Transparent PNG, Explained (and Why Your Cutout Has a White Halo)

If you have ever cut out a subject, dropped it onto a coloured background, and noticed a faint white halo around the edges, you have hit the classic transparent-PNG problem. It is not a bug in your tool — it is a side effect of how transparency is encoded in PNG files and how most image editors handle it. The good news: the fix is 30 seconds and does not require Photoshop.

How transparency works in a PNG file

A 32-bit PNG has four channels per pixel: red, green, blue, and alpha. The alpha channel controls how opaque the pixel is: 0 means fully transparent, 255 means fully opaque, anything in between is partially see-through. When a background-removal tool outputs a transparent PNG, it sets the background pixels to alpha 0 (fully transparent) and the subject pixels to alpha 255 (fully opaque). The edges — where the subject meets the background — get a gradient of alpha values, which is what gives a clean cutout its soft, natural edge.

Why the white halo appears

The halo happens when the edge pixels are not fully transparent. If a pixel at the edge of the subject is set to alpha 250 (almost opaque) but its colour is still the original background colour (white, in most studio shots), then when you drop the cutout onto a dark background that pixel still shows up as faint white. The fix is to either (1) push the edge pixels all the way to alpha 0, or (2) replace their colour with a neutral that matches the new background.

The 30-second fix in BGRemover

BGRemover has a 'defringe' toggle in the export panel. Turn it on, and the AI replaces the colour of any partially-transparent edge pixel with the colour of the adjacent opaque pixel. The halo disappears instantly. For most images, the default settings are correct; for stubborn halos on dark backgrounds, the defringe is the lever to pull.

When the halo is in the original image (and what to do)

Sometimes the halo is in the source photo, not in the cutout. Studio shots with strong backlighting produce a thin glow around the subject, and that glow ends up baked into the edge. The fix is to remove the backlight in the original photo (re-shoot if possible) or use the brush tool in BGRemover to manually paint the affected edge to fully opaque. The brush is the right tool for one-off halos; the defringe is the right tool for systemic halos.