Film-Tech Forum ARCHIVE: Cropping & Keystone Correction

This is topic Cropping & Keystone Correction in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=16;t=002912

Posted by Jesse Crooks (Member # 9389) on 01-08-2017, 06:46 PM:

Can anyone tell me what the tolerance threshold is for cropping when configuring lens files? I saw a previous Film-Tech thread that suggested less than 5% was considered acceptable. I tried to check the SMPTE standard that I thought might specify it, but hit their pay wall.

We have a fixed-height screen in which the scope image is getting cropped just shy of the sides of the flat pixel array, as indicated by the test pattern. I'd estimate that we're losing a total of about 4% of the full image width. I assume that the screen wasn't wide enough to accommodate the scope image at that height (it's a historic single screen theater) and that cropping it was considered better than letterboxing it.

And can anybody explain the options for keystone correction since DCI doesn't allow for software-based solutions? I won't be doing it myself but I'd like to understand it, and more importantly understand what the optimal calibration looks like for a screen with a pretty steep throw. Most of our screens have B-series Barcos, if that matters.

Posted by Mike Schulz (Member # 4297) on 01-08-2017, 07:37 PM:

If you are playing "Rogue One" or "Moana" right now, check the hard drive for the Disney Digital Framing Charts. They usually include it on all of their feature drives and they are great for doing quick focus and alignment checks. It has a few different colored border bars and it will tell you specifically which ones must be visible and which ones you can crop out if you need to.

Posted by Leslie Hartmier (Member # 7053) on 01-08-2017, 08:32 PM:

Depending on what server you have, you might also have framing charts right on your server. For example, GDC includes 'DCI XYZ Framing Chart'.

Leslie

Posted by Justin Hamaker (Member # 2165) on 01-08-2017, 09:21 PM:

It's not actually keystone correction. All you are doing is setting the crop position for each corner of the screen. Because each corner has it's own adjustment for the X and Y axis, you can set the crop so it correctly fits your screen. But you are definitely cropping the image rather than changing the shape to fit your screen.

As for the cropping, I have three common height screens where the scope picture is cropped anywhere from 25 to <50 pixels on either side. I have never noticed any issue with key elements being cropped.

If you're concerned, you might check your side masking to see if there is any kind of adjustment so you can make it a little bit wider or reduce the height by a couple inches.

Posted by Marco Giustini (Member # 4544) on 01-09-2017, 04:30 AM:

As mentioned, find a framing chart. On any given format you should be able to see either top/bottom or left/right borders - both would be ideal. If you don't see any of those, then your picture is excessively zoomed and cropped.

Posted by Steve Guttag (Member # 268) on 01-09-2017, 11:52 AM:

The problem in the theatre he mentions is not too uncommon. In that, the screen has a motorized masking system (but it would apply to a manual one too). The overall dimensions of the screen are shy of actually 2.39, therefore the sides of the scope image will be cropped if the image fills the height. This site also doesn't close its masking down far enough for 1.85 so on that format the height is cropped.

I generally give the customer the option of zero/minimal cropping or fill the screen and without exception, I've been told to fill the screen (because it just looks stupid to have a masked screen and to still not fill it) and amateurish to not mask the screen altogether.

DCI spec used to be:

quote:
8.3.3.3. Screen Characteristics
The screen is required to be non-specular and equally reflective over the entire visible
spectrum. The screen is required to have variable black masking, adjustable to tightly
frame the projected image. This is required to minimally include 1.85:1 or 2.39:1 image
formats. Masking for other formats is optional.

As of DCI 1.2, they punted that section and said it was an SMPTE thing. Whereas DCI was thrown about as a condition of obtaining VPFs...SMPTE has NEVER been a regulation body, just an engineering one with the mere power to recommend or codify existing practice. It has never had an enforcement aspect to it.

Posted by Rick Raskin (Member # 1561) on 01-09-2017, 01:03 PM:

Does any standards body have enforcement powers? In my experience the marketplace is the best enforcer. Make non-compliant equipment and you'll risk sales.

Posted by Justin Hamaker (Member # 2165) on 01-09-2017, 04:08 PM:

I don't think the equipment is the issue; it's theatre being built without masking and/or common width screens. And sloppy installations where the screen files are not properly set to fill the screen.

Depending on how much might need to be cropped on the scope picture, I could see shifting the image down slightly so you are on the bottom masking and the fit the picture edge to edge. A couple inches of blank screen at the top of the screen wouldn't be the worst thing - especially if you would have to crop more than 5% to completely fill the screen.

Posted by Steve Guttag (Member # 268) on 01-10-2017, 06:04 AM:

I was mistaken, the Flat image is fine but yes, the scope screen isn't wide enough. The screen is also curved (slightly) so you get a geometric element in there, particularly if you undersize it. And leaving dead screen just highlights a misalignment. I would be more inclined to recommend adding a fixed upper masking to slightly lower the top of the image until the effective ratio is an actual 2.39:1

Posted by Philip Wittlief (Member # 1724) on 01-13-2017, 03:34 PM:

Along this same topic, is there a calculator/simulator out there for dealing with the curved screen distortions. Looking for what actual screen dimensions will end up with the least cropped picture on curved screens.

Posted by Steve Guttag (Member # 268) on 01-13-2017, 06:12 PM:

I think Harkness does have a curved screen simulator, actually. Though I think it is geared towards simulating light evenness more than showing cropping.

That said you can do it the old fashioned way and do the math or, grab an old Schneider Theatre Design program and put in your screen dimensions and radius and then ignore the focal length (since you will be zooming the lens anyway) and fit the screen and it will let you see about what the barrel distortion will be...even taking into account lens shift and keystone.