Auto image trimmer

What it cuts, and how it knows

A picture exported from a design tool, cut out of a photo or scanned often carries more empty space than subject: a logo afloat in a transparent square, a product on a wide white ground. Trimming finds where the subject begins and ends on each side, and keeps exactly that frame.

• The background is read from the picture’s border: transparency when the border is empty, else the color most of the border has, whatever that color is.
• Every pixel that differs from it by more than the tolerance belongs to the subject; the frame is the smallest one that holds them all.
• Nothing is guessed about what the subject is, so nothing of it is cut. This is not background removal: inside the frame, the picture is untouched.

Tolerance, margin and square

A background is rarely as even as it looks. Around anything saved as JPG it wavers by a few levels; a scan carries the grain of its paper; a cut-out can end in a shadow too faint to see. The tolerance says how far from the background a pixel must be to count. At 0, anything that is not exactly background is kept; the starting value, 4%, reads through the noise of a JPG; higher values cut faint shadows and uneven grounds as well. On a transparent background, which has no such noise, the tolerance counts for a fifth: a soft shadow is kept to its last visible pixel unless you raise it.

A margin leaves room around the subject, in pixels or as a share of its longer side; where the picture had less room than that, the margin is made of the same background. Square centers the subject in a square, which is what an app icon, an avatar or a grid of products asks for; it is also a page of its own, the auto image square trimmer.

On a scan, one dot of dust in a corner is enough to hold the frame open. “Ignore specks” counts a row or a column as part of the subject only from a few pixels up.

What comes out

The file keeps its format when your browser can write it (PNG, JPG, WebP, AVIF), and its size in pixels is simply that of the frame: nothing is resampled. A JPG, a WebP or an AVIF is written again at high quality, since none of them can be cut without being encoded again; choose PNG to stop the loss there. What the camera wrote into the file (place, date, device) is not carried over.

To cut a picture to a circle, a star or another shape instead, use the shape crop; to change its format, the image converter.