Copy a circle of image A into image B

Question

I have two images:

• a.jpg
• b.jpg

Both images are square (100x100 pixel). I want to cut a circle with a radius of 50 from image a.jpg and paste it in the middle of image b.jpg. I want to save the result in c.jpg.

How can I do this with Linux command line tools? I need to do it within a shell script.

Answer 1

Many different techniques can be used. ImageMagick has FX language that can perform complex calculations.

convert a.jpg b.jpg -fx 'Wi=w/2; Hi=h/2; hypot(Wi-i, Hi-j) < 50 ? u : v' c.jpg

For example...

convert -size 100x100 PLASMA: a.jpg

convert -size 100x100 GRADIENT:LIME-ORANGE b.jpg

convert a.jpg b.jpg -fx 'hypot(50-i, 50-j) < 50 ? u : v' c.jpg

Update with another technique.

A faster approach can be leveraging image mask(s) of the shape you wish to crop, and compose/composite it between both images. It'll require a format that supports alpha channels, but only for the initial work. For example...

Create a circle mask, and copy values to alpha channel.

convert -size 100x100 xc:White -fill Black \
        -draw 'circle 50 50 50 5' -alpha Copy mask.png

convert \( a.png mask.png -alpha Set -compose Dst_Out -composite \) \
        b.png -compose Dst_Atop -composite c.png

Answer 2

Eric's approach is much more succinct, and probably preferable, but here is another way anyway. I am being very environmentally aware and recycling ;-) his start images:

magick b.jpg \( a.jpg \( +clone -threshold 101% -fill white -draw "circle 49,49, 49,99"  \) -channel-fx '| gray=>alpha' \) -flatten result.png

That says... "Load b.jpg as the background. Load a.jpg and then create a transparency mask by cloning the entire a.jpg setting it black and drawing a white circle in it and pushing it into the alpha channel. Then flatten that over the top of b.jpg".

The result is the same as Eric's.