From how I understand it, if your spherical camera is 50’ above ground when you shoot the render, your render camera will give better results if it is 50’ above ground when using that spherical image as a background in that image, with a shadow catcher adding shadows from what it now in this scene onto the background.
Less wordy version:
Say you want to render a car coming from a distant edge and zooming up through the view of the render camera in the final animation.
Put your car in the scene and set up the camera how you’d like it. While you’re at it, envision exactly what your background should be. Follow me so far?
Now select the camera that you’ve just aimed and go to the motion tab and look at the number in the height (z, or blue) box. Rotation and stuff won’t matter so much - as you’ll be using a spherical image. The height that this camera is at is the height that your spherical camera should be at in order for a shadow catcher plane to work properly between the two. You’ll likely be three to seven feet off the ground for this example.
Just know that, due to the enormity of what Carrara considers to be the entire surrounding spherical background, when people say that you need to be at 8000 x 4000 pixels to help cut down on distorted pixels toward the feet of the render camera’s cameraman, that is about as minimal of a resolution as you’d want for this sort of example, which takes quite some time to render, if you’ve got a fairly detailed surround.
This is where you can help fudge the entire process by counting on using something as a ground to hide the bottommost portion of the projected spherical image in the BG, if the camera is to come anywhere near pointing down.
In your examples , above, you can increase the probability of having an undistorted image towards the “looking down” position by raising the spherical camera higher. But if you render that into a background spherical projection, you’ll need to keep your camera above the subject in the subsequent scene or it will be floating above the spherical images ground - floating in the air.
So to help all of this along, we’ll do two things. First, we’ll use the measurement we took for the car driving towards us for that shot and we’ll build our scene and set the spherical camera to that height and make sure that it is looking right-side up.
Before we call this done, let’s load that car back in and set it along it’s motion path, passed that final camera that we’ve used for our height measurement. This time, now that we have our completed scene, we can double check our measurement for best results.
But now that we have our car in motion, why don’t we add a new camera and parent it to the car - setting it’s view to a dramatic angle, changing ever so much over the course of the animation. Now you can render a backdrop image for use in that shot. Backdrop images are much easier to deal with in this regard because they only render what will be seen in the final animation. So these images are faster, even at high res, and can then be used to render animated backdrops - which is incredibly inefficient for something as giant as a spherical image.
Speaking of practicality, you may just want to render the car at the same time as that motion backdrop and be done with it. I just wanted to show that possibility - which can then be used for many shots that need the same motion in the background - like outdoors scenes with birds flying or kids playing for use on interior scenes - this is what’s happening outside, etc.,
So the scene is built and we’ve checked the height of our final rendering camera against the height of the spherical camera. The huge benefit of this is that it doesn’t need to be an animation - as it surround the entire scene with something - which will also assist in Global Illumination results, reflections, etc.,
8000 x 4000 is very large. Going a few numbers higher will make very subtle differences and going a lot larger begins to remove the benefit of speed - as it will take a very long time to create - for a super-detailed surround.
So just go for it and keep from aiming the camera towards your feet, without having something between the camera and the BG image!