How to utilize grain in stereoscopic productions

The use of grain is probably one the most disregarded topics in stereoscopic productions.
And yet – it can most certainly enhance the perception of a stereoscopic movie. So I thought I take the time to write about the experiences I made concerning grain management and also give you some examples to see and judge for yourself.
So, why did I even come to the point of caring about grain?
If you read any of the articles in this blog you might have noticed that I made a stereoscopic movie with the Panasonic AG 3DA1 twin lens camera to get some experiences with stereoscopic postproduction techniques. Due to the fact that renting the camera waspretty expensive for a student I wasn´t able to get an extra SDI recording device which is why I had to rely on the inbuilt AVCHD Recorder. I actually thought it would not make a huge difference but after processing the clips through colour correction the artifacts were pretty obvious which is why I decided to go back to the start and try to denoise the plates as best as I could to then ad back the noise on top.

Below you can see the denoised plate on the left and the original image on the right.
To view the images contained in this article in full size, please click on the image.

Adding grain or rather a noise pattern actually isn´t that big of a deal. But how would you do that in a stereoscopic movie? Here are my experiences and a lot of pictures, so grab your anaglyph glasses and compare for yourself!

For the sake of making the effect more obvious I exaggerated the noise-pattern massively.
The effect would otherwise be impossible to see with with an anaglyph viewing method.

In the first image below you can see what a normal image would look like whiteout any grain treatment. The noise patterns are unequal for both views and thus aren´t perceiving any depth information. Because there are no correlating patterns, our brain can´t fuse the noise patterns and it produces eyestrain and an uncomfortable feeling that is more obvious in this exaggerated version.

In the next to pictures you can see a planar noise pattern. The noise Pattern acts like a window that can be either on the screen plane thus allowing elements to brake trough(left image) or in front of the closest object to minimize false depth interpretations(right image).

The most elegant method I find – also the most extensive one – would be to map the noise pattern to the proper depth positions of the contained objects. This can be done with a disparity map. A disparity map displays the disparity of corresponding pixels in a single 32bit float image. This image can then be used to shift the pixels of the noise pattern accordingly to generate a depth perception. The effect can be compared to the images of “the magic eye” books which used to be popular in the early nineties. Below you can see the picture of a disparity map as well as the stereoscopic noise-pattern and the combined picture.

The disadvantages – besides the huge amount of time it takes to generate such a stereoscopic noise-pattern – are also the accuracy of disparity maps. Parts of the image that don´t contain enough information to let the algorithm find enough corresponding points will result in false depth interpretations.

Another important question is: Do we even need grain in stereoscopic movies? Digital projections methods in movie theatres are already taking away the once so appreciated filmlook. 48-FPS  and 4k projections now seem to be a way to go. But there are still people who would rather shoot on film and then convert to stereo3D later on – like Men in Black III.

I am excited to see where stereoscopic cinema is going!

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