Question

I’m looking for a way of converting an image so that all non-transparent pixels (those that have alpha != 1) into black and transparent pixels untouched (or converted to white). The closest I got was with the below imagemagick command:

convert <img> -colorspace Gray <out>

However this still gives me some gray colors instead of a complete black. I have tried all colorspace options and none does the job.

Any idea how I could achieve this with imagemagick or with similar tools (or with a PHP library if it exists)

Answer 1

I know this question is old, but now I’ve stumbled on it I might as well answer it.

The ImageMagick command you want is:

convert <img> -alpha extract -threshold 0 -negate -transparent white <out>

I’ll breakdown what it’s doing as well.

1. -alpha extract – Take the alpha mask of the image. Completely transparent pixels will be black, completely opaque ones white.
2. -threshold 0 – Increase all channels to their maximum value if they are greater than zero. In this case, it will make every pixel white except the ones that are completely black.
3. -negate – Invert the image. Now our blacks are white and our whites are black.
4. -transparent white – Set white pixels to be transparent. This can be excluded if you’d prefer your originally transparent pixels to be white.

Before

After