What really happens to transparency when PNG becomes JPG?

Transparency is data, not an empty area

A PNG can store an alpha value beside every pixel. At full opacity the pixel is solid; at partial opacity the background shows through; at zero opacity it is invisible. Logos, icons, cut-out products, and soft shadows rely on these values even when the transparent region looks like blank space in an editor.

Baseline JPEG stores color and brightness information but no alpha channel. A PNG-to-JPG converter therefore has to composite the PNG over a solid background before JPEG encoding. That background becomes permanent pixel data in the new file.

Choose the background before you convert

Use white when the image will live on white pages, black for dark layouts, or a background matching the final design. If the destination can change between light and dark modes, JPEG is usually the wrong delivery format for a transparent asset; keep PNG or use transparent WebP instead.

Soft edges deserve special attention. Anti-aliased pixels around a logo are partially transparent. Flattening them on white and later displaying the JPEG on dark gray can produce a pale fringe. Composite onto the real destination color to prevent that mismatch.

Quality affects edges and text

JPEG divides an image into blocks and approximates detail to reduce size. Photographs tolerate this well, but flat colors, small text, and sharp logo edges reveal artifacts sooner. Start near quality 90 for graphics and inspect the result at 100% zoom before reducing quality further.

Converting the JPEG back to PNG later stops additional loss on that save, but it cannot reconstruct the discarded transparency or edge detail. Preserve the source PNG as the editable master.

• Photograph with no transparency: JPEG is usually appropriate.
• Logo, icon, screenshot, or text-heavy graphic: PNG is usually safer.
• Transparent web asset where size matters: test WebP while retaining the PNG original.

A repeatable check

Open the PNG over a checkerboard or contrasting background and look for transparent or semi-transparent edges. Convert one copy on white and another on the destination color. Compare both at normal size and at 100% zoom, then keep the smaller file only if its edges and text remain clean.