ImageMagick convert to png8 (paletted PNG with binary alpha) using background color for partially transparent pixels

Question

I have a 32-bit PNG image like this:

Displayed here with a checkerboard background for visibility, but instead of the checkerboard the image is actually transparent (for your reference: the original 32-bit image). As you can see, around the edges and towards the right, the red pixels are gradually fading out from opaque to transparent.

If I would display this 32-bit image on top of a blue background, the pixels around the edges and towards the right would gradually fade from red to blue.

Now I need to convert this image to an 8-bit PNG with binary alpha. I want the fully transparent area to remain fully transparent, but the partially transparent pixels to gradually blend with the intended background/matte color, in this example blue.

So something like this:

However, I can't seem to figure out how to do this with ImageMagick (using ImageMagick 7.0.8-26 on Mac). I tried as follows:

magick original.png -background 'rgba(0,0,255,0)' png8:output.png

But this results in:

It seems to ignore the blue background color altogether, and just convert the transparency to binary (probably taking fully opaque for ≥128 or fully transparent for <128).

Also if I specifiy rgb(0,0,255) as background color (i.e. no alpha) the result is the same.

Does ImageMagick have some smart 'background blending for partially transparent pixels' option that I don't know about?

Otherwise, I'm guessing I should somehow extract a binary bitmap where the original image is fully transparent (i.e. has alpha value or opacity 0), then flatten the image on a background of my choosing, and then re-apply the boolean alpha bitmap again. But not quite sure how to do this.

Answer 1

Here is one more way in ImageMagick.

convert img.png \
\( -clone 0 -alpha extract -channel rgba \
    \( xc:none xc:blue +append \) -clut +channel \
        -channel alpha -threshold 0 +channel \) \
+swap -compose over -composite PNG8:result.png

OR FOR Imagemgick 7

magick img.png \
\( -clone 0 -alpha extract -alpha copy -channel rgba \
\( xc:none xc:blue +append \) -clut \
-channel alpha -threshold 0 +channel \) \
+swap -compose over -composite PNG8:result.png

Read the image.

Clone it, extract the alpha channel, create transparent blue color map and apply to the alpha channel, then threshold the alpha channel.

Swap the two images and composite the original over the processed alpha channel.

Save the output.