Hi Ramv,
Reregistering was my first solution too. But I think there is a cleaner method -- at least it works for me.
Say you have two images (1 and 2) that are the short exposures integration and long exposures integration.
For me, both are registered (star aligned) using the same reference. Both are taken with the same scope, focal length and camera. And other limitations may also apply.
a) Start a dynamic crop on image 1. Create the size of the crop (image 1) you desire, but do not execute it yet.
b) Drag the blue triangle from the original dynamic crop window onto your PI workdesk. This creates another instance with the same size parameters.
c) Execute the crop on the image 1. Then close the original dynamic crop operation.
d) Bring image 2 to the front. Open the cloned dynamic crop operator and execute. This should result in the same crop size for image 2 as image 1.
You should be able to do individual linear operations (Background neutralization, DBE, etc.) to either of the two images after that and then successfully execute the HDR Composition operator.
Good luck.
Mark
