Process image to find external contour

Question

I have hundreds of PNG images for which I have to generate corresponding B&W images that show the outer outline of the object. The source PNG image has alpha channel, so

Answer 1

I would recommend ImageMagick which is available for free from here. It is included in many Linux distibutions anyway. It has Python, Perl ,PHP, C/C++ bindings available as well.

I am just using it from the command-line below.

convert donut.png -channel A -morphology EdgeOut Diamond +channel  -fx 'a' -negate output.jpg

Basically, the -channel A selects the alpha (transparency) and applies the morphology to extract the outline of the opaque area. Then the +channel tells ImageMagick I am now addressing all channels again. The -fx is a custom function (operator) in which I set each pixel of the output image to a - the alpha value in the modified alpha channel.

Edited

The following may be quicker than using the above fx operator:

convert donut.png -channel RGBA -separate -delete 0-2 -morphology EdgeOut Diamond -negate output.png

Result:

If you have many hundreds (or thousands) of images to outline, I would recommend GNU Parallel, available from here. Then it will use all your CPU cores to get the job done quickly. Your command will look something like this - BUT PLEASE BACKUP FIRST and work on a copy till you get the hang of it!

parallel convert {} -channel A -morphology EdgeOut Diamond +channel -fx 'a' -negate {.}.jpg ::: *.png

That says to use everything after ::: as the files to process. Then in parallel, using all available cores, convert each PNG file and change its name to the corresponding JPEG file as the output filename.