ALT vs DCI aspects and masking

[Ryan Gallagher]

Please correct my logic. (Christie CP2220)

I, perhaps falsely, expect there to be a way to get a 1080p 1.85 ALT signal to fit the same masking as a DCI 1.85 1998x1080 2K DCP. Without doing any change to zoom that is.

Is this incorrect on it's face? We had a rare pairing of ALT 1.85 and DCI 1.85 content last night (similar pairing for tonight as well) and, perhaps expectedly, the ALT it just a hair narrower when using a 1920x1038 source file with "letterboxed" checked. To my eye they appeared the same height but that might have been an illusion since the difference there may have been minor.

Is something amiss with my channel configuration for ALT? Or should I just expect a tiny zoom and ILS adjust to be required to fit the ALT 1.85 to the DCI masking? I did try looking at 1920x1080 source too but that just made it even narrower. We just moved our masking for the 2nd film but feels like that should not be necessary.

I thought there was some way to tell it to take the ALT source and render it full raster height of 1080 if you told it the source file was 1038 tall for example, which should have scaled the width to match the aspect up to 1998ish. But perhaps that is not how source files versus the native 2K raster behave? Maybe that "letterbox" checkbox is throwing me off?

[Ryan Gallagher]

Having said that I realize that even if it is "doable" with the right channel config, it would involve pixel interpolation scaling up from the source size. So perhaps the "proper" way is having the other ILS configuration and zoom leaving the pixels 1:1 with a 1920x1080 source file with letterbox checked (to force the pillars to exist beyond 1920 width?).

ALT features are pretty rare for us, so I was was willing to compromise if the scaling method was in fact a thing, just to avoid even more marks on our manual zoom lens.

Edit for clarity: We DO currently use a separate screen file for the ALT 1.85 channel, but I confirmed the keystone crops were not why it was slightly narrower on screen by temporarily removing the side crops.