ImageMagick: How to create a black rectangle with opacity 40%?

Question

I work on a longer workflow to design elegant maps (project @Wikiatlas).

With ImageMagick shell command (convert?)...

How to output a 1200px wide, 1000px high black png image with an opacity of 40% ?

Answer 1

Have a look at section Semi-Transparent Colors on the Color Basics and Channels page of the ImageMagick Examples:

convert -size 1200x1000 xc:'rgba(0, 0, 0, 0.4)' out.png

This uses the xc pseudo-format with an RGBA color.