Merge diff. size plates (total newbie question)

Hi, Nuke user here...

I'm trying to do a simple A over B merge, where the A plate is a building keyed out and then cropped to remove the extra stuff I don't need and make the transforms simpler*. I can't do it!

When I merge my A plate over my now-larger B plate, everything outside the A plate is black. It appears Fusion uses the DoD of the front plate as a crop on the merge node, or something similar.

I then thought I'd crop the plate to my project resolution as a hack, but the crop tool insists on setting the alpha of the outside pixels to 1!

Obviously I'm not getting something. Help appreciated, how would you merge A over B when the plate sizes aren't the same?






*(I wanted to move the pivot to the base of the building in questin, but the on-screen XY transform gizmo doesn't follow the pivot, so I decided to crop to at least keep the gizmo somewhat visible)

after the key you need to add a "set canavas color" with the default settings (everything at 0)

Apart from Michael's very good suggestion, you could also use a rectangular inverted garbage matte on the key to make everything outside the rectangle transparent.

As for setting the pivot, in cases like these I usually use merges set to the default setting (centered on screen) with an Xf (transform) node right before the merge. In the Xf you can set the pivot where you want it and scale/rotate etc around it. Since Fusion concatenates the image there is no loss in image quality if you do this straight for the merge (the actual merging (and resizing/samping)of the image will only happen at the merge tool)

Even if you view the result of the merge tool, but select the transform tool, the onscreen gizmo will move to the resulting position making it usually a pretty userfriendly experience.

Does this make sense to you? If you can share two still images we could explain it visually as well.
Hope this helps.