how the area will be straightened; its lines should run along the window columns and floors.

   Don’t use the building’s outline or the edge of the photo: on the sample, the outer edges belong to corner walls that turn away from you, so they lean more than the facade does. The edge handles move a whole side at once, and a drag that doesn’t start exactly on a handle moves the whole frame.

   Tip Lost track of the corners? Click Reset on the options bar to start the frame again. Esc removes the frame; the tool stays selected.

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4. 4

   Check the output size

   The options bar shows the size of the result. Width and Height at 0 mean automatic: Lil Cactus averages the frame’s sides, which gives about 1036 × 885 px on the sample. That can’t know the building’s real proportions, and a steep upward shot comes out squashed, with windows too wide. There is no live preview of the output, so type a Height and apply: 1450 px works for the sample. If the windows still look too wide or too tall, undo with Ctrl+Z or ⌘Z. Undo leaves no frame, so click Reset on the options bar, place the corners on the window columns again, type another Height and apply. For a document, type its real proportions, such as 2100 × 2970 for A4.

   

   

5. 5

   Apply the crop and save

   Click the ✓ at the right end of the options bar. Enter or ↩ applies too, but not while the cursor is still in the Height field, so clicking the ✓ is the simplest. The photo is cropped to the frame and straightened: the window columns are now vertical and parallel, as if you had photographed the facade square-on from halfway up.

   Export a new file with File› then Export› then JPEG…, or use File› then Save (Ctrl+S or ⌘S) to write the result back into the JPEG you opened; the first time, the Save Image dialog asks for its quality.

Keep the whole frame: Lens Correction

Perspective Crop throws away what lies outside the frame. To keep the street and the neighboring buildings, choose Filter› then Lens Correction… and click the Perspective tab:

• Vertical perspective (−100 to 100, default 0): positive values widen the top of the photo and narrow the bottom, which straightens verticals that lean in toward the top. Negative values do the opposite.
• Horizontal perspective: the same for a wall photographed from the side.
• Angle: rotates the photo to level a tilted horizon.
• Scale (default 100%): zooms the result; raise it to fill the transparent edges.

The sample leans so strongly that even the maximum, 100, still leaves the outer columns leaning in by about 10°, which is why Perspective Crop is the main method here. Lens Correction suits a gentle lean. Tick Show alignment grid to compare lines against true verticals. Correcting perspective pulls the bottom corners inward, leaving transparent wedges at the edges: raise Scale until they disappear (135% at a Vertical perspective of 100) or crop them away afterward with the Crop tool. The Lens tab’s Distortion slider straightens lines that bow outward from a wide-angle lens.

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Free-hand: Edit › Transform › Perspective

For fine control, choose Edit› then Transform› then Perspective: dragging a top corner outward moves the other top corner outward by the same amount, which is exactly what converging verticals need. Edit› then Transform› then Distort moves each corner on its own. Zoom out first with View› then Zoom Out so there is room to drag past the edges of the photo. Press Enter or ↩ to apply. Turn on View› then Show› then Grid first if you want lines to check against.

Why buildings lean in photos

A camera records straight lines as straight, but parallel lines only stay parallel in the photo if they are parallel to the sensor. Tilt the camera up, and the building’s verticals point away from the sensor: like railway tracks running into the distance, they meet at a vanishing point above the photo. Perspective correction moves that vanishing point back to infinity by stretching the top of the image more than the bottom. Leave space around a building when you shoot it: some of the frame is always lost.

Fix common problems

The building looks too tall and thin, or squat

Perspective correction can’t know the building’s real proportions. In Perspective Crop, set Height by hand; with Lens Correction, use Image› then Image Size… afterward with the chain link off to adjust height alone.

Verticals still lean in toward the top

The correction is too weak: the top corners sit outside the lines that are truly vertical. Move them closer together, onto the column edges, or raise Vertical perspective.

Verticals now lean outward

The correction is too strong: move the top corners apart or the bottom corners closer together, or lower Vertical perspective.

Lines still bend

That is lens distortion, not perspective. Correct it first with Distortion on Lens Correction’s Lens tab, then fix the perspective.

What to do next

• Crop to a final shape: how to crop an image.
• Widen the photo without stretching: content-aware scale.
• Resize the result for the web.

Coming from another editor

Photoshop

The Perspective Crop tool shares a button with the Crop tool, and Filter › Lens Correction… has Vertical and Horizontal Perspective.

GIMP

The Perspective tool (Tools › Transform Tools › Perspective). In Lil Cactus the closest equivalents are the Perspective Crop tool and Edit › Transform › Perspective.

Perspective correction: questions and answers