as far as I understood, the cage tool is NOT assumed to transform an image but part (or parts, one after the other) of an image.
I made a very stupid test but in the direction I said.
I write the word "test" on a white background.
The I put a cage around one of the letter (four anchor points).
I moved as I wanted the anchor points.
At the end of the distortion of the first letter I pressed "enter" and the modification was saved.
I did the same for the second letter.
Then with the third letter.
I got my final text distorted as I wanted to get it.
I know, it's a useless example, but I made it to clarify that the tool operates only on a well defined section of an image.
Hope could be of some help for somebody...
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before-cage.png [ 8.64 KiB | Viewed 235612 times ]
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after-cage.png [ 27.81 KiB | Viewed 235612 times ]