The behavior typically is neither pure CIH or CIW by default, it's just "use most of the screen area without discarding any image". If you were to play old 4:3 video content, the width would get reduced and you would get pillar boxing.
Essentially, if the image is wider then your screen, you get CIW, and if its less wide then your screen, you get CIH. So if you have a 2.35:1 screen and play 16:9 content, what I would expect to happen is the height to fill the entire image, and the width to get adjusted accordingly. Isn't that what you want?
The screen is CIH in such a setup because all content is generally as-wide or narrower then your 2.35:1 screen. But that wouldn't need any special handling and already do just the right thing.
Thanks for the quick reply, Hendrik!
To address your post, no, that is not how a 2.35:1 CIH screen behaves with front projection. If I use the projector's lens zoom to utilize the full height of a 2.35:1 image, then a 16:9 image spills over the top and bottom of the screen projecting content that is clearly off the screen. If I adjust the image for CIH of a 16:9 image, then I get pillar bars on the 2 sides, as expected, BUT if a 2.35:1 image is projected with that mechanical lens zoom, then I get a 2.35:1 image
within the 16:9 frame, giving me black bars on all 4 sides (2 pillar bars from the 16:9
frame and top and bottom black bars from the content within that frame). This results in a 2.35:1 image that is way too small and leaves a LOT of screen real estate unused.
After thinking about this more overnight, I realized that the problem is that the frame AR is very often different than the content AR, which is probably just a different way of saying what you just told me. For a CIH algorithm (if "algorithm" is the correct term to use here) to work, then it must be able to detect the
content AR and ignore the frame AR, maintaining constant height of the
content. I don't know if this is possible, but it certainly would not be as simple as I first thought.
Then there is the matter of subtitles and their placement. Very often subs are partially or wholly in the lower black bar of 2.35:1 content in a 16:9 frame. Totally ignoring 4:3 content, this works fine for CIW setups that have a 16:9 AR, though personally I find it annoying. Once again, for the CIH algorithm to work properly with subs, the subs would need to be moved upwards from the lower black bar to reside at the bottom of the content, as the top and bottom black bars would not exist in such a setup.
This is definitely not as easy as I first thought.
As far as how I deal with things right now, with my current 2.35:1 CIH screen I am forced to create precise mechanical lens zoom presets for each AR in order to obtain the largest images possible with each AR, which in most every case will be to use the entire height and then allow the width to fall where it may. I have come across very lttle content any wider than 2.35:1 (like 2.39:1 or 2.40:1), and when I do I just let the little extra width fall off the sides of the screen into the black velvet border. The biggest problem is with content that changes AR on the fly. I always have to use the lens preset that accomodates the
highest AR (usually 16:9). If I use the lens preset for 2.35:1 when the content AR is 2.35:1 then I get to use my entire screen, but when the content AR changes to 16:9 or 2:1, then content gets displayed above and below my screen...very annoying. The best way I have found to deal with this type of AR that changes on the fly (like IMAX titles) is to use the entire screen for 2.35:1 and then blank out everything above and below the screen so that there is no content being displayed when the content expands to 16:9, all performed by the projector. I lose content above and below the screen, but it is the best compromise so far.
Or, I could replace my screen with a 16:9 screen and totally ditch the idea of CIH. The problem there is that my room is much wider than it is tall, so my current 158" 2.35:1 screen would need to be replaced by a 167" 16:9 screen in order to see 2.35:1 content at the same size as I currently see it. The height needed for a 167" 16:9 screen is 87.5" (including frame) and I only have 86" from floor to ceiling....