Fix Image Perspective

Why a photo of a screen looks wrong

A camera pointed at a flat object from an angle sees the near edge bigger than the far edge. The rectangle you photographed lands in the picture as a four-sided shape with no right angles: a trapezoid if you stood off to one side, something less regular if you were also above or below it. Rotating cannot fix that, because no single rotation makes all four edges parallel again. Cropping cannot fix it either, because the corners are in the wrong places relative to each other.

What fixes it is the transform a camera applied in the first place, run backwards. In maths it is called a homography: a 3 by 3 matrix that maps any four points on one flat plane to any four points on another, while keeping straight lines straight. Four corner pairs are exactly enough to pin down its eight unknowns. This page builds those eight equations from your four handles and the four corners of the output rectangle, solves them, and then for every pixel of the output works out which point of your photo it came from.

What happens to the pixels

The output is drawn backwards, one pixel at a time. For each pixel of the flat result, the matrix gives a position in the original photo, usually between pixels, and the colour is blended from the four photo pixels around that position (bilinear sampling). Drawing it this way round means every output pixel gets a value, with no gaps or holes, which is the problem with pushing source pixels forwards.

The far end of a skewed object had fewer pixels to begin with, so straightening it stretches those pixels. Text near the far edge will be softer than text near the camera. Nothing can restore detail the camera never recorded, but a straight result at the right proportions is far easier to read and far better in a thumbnail than the skewed original.

Getting the shape right

By default the output width is the longer of the top and bottom edges you marked, and the height is the longer of the two sides. That keeps every bit of resolution the photo has, but it is not always the true shape: the far edge of a skewed object is foreshortened, so a 16:9 screen shot steeply from the side can come out narrower than 16:9. If you know what the object is, pick its shape from the list. A TV or laptop screen is almost always 16:9, a sheet of paper is A4 or US Letter, a phone screen held upright is close to 9:16. The longer side stays at the size measured from your corners and the other side follows the shape you chose.

• Put the handles on the outer corners of the thing you want, not on a frame or shadow around it, unless you want those in the result.
• Use the magnifier. A handle one pixel off on a small photo shows up as a slight slope along a whole edge.
• The handle order does not matter. The page sorts the four points into top left, top right, bottom right and bottom left before solving, so the result is never mirrored. It does refuse a shape where one corner is pushed inside the others, because no flat rectangle could have been photographed that way.

Using it for a thumbnail

A straightened screen is the cleanest way to show a game, an app or a website in a thumbnail without a screen recording. Shoot the monitor, flatten it here at 16:9, then open it in the editor to put your face and title over it. A flattened page or receipt works the same way for "I read the fine print" videos. If the result is soft, the sharpen tool brings back some edge contrast, and the Thumbnail Autopsy tells you whether the text still reads at feed size.