What this page changes
Two things start differently from the front page. Quality opens at 95 instead of 86, and the size panel is gone: the JPG is written at the PNG's own width and height, so a misplaced click cannot shrink it. Everything else — the background under transparent pixels, the edge repair, the measured size — is the same converter.
The Quality panel prints the colour sampling the browser actually wrote, read out of the JPEG's frame header. On artwork with fine coloured detail — red text, thin coloured lines — the classifier may suggest the picture wants full-resolution colour; a button then tries the higher steps up to 100 and keeps the lowest that comes back 4:4:4, if any does.
Honest limits
The browser decides the colour sampling.
The canvas encoder in a web page accepts a type and a quality, and nothing else. So this page cannot promise 4:4:4; it can only read back what was written and tell you. Some browsers write 4:2:0 at every quality, and when that happens the page says the PNG is the way to keep the colour.
- High quality is not lossless. For an exact copy, keep the PNG.
- A flat graphic or a screenshot can come out larger as a JPG than as a PNG. The verdict beside Save shows both figures when it does.
- Nothing from the PNG's metadata is carried over; the pixels are what is converted.
Questions about quality
- Can a JPG be lossless?
- No. JPEG always rounds something away, even at quality 100. What high quality buys is rounding small enough that you cannot see it at normal viewing size. If the file must be exact, it has to stay a PNG — and when the JPG would be larger anyway, the page says so and offers the PNG back.
- Does “same size” mean pixels or kilobytes?
- On this page it means pixels: the JPG has exactly the PNG's width and height, and there is no resize panel to change that by accident. The kilobytes are printed, measured, on the Save button. They are usually far fewer than the PNG's for a photograph and sometimes more for a flat graphic.
- What is “full colour” here?
- JPEG can store colour at half the resolution of brightness (4:2:0) or at the same resolution (4:4:4). The browser's own encoder chooses, and there is no setting a web page can pass it. This page reads the choice back out of the written file and prints it, and on a picture that wants 4:4:4 it can try higher qualities to see whether the browser switches.
- Why 95 and not 100?
- Between 95 and 100 the file grows quickly and the picture barely changes. 95 is the point most people stop seeing a difference on photographs and artwork alike. Move the slider to 100 if you want the last step; the size is measured either way.
Next steps
- When to keep the PNGThe cases where the JPG is bigger, softer, or loses something needed.
- PNG to JPG in KBWhen a form sets a size limit and quality has to give way.
- Transparent PNG to JPGWhen the hard part is the edge, not the quality.
