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I didn't check a Mask option, but seems to me more natural to use an Object as a Mask and Shadow Catcher in once, so shadow follow the shape.

Alpha Over node seems to be working as expected.

Notes:

• Image in my 3Dviewport is an Empty object for a reference it doesn't influence a render. For the same effect (and probably easier for video) is to use image as Bacgroud for Camera (Camera Properties > Background Images) that can be video as well.
• With Object ID pass or Cryptomatte you can separate shadow and adjust if needed.

I didn't check a Mask option, but seems to me more natural to use an Object as a Mask and Shadow Catcher in once, so shadow follow the shape.

Alpha Over node seems to be working as expected.

Notes:

• Image in my 3Dviewport is an Empty object for a reference it doesn't influence a render. For the same effect (and probably easier) you can use image as bacgroud for camera (Camera Properties > Background Images) that can be video as well.
• With Object ID pass or Cryptomatte you can separate shadow and adjust if needed.

I didn't check a Mask option, but seems to me more natural to use an Object as a Mask and Shadow Catcher in once, so shadow follow the shape.

Alpha Over node seems to be working as expected.

Notes:

• With Object ID pass or Cryptomatte you can separate shadow and adjust if needed.

I didn't check a Mask option, but seems to me more natural to use an Object as a Mask and Shadow Catcher in once, so shadow follow the shape.

Alpha Over node seems to be working as expected.

Notes:

• Image in my 3Dviewport is an Empty object for a reference it doesn't influence a render. For the same effect (and probably easier for video) is to use image as Bacgroud for Camera (Camera Properties > Background Images) that can be video as well.
• With Object ID pass or Cryptomatte you can separate shadow and adjust if needed.

I didn't check a Mask option, but seems to me more natural to use an Object as a Mask and Shadow Catcher in once, so shadow follow the shape.

Alpha Over node seems to be working as expected.

I didn't check a Mask option, but seems to me more natural to use an Object as a Mask and Shadow Catcher in once, so shadow follow the shape.

Alpha Over node seems to be working as expected.

Notes:

• With Object ID pass or Cryptomatte you can separate shadow and adjust if needed.