Introduction
This document describes what is the Vignetting effect, and how to correct it in Aximmetry.
By correcting the Vignetting effect you can achieve better quality keying in case of fixed zoom Tracked Camera and Virtual Camera setups.
Vignette correction can also help you achieve better quality color correction when creating LUTs in the case of LED wall setups.
What is Vignette?
The effect called Vignette can cause a “darkening” effect towards the edges of the image of the camera. In the picture below you can see what a white surface can look like in your camera.
Vignette is caused by the characteristics of the camera and lens. Because of the vignette, during keying, the individual green pixels can have different shades in the input from your camera, even if your green background is evenly lit and it is one homogeneous surface.
The Vignette Correction could solve this problem by removing the darkening towards the borders of your camera's picture.
How to apply Vignette Correction in Aximmetry?
For Vignette Correction to work, you need to make a capture with Aximmetry of the corresponding Input. You can find it under Inputs Control board \ Input (n) \ Capture Vignette.
This capture should be filled with a homogeneous white surface.
For the best quality, you should use an LED wall as the white surface. For the best possible result, the captured surface should not be in focus while taking the capture.
If you don't have an LED wall, we suggest using a big screen, for example, a TV screen. In this case, you will probably have to move your screen up to the level of your camera and quite close to your camera
You can easily put out a white screen on a monitor with Aximmetry, just use the Solid Color module:
If you don't have a big enough homogeneous white light source, then you can try using white cardboard (the cardboard should have an even surface). The white cardboard should be evenly lit with diffused white lighting to avoid the appearance of any color gradient.
To get the best results, your camera's picture should be close to overexposure. But, any part of the picture being overexposed will significantly reduce the quality of the Vignette Compensation. To determine if the camera is overexposed or not, we suggest turning on the zebra ( https://en.wikipedia.org/wiki/Zebra_patterning ) in your camera. When the zebra is turned on, you should increase the exposure until the zebra appears, and then turn the exposure just a little bit down until all of the zebra disappears. Also, while the zebra is visible, it can give a good idea if your white surface is homogeneously white and evenly lit as the zebra should only appear in the middle of the picture where there is no vignette. If the zebra pattern is more numerous in one direction, then your white surface is probably not evenly lit.
The Difference that Vignette Correction Makes
In this session, you can see a direct comparison of the keying with, and without Vignette correction. The pictures were made with the same camera, lens, and lighting setup, the only difference is the Vignette correction.
Preview during keying without using Vignette Correction:
Preview during keying using Vignette Correction:
Keyed result without using Vignette Correction:
Keyed result using Vignette Correction:
As you can see, after applying the Vignette correction, the Keying is much clearer around the corners of the green screen, therefore there is no need to increase the ‘Low Cut’ value any further (as it would be, when not using Vignette correction).
NOTE: Vignette Correction currently only provides precise results for fixed Zoom. Different zoom levels will change the vignette. So in case the zoom level changes, we recommend making a new vignette for the best possible keying results.
