How can I rotate a transparent png by 45 degrees using imagemagick and keep the new image transparent?

Question

I have a 16x16 transparent png and I did

convert -rotate -45 a.png b.png

This rotated it and created a new image b.png which is of size 22x22 and which when I use against a background shows the original image (16x16) rotated with the underlying background but the new filling that came about shows up with a white background.

How is it possible to have the new filling too be transparent?

If that is not possible, than how can I have all the background of the new image be one color?

Answer 1

Use the -background option to specify a transparent color with alpha set to zero:

convert -background 'rgba(0,0,0,0)' -rotate 45 a.png b.png

Answer 2

I was also having the same issue, however I was using the command like so:

convert a.png -rotate 45 -background transparent b.png

It needed to be:

convert -rotate 45 -background transparent a.png b.png

So, this actually helped a little, thanks :)