About Parasitic light

Parasitic light example

What is parasitic light? The easy answer is any unwanted light which is added to the original image and radically decreases the contrast ratio. In fact, there are many different sources of parasitic light: from the exterior (ambient lightning, exterior light leak, phone flash, EXIT signs, etc...), from the black level of the projectors and from the image itself, which we call auto-parasitic light or cross-bouncing. This phenomenon can be seen when the light emitted from the screen is reflected back on pale surfaces. With a flat screen, this problem is easy solved with dark walls, ceillings and furnitures.
 
In a dome this is much more problematic since all the "walls" are white, the light of the image on the front of the dome will increase the black level of the rear and vice-versa. It's hard to find information on the internet regarding this issue, so we created this tool in the hopes of helping the fulldome community to have a better understanding of how parastic light will affect their productions - without a dome!

 

Online Auto-Parasitic light Calculation tool

Upload a frame

Here you can upload a single frame from your project and our server will automatically simulate the parasitic light produced by the image when projected in the dome and add it to your original frame. In this way, you will have a more realistic idea of the final image quality.

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Results

How it works

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Parasitic results examples

parasitic results
Here are some examples of how auto parasitic light can affect your images.
 
In the first row, the parasitic light is mostly on the opposite side leaving the projected image with a good contrast.
 
In the center row, the black background and small high contrasted objects produce very low polution and we see almost the same results in the dome.
 
In the last row, our footage is very bright throughout the image, producing a lot of pollution which heavily affects the black level and contrast.
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How it works

How parasitic light is computed
The mathematical concept can be pretty simple to understand with these two general steps.
 
First (1), not all pixels of a dome master are the same size on the screen. Those in the center are a bit streached and take more surface than those on the outside. So there is compensation of the impact of every pixel by adjusting the relative luminosity.
 
The second step (2) is to sum, for every pixel, the luminosity emmited by every other pixel in the dome. The sum takes into account the relative orrientation between each pixel.
 
This calculation assumes the following :
⇒ There is no reflexion from the floor or any other surfaces in the dome - only the dome screen.
⇒ The screen is a perfect Lambertian surface.
⇒ It only computes a single pass, which means that it doesn't take into account the pollution caused by the parasitic light itself.
 
But despite this simple explanation, the complexity resides in the optimisation of the algorithm which is approximatly O(n4) growth relative to the resolution. This means that if it take 2 seconds to resolve a 128x128 pixels frame, it will take 32 seconds for 256 pixels, 2h20 for 1k, 36h for a 2k or 24 days for a 4k frame.... I don't think you wish to wait so long. With our optimisation, every picture size take approximatly the same amount of time to compute.

Some awesome tips

What you can do

⇒ Keep the background dark.
 
⇒ For bright footage, try to darken the regions out of the interest area and increase artificially the contrast level.
 
⇒ Make high constrasted objects.
 
⇒ Images near the zenith will produce less parasitic light since it emits towards the floor. Try to use this to your advantage by positionning your area of interest on a higher position.
 

What you can't do

⇒ You cannot minimize auto parasitic light with more powerfull projectors and the "contrast ratio" is far away from competing with this phenomenon.
 
⇒ A grey or "high contrast" screen will not help for auto parasitic light. But it can help to reduce parasitic light from other sources such as ambiant light or light leak from the outside.
 
⇒ Since the effect of light is purely additive, you cannot apply an "anti-parasitic" filter on your image based on the result of this tool. You simply cannot make black blacker than black.

What you could do

⇒ If you can, keep the floor, furniture and unused screen surface covered with light absorbant material.
 
⇒ Choose projectors with low black level.
 
⇒ Keep ambiant light low and minimize outside light sources from entering.
 
⇒ Of couse, use this tool as many time as you like to find the best results.
 

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1473 rue Évangéline
Sherbrooke, QC, Canada
J1H 5Y7

Phone

Mobile: 1-819-574-3766
 
 

Email Address

info@nestimmersion.ca
 
 

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