All Things Color for Film and Digital Cinema
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3D Theatrical Projection Problems with Floating Windows – Keystoning

As noted in a prior post on “Floating Windows” there is somewhat of a problem with the theatrical screening of 3D whereby cinema screens cannot for various reasons project a full screen DCI compliant image without physical image masking. Below I outline some possible solutions to this problem.

The main problem with both the theatrical presentation of 2D and 3D Feature Films is the tendancy for a cinema to be designed where the projection device is situated at the rear upper level of the auditorium. This old model of cinema design requires that the image be projected downwards over the distance of the auditorium to the screen. The physics of this basic model there for assume that the image will be somewhat keystoned by the time it reached the screen. To then counter the effect of a trapeziod shaped projection image on the screen one would then employ cinema black masking and essentially over shoot the image into the blacks until a nice pleasing rectangle is created.

For this very reason a ‘projection safe’ or ‘action safe’ model is employed by 2D image creators whereby, just like television, images were made to be displayed with a 5% ‘masking’ tolerance. Some screens may see the full image and most would see somewhere between that and roughly 5% – 10% in.

You can see now that if in a 3D presentation you are employing the use of ‘floating windows’ to counter 3D edge violations or simply play with the 3D screen dynamics these will more than likely be lost. Unfortunately as these windowing techniques are employed to assist with the audiences comfort factor when viewing 3D materials their exclusion from the presentation will make the experience slightly more uncomfortable.

Lets look at the below screen setup.

  • Screen Width – 30 Ft Screen
  • Screen Resolution – 2048
  • Auditorium Depth / Projector Throw- 100 Ft
  • Tilt of Projector – 10 Degrees

Given the above scenario, without masking, our image is now 30 foot across on the top of the image and 29 foot along the bottom. 28/30 = 93%

Top Width = 68″9′

Bottom Width = 76″1′

Keystone % = 100 * (top width-botom width)/top width = %9

9% of 2048 = 184 pixels.

Generally FLoating Windows will not be more than the max Positive Disparity which ona 30ft screen would not be more than 28 pixels Either side.

Well…with the calculations above we just lost our floating windows.

Bugger!

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