Image Converter

Re-saving a JPEG costs pixels and does not cost bytes

Which is why nobody notices. Take a 1200 × 630 image, save it at quality 85, open that and save it again at the same setting, eight times over. The drift column is the average distance every colour channel has moved from where it started.

The file size moves by 0.17% across the whole run, and it moves up as well as down, which is how you can tell it is noise rather than a signal. Meanwhile one channel has shifted by 57 out of 255 — a fifth of the range — and nothing in the file size hints at it.

Most of the damage lands in the first few passes and then slows, and it concentrates where the compression works hardest: edges, lettering, fine texture. A photograph re-saved eight times looks much the same. A screenshot of text does not. Quality 85 is not a floor you settle onto — every pass takes a fresh bite.

WebP is roughly half the size at every setting

The same image encoded to both, at matching quality numbers.

The margin widens as the quality rises, which is the opposite of what you might expect. One caveat worth stating: the two quality scales are not the same thing — 85 in one encoder does not mean what 85 means in the other — so this is not quite like for like. The gap is large enough that no reasonable re-alignment closes it.

Lossless PNG of the same picture is 69.4 KB, which lands between JPEG at 95 and JPEG at 100 — 57% more than JPEG at 85 for no visible gain. PNG earns its keep on flat colour, line art and screenshots, and loses badly on photographs.

Transparency becomes black, and not for the reason usually given

JPEG has no alpha channel, so something has to be decided about the transparent pixels. A PNG does keep a colour behind them and it can be anything. Encoding three of them — red, blue and black, all at zero alpha — gives black every time.

So "it showed the hidden colour" is not the explanation — the stored colour is discarded and replaced. This converter fills white before drawing whenever the target cannot carry transparency, which is why it does not happen here.

Filled first: image/jpeg and pdf. Left alone: image/png and image/webp, both of which carry an alpha channel of their own.

One last conversion worth naming. A PDF measures in points, seventy-two to the inch, while an image measures in pixels at ninety-six — so a pixel is exactly 0.75 of a point. Matching the page to the image gives 900 × 472.5 points, or 12.5 inches wide, which is wider than A4 and Letter both. Matching the page and fitting to a page are genuinely different requests.