What this mode does
A print photographed at an angle is a quadrilateral, not a rectangle. Four corners are enough to describe it, and a projective transform maps those four points back onto a rectangle with the print’s own proportions. Every pixel of the result is sampled from the phone’s picture by bilinear interpolation; nothing is drawn.
Glare is the other phone problem. It is found as blown-out patches where all three channels sit at or near white, at least two dozen pixels across, grown by two pixels for the soft edge. It is counted on the bench, and filling it is your choice.
When this is the wrong tool
- A curled or warped print is not flat, and four corners cannot straighten a curve. Flatten it under glass and photograph it again.
- Glare that covers a face or a key detail is lost picture in this photo, but not on the print: retake it.
- A flatbed scan needs no straightening. Open it on the main bench.
Questions about phone photos of prints
Can I restore a photo I took of an old photo with my phone?
Yes. A phone picture of a print has two problems a scan does not: the print is usually at an angle, and there is often glare from a lamp or window. This page fixes the angle first, from the print's four corners, then opens the normal repair bench with the glare found and counted.
How does it find the corners?
From the edge of the print against what it lies on. The photo is reduced, split into lighter and darker by Otsu's method, and the region holding the centre of the frame is taken as the print; its corners are its extreme points along the two diagonals. When nothing print-shaped stands out, the corners start just inside the frame and the page says so. Either way you drag them into place, with the mouse, a finger or the arrow keys.
What size is the straightened photo?
The print's measured shape: the average length of its opposite edges, in the pixels your phone captured. It is never enlarged beyond what the phone recorded, and the album page or table around the print is cropped away.
Can it remove glare?
It finds it: blown-out highlight patches where all three channels are nearly white, large enough to be a reflection rather than a catchlight. Glare hides picture that is still on the print, so the first advice is a second photo from another angle with the flash off. Marking the glare for the fill is offered second, and the fill then invents what was under it from what surrounds it.
Is a phone photo as good as a scan?
Rarely, but it can be close. Shoot in shade or even daylight, flash off, with the phone parallel to the print and filling the frame. HEIC from an iPhone opens here directly.
