PNG to JPG

When this is a good idea

For photographs, JPEG at quality 85 is typically 70 to 90% smaller than the same picture as PNG, with no visible difference. For screenshots of text, interfaces and diagrams, it is the wrong move. JPEG smears sharp edges and colored text, and the file often is not much smaller anyway. Keep those as PNG, or use WebP.

Transparency

JPEG has no alpha channel. Transparent areas are composited onto white by default here. Untick the option to get black instead.

This is the single most common surprise in the conversion, and it is worth understanding rather than working around. A logo with a transparent background is not a logo on white; it is a logo on nothing. JPEG has nowhere to store "nothing", so something has to be chosen. Left to its own devices FFmpeg typically fills it with black, which is why converted logos so often come back sitting in a dark rectangle. Flattening onto white is the default here because it is what almost everybody wants and almost nobody thinks to ask for.

If the image is going onto a colored background, neither choice will look right, and the answer is to keep it as PNG or WebP rather than to fight it.

How to tell if your PNG is a photograph

The distinction that decides whether this conversion is a good idea is not the subject, it is whether the image has hard edges. A photograph is continuous tone, gradients everywhere, no two adjacent pixels quite the same; JPEG was designed for exactly that and handles it beautifully. A screenshot, a chart, a logo or anything with text is the opposite: large flat areas separated by sharp boundaries, which is what JPEG's block-based compression handles worst. Look at the picture at 100% and ask whether there is any text or any hard line in it. If there is, keep PNG or use WebP, which does both jobs well.

Quality, and what the numbers mean

Quality 85 is the standard sweet spot and is the default here for good reason: on a photograph the difference from 100 is essentially invisible, and the file is several times smaller. 90 to 95 is worth it when the image will be printed or examined closely. Below 70 the artefacts around edges become obvious even on photographic content, and below 50 there is no reason to be using JPEG at all rather than a smaller resolution at a higher quality, which almost always looks better.

The size difference is also non-linear. Going from 100 to 85 typically saves more than half the file. Going from 85 to 70 saves perhaps another fifth, for a much more visible cost. Most of the benefit is in the first step.

Common questions

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