Upscale an Image 2× or 4×

Resampling is interpolation, not invention

Enlarging computes new pixels from the ones around them. A good filter — Lanczos, here — weights a neighbourhood of source pixels to estimate each new one, which keeps edges clean where a naive method would produce stair-stepping or blur.

What it cannot do is add detail that was never captured. If a face occupies forty pixels in the original, no filter recovers the eyelashes, because that information does not exist in the file. Enlargement makes an image bigger and, done well, keeps it looking deliberate rather than stretched.

Why this is not an AI upscaler, on purpose

Model-based super-resolution genuinely can hallucinate plausible detail, and on the right image it is impressive. In a browser tab it is also slow enough to lock the page for tens of seconds on a large photo, and memory-hungry enough to crash a phone.

There is a second, less discussed cost: an AI upscaler invents detail, and invented detail is wrong detail. On a document, a licence plate or a face, that is a liability rather than a feature. A resampled enlargement is honest about what it knows.

Sharpening after, not before

Enlargement softens edges slightly no matter how good the filter is, so a gentle unsharp pass afterwards restores the appearance of crispness. Applied before enlargement, the same sharpening gets magnified along with everything else and turns into visible haloes.

Overdoing it is the common mistake. Sharpening amplifies noise and JPEG artefacts as readily as detail, so an already-compressed source will show its blocking pattern long before it looks sharp.

When enlargement is the wrong answer

If you need a larger image for print and have access to the original file, go back to it. A camera original or a vector source beats any enlargement of a downscaled copy, and the difference is not subtle.

• Logos and icons: find the SVG and rasterise it instead — infinitely better than any enlargement.
• Screenshots of text: retake at a higher zoom rather than enlarging.
• Heavily compressed images: compress artefacts enlarge too, and sharpening makes them worse.
• Print: 2x from a good original is usually plenty; 4x from a thumbnail will not rescue it.