Why does my transparent PNG turn black when converted to JPEG?

JPEG has no alpha channel. Transparent pixels usually store black underneath, which is revealed when the alpha is discarded. Fix it with -background white -alpha remove -alpha off.

What is the difference between -flatten and -alpha remove?

Both composite against the background, but -flatten also applies layer offsets, so an image with a virtual canvas can shift or gain padding. -alpha remove only touches the alpha channel and is more predictable.

Why do my rounded corners come out black?

The rounding is stored in the alpha channel, and JPEG cannot hold it. Write the result as PNG or WebP instead.

Transparent PNG becomes black after converting to JPEG

JPEG has no alpha channel, so transparent pixels keep whatever RGB is stored underneath — usually black. The correct -alpha remove fix, and why -flatten can misbehave.

You converted a PNG with transparency to JPEG and the transparent areas came out solid black:

magick logo.png logo.jpg    # transparent background → black

Why

JPEG has no alpha channel. When ImageMagick discards the alpha, the underlying RGB values remain — and in most PNGs written by design tools, fully transparent pixels are stored as black with zero alpha. Remove the alpha and you reveal the black.

This is not ImageMagick misbehaving. Any tool that flattens without being told a background colour has the same problem.

The fix

magick logo.png -background white -alpha remove -alpha off logo.jpg

In order:

-background white — the colour to composite against. This is a setting, so it must come before the operators that use it.
-alpha remove — composites the image over that background.
-alpha off — drops the now-redundant channel so the file is smaller.

Why not -flatten?

magick logo.png -background white -flatten logo.jpg

This usually works, and you will see it recommended everywhere. The difference is that -flatten also merges layer offsets. If the PNG carries a virtual canvas — very common after a crop, and standard in GIF frames — -flatten positions the image according to that offset and you get unexpected padding or a shifted result.

-alpha remove only touches the alpha channel, so it is the more predictable choice. If you do use -flatten, add +repage first.

Any background colour

magick logo.png -background '#0d9488' -alpha remove -alpha off out.jpg
magick logo.png -background none -alpha remove -alpha off out.png

Batch a whole folder

mkdir -p jpg
for f in *.png; do
  magick "$f" -background white -alpha remove -alpha off \
    -quality 88 "jpg/${f%.png}.jpg"
done

The related surprises

Rounded corners go black. Same cause — the mask lives in the alpha channel. Output PNG or WebP instead.
A PDF page renders black. PDF pages have a transparent background, so converting straight to JPEG produces the same result. Add -background white -alpha remove. See the PDF page.
The image goes white instead of black. Some PNG encoders store white under transparent pixels. The same fix applies — you are making the result explicit rather than depending on what the encoder happened to store.

Check before you convert

magick identify -format '%A %[opaque]\n' logo.png

The first value reports whether an alpha channel exists, the second whether every pixel is opaque. A file can have an alpha channel that is entirely opaque, in which case a straight JPEG conversion is safe.