The problem is in my scene the actual horizon line (at infinity) doesn’t meet the end of my ground plane. When I enable the Sun Disc in the Sky Texture background (Nishita sky) the horizon line is clearly above as the sun is cresting the horizon. I also want to blur the sun image.
I figured I could render the scene with a transparent film background and then render the Nishita sky background without any objects in the scene after having rotated the camera such that the horizon line would have dropped. Then I can blur the background and composite it beneath my scene (after having blurred it). Is this what a new scene is for?
OK, I think I got it. I duplicated the scene, turned off the compositor on the original scene, made sure I had a transparent film background.
In the new scene I rotated the camera 1.5 degrees on the global X axis, turned off all objects and lights, and then in the compositor I blurred this “background only” scene with a gaussian blur node and used a mix node with the original scene. The Render Layers node allows you to choose a scene. The blur node needed to have “Relative” checked and put between 2 and 3 % for the X and Y. The only problem is that my horizon line wasn’t completely horizontal (tilted camera) so the blur distorted the sun disc a little.
Ok, I deleted all the objects from the new scene besides the camera to save on space. But whenever I rotate or move the original camera, I have to duplicate it and put it in the new scene. I figure there should be a way to do this with drivers or constraints. I tried drivers but it was giving me wrong rotations. Would love to hear how to make the new camera copy the original camera’s X rotation and add 1.5 degrees.