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I wonder if there is any way to crop a image based on alpha data like this:

enter image description here

The reason I want to do this is that I want to crop the White area of another image (of the same dimension to the Alpha image above), then stretch it to fill the whole area, or fit to other expected dimension. And this has to be done in Compositor.

For example, this is a MANUAL way to crop then fit to expected dimension:

enter image description here

Blend File

However, to decide the value in Crop node is not so easy in order to get rid of unwanted part. So I wonder if there is any way (math or something?) that can be smart enough to save that work.

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  • $\begingroup$ Related: blender.stackexchange.com/q/2376/599 $\endgroup$ – gandalf3 Jul 20 '14 at 6:10
  • $\begingroup$ @gandalf3 Thanks. I know that post, but want I really want is to decide (or "guess") the cropped dimention based on the white area. All I know is it is a nice rectangle, but the exact dimension? No idea. $\endgroup$ – Leon Cheung Jul 20 '14 at 6:20
  • $\begingroup$ I think the title contains everything: "a real crop based on Alpha data". $\endgroup$ – Leon Cheung Jul 20 '14 at 6:30
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    $\begingroup$ Could you show a walk-through(using images) of what you would like to see using manual steps even if you have to use a 2D Editor to accomplish each step? BTW. It's good to see you back again. $\endgroup$ – MarcClintDion Jul 20 '14 at 14:06
  • $\begingroup$ @MarcClintDion Thanks for that :D I've updated the question. Hope you can get what I'm trying to do. $\endgroup$ – Leon Cheung Jul 21 '14 at 3:05
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Use this node setup:

enter image description here

In the texture node, you open the image you want to use as a mask, then plug the alpha output from that to the scale node's input for X and Y (the scale node's "Coordinate Space" is set to relative), you grab you RenderLayer and shove it in the image input for the sacle node, and finally connect that to the composite node.

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  • $\begingroup$ Hi, you can do it because the "Use Alpha" is toggled. I bet if it is untoggled, you can still see it is "uncropped" actually. So, this is not a real crop as I mentioned. I want the true change on dimension. $\endgroup$ – Leon Cheung Jul 22 '14 at 1:12
  • $\begingroup$ Yeah, if it's not on, the area which would be transparent is actually black. $\endgroup$ – someonewithpc Jul 22 '14 at 8:31

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