Why your PNG turned white: transparency, explained

What transparency actually is

A normal colour pixel holds three numbers: how much red, green, and blue. A pixel in a format that supports transparency holds a fourth, called alpha, which says how opaque that pixel is. Alpha 255 is fully solid, alpha 0 is fully invisible, and everything in between is a partial blend — which is what gives a good cutout its soft, non-jagged edge.

That fourth number is the whole story. A format either has somewhere to put it or it does not. When it does not, the alpha information cannot be carried across, and the converter has to decide what colour those see-through pixels should become.

Which formats keep it

What our converter does with the alpha channel

When the target cannot hold transparency, the image is not simply stripped of its alpha — that would leave whatever colour data happened to be hiding underneath, which for many files is black or garbage. Instead a solid white canvas is created at the same size, and your image is composited onto it using its own alpha as the mask. Fully transparent pixels become pure white, half-transparent edge pixels become a half-blend with white, and solid pixels are unchanged.

The practical consequence is that the anti-aliased edge of your cutout is now baked against white. If you later place that image on a dark background, you will see a pale halo around it. That halo is not an artifact of the conversion being bad; it is the correct rendering of your image against the background it was flattened onto.

A few formats get special handling because they are more restrictive still. MSP and PBM are one-bit formats, so the image is flattened onto white and then dithered down to pure black and white. Palm is palette-only with no alpha, so it is flattened first and then reduced to 256 colours. BLP, the Blizzard texture format, is palette-based but does keep an alpha channel, so it is quantized to 256 colours with the transparency preserved.

The mistakes worth avoiding

• Do not convert a logo to JPG and then convert it back to PNG expecting the transparency to return. The alpha was discarded, and the white is now part of the picture. You need the original file.
• Do not use GIF for a cutout with soft edges. GIF transparency is a single on/off colour key, so the soft edge either disappears or leaves a fringe of near-background pixels.
• Do not assume a white background is safe. If the site or document you are placing the image on has any other colour, the white box will show.
• Do not judge transparency by looking at the file in a viewer with a white interface. Open it over a dark background, or check the format, before you conclude the cutout survived.

What to do instead

If you need a smaller file and you need the transparency, convert to WebP rather than JPG. WebP keeps full alpha and is usually smaller than the PNG you started with — for a logo or an icon it is very often the right answer and almost nobody reaches for it.

If the destination genuinely will not take anything but JPG, flatten deliberately rather than accidentally: open the image in an editor, place your own background in the colour the final context will use, and export that. You will get an edge that blends correctly instead of a white halo.

And if you are producing the original artwork, keep the layered source. Every conversion in this article is one-way.