Crop Images

What cropping costs, and what it does not

Cropping discards pixels and keeps the rest exactly as they were — there is no resampling, no scaling and no interpolation, so the part you keep is pixel-identical to the original. The one thing that does happen is re-encoding: the remaining pixels have to be written into a new file. For a PNG that is lossless and costs nothing. For a JPG it is a second generation of lossy compression, applied to pixels that already carry the first — usually invisible at a sensible quality, and worth knowing if the picture has already been through several rounds of saving.

The consequence people miss is resolution rather than quality. Cropping to the middle third of a photograph leaves you with a ninth of the pixels, and no amount of enlarging afterwards puts them back. A 12-megapixel phone photo cropped hard for a close-up can easily end up below what a print or a full-width web banner needs — so crop from the largest original you have, not from a copy that was already resized for a website.

Ratios are there because most destinations have opinions. 1:1 for a profile picture, 16:9 for a video thumbnail or a slide, 4:5 for the tallest portrait a social feed will show without cutting it — picking the ratio before framing means the crop you see is the crop that survives, rather than something the platform trims again on your behalf. Cropping freely and letting the destination decide is how faces end up half out of frame.

The file is written back in its original format, so a PNG stays a PNG with its transparency intact and a JPG stays a JPG. Camera and location metadata do not survive the re-encoding, which is usually what you want for a picture heading somewhere public.

When you want something else

For a folder of images that all need the same crop — a batch of product shots from a fixed camera, or scanned pages with the same border — cropping one at a time is the wrong shape of work. magick mogrify -crop 800x600+100+50 +repage *.jpg applies identical geometry to everything in a directory, and libvips does it using far less memory on large files.

For a JPG specifically, there is a genuinely better method that no browser can offer: jpegtran -crop 800x600+96+48 -copy none in.jpg out.jpg crops without decoding and re-encoding at all, so there is no second generation of loss whatsoever. The catch is that it can only cut on the format's own 8- or 16-pixel block boundaries, so the crop lands near where you asked rather than exactly — a fair trade when the picture matters more than the last few pixels.