Film-Tech Forum ARCHIVE: keystone correction for D-cinema projector

This is topic keystone correction for D-cinema projector 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=001356

Posted by Joju Zacharias (Member # 7400) on 01-22-2013, 07:04 AM:

Hi, I don't work in a theater but I visit projection booth of all the cinema theater's I go to, also this is my first time in film-tech forum, so if I break any rules with my words please go easy on me.

Almost all the theater I seen the projector is located higher than the center of the screen, so the image is trapezoidal, it doesn't fill the screen neatly. I know that it's not recommended to correct it using picture warping softwares. Is it possible to use a "flexible mirror" to correct the image(like they use in christie duo). spending all the money on expensive projectors for a trapezoidal image doesn't seem good enough.

Posted by Gavin Lewarne (Member # 5397) on 01-22-2013, 07:57 AM:

was this on a common height or common width screens?

We have a common height screen, meaning adjustable side masking. We completely fill the height of the screen in both flat and scope, and only had to digitally mask (in the projector) just a handful of pixels on just two corners. The only time you see a trapezoid is on letterboxed flat material (eg trailers for a scope film in the flat format) and even then its exceptionally slight. Using the digital side masking feature of the projector, we have perfectly perpendicular edges of the picture on the left and right, again, only masking just a minuscule number of pixels

We were very careful during installation to make sure that the vertial and horizontal line patterns were as straight as possible.

Our projector is above and a bit to the right of the centre line, something we cant change due to the design of the projection room and the fact we needed to keep our 35mm for archival prints and arthouse one-offs.

We did find during install that correctly scheimpflug'ing the lens resolved most of the wonky picture issues, and we have excellent focus in all four corners , and centre of the picture in flat and scope.

Posted by Joju Zacharias (Member # 7400) on 01-22-2013, 08:11 AM:

it was a constant height screen, but they show all the flat movies in scope(cropped). the projector is located almost above the level of the screen, the picture is so trapezoidal that they cannot zoom the picture too much to fill. so they fill the bottom corners of the screen and you can see blank screen on top corners. it is not a good idea to zoom out the picture any more because they will loose some amount of picture from sides and it will ruin 3d movies, "floating windows" on the side of the image is very important for 3d effect.

Posted by Gavin Lewarne (Member # 5397) on 01-22-2013, 08:27 AM:

Wow that sounds pretty crppy

Posted by Carsten Kurz (Member # 5396) on 01-22-2013, 03:27 PM:

All DCI projectors (and a lot of home video projectors as well) have 'lens shift' - that allows to shift the image vertically and horizontally without any electronic correction ('keystone correction'). Goes by shifting the lens in parallel with the screen/imager plane.

That feature will usually cope with most projection settings. Some projectors/lenses allow for more than 100% up/down movement. If you overdo it, vignetting or convergence issues can occur, but most installations do not need extreme values of lens shift. Some places, though, show notoriously bad layout.

Electronic keystone correction is not available on any DCI compliant projector. Only keystone masking is possible.

http://en.wikipedia.org/wiki/Perspective_control_lens

- Carsten

Posted by Marco Giustini (Member # 4544) on 01-23-2013, 01:56 PM:

They run Flat films in Scope and you bother about the shape of the borders? If the projector is just over the screen, you should not have such a distortion. Use more lens shift!
You may have the wrong lens as well. You should be able to zoom until your corners - all of them - reach the corners of the screen.

Then, crop the picture to have the edges parallel to the sides of the screen. I know it's not nice to crop, but much better to see what you are describing.

Glad that DCI do not allow for keystoning correction!