Make transparent regions using a mask

Question

I have a batch of images that need a transparent background. I am able to create a black/white mask of the lighter and darker regions and want to use this mask to keep the pixels, which are white in the mask unchanged and set all pixel to transparent, which are black. The best outcome so far I got with

convert $FILE -rotate "-90<" $ROTATED
convert $ROTATED \
  +dither \
  -colors 2 \
  -fill black \
  -draw 'color 0,0 floodfill' \
  -flop \
  -draw 'color 0,0 floodfill' \
  -flop \
  -white-threshold 0 \
  $MASK
convert $ROTATED -mask $MASK -compose copy-opacity -composite $OUT

But the last command just "ghosts" the whole image. How can I "cut out" the black pixels and keep the white pixels unchanged?

This is what I get so far.

Answer 1

You simply need to remove the "-mask" from your command line leaving your mask image (and add -alpha off). So the following works fine for me in ImageMagick 6.

Input:

convert star.png \
+dither \
-colors 2 \
-fill black \
-draw 'color 0,0 floodfill' \
-flop \
-draw 'color 0,0 floodfill' \
-flop \
-white-threshold 0 \
mask.png
convert star.png mask.png -alpha off -compose copy-opacity -composite out.png

Mask:

Result:

Download the result to see that the background is fully transparent.

If using Imagemagick 7, then change convert to magick

ADDITION

Here is one way to do that with MPR. Note the +swap.

convert star.png \
-write mpr:star \
+dither \
-colors 2 \
-fill black \
-draw 'color 0,0 floodfill' \
-flop \
-draw 'color 0,0 floodfill' \
-flop \
-white-threshold 0 \
mpr:star \
+swap \
-alpha off \
-compose copy-opacity -composite \
out.png

You can also do it with a clone and parentheses.

convert star.png \
\( +clone  \
+dither \
-colors 2 \
-fill black \
-draw 'color 0,0 floodfill' \
-flop \
-draw 'color 0,0 floodfill' \
-flop \
-white-threshold 0 \) \
-alpha off \
-compose copy-opacity -composite \
out.png

I get the same result as above.