How To use the Color Difference Keyer

This how to will guide you to a more successful way to use and adjust the Color Difference Keyer. The most important part is to understand that we do not have a Chroma Keyer - we are using a Color Difference Keyer which will, as the name says, check between the seperate color channels, Green vs Red and Blue (depending on the selected Key color), while the Chroma Keyer uses a different algorithm and creates the key based on differences in the color space (HUE). Internally we are still converting to HUE and back to RGB to make it possible to use a shift function to adjust the green.

Introduction

You will see how to setup the Color Difference Keyer as an IPP Effect and to use it in an Internal Keying situation which is commonly used with SDI boards. You can use the Color Difference Keyer also just to key out green screen material which you will use for a simple presentation or such as a Power Point to have green background and embed this into a cool Ventuz environment. The advantage is that the presenter still has the chance to concentrate on his material while you create a stunning environment for the presentation look and feel.

Color Difference Keyer as IPP

You can apply the Color Difference Keyer on the Layer level or by using a FX Node. Just click  the + and add the Color Difference Keyer.

This will add the Keyer and all material that is applied to this Layer and contains green - as by the current default settings, will be keyed out. Basically you would work your way from top to bottom to adjust the properties for the color difference keyer. Usually you would also apply the IPP Effect onto a Live Video input. Check that you have proper lighting conditions, because this is the key to a successful keying.

Adjust the ScreenColor to match your current setup. If you are using a green screen you would not need to do anything, since this is the default. You have Blue and Red available as well. If it doesnt match a pure Green for example, you can adjust the hue by using the HueShift. This way it is also possible to match a complete different color.

With the KeyBalance you can slightly shift the Input signal to the blue and red channel - assuming you are using a green ScreenColor, else it would be against green and red and so on.

At this point we suggest to switch the Preview from Live to Key. By doing so, you are able to see the real keyed out signal as a gray scale image in the renderer window. This makes even small adjustments and fine tuning possible and will let you preview way better the resulting key.

After you have switched the Preview we adjust the settings from Top to Bottom. In general you would only adjust rarely and if, just slightly the KeyPreGamma, KeyMin/Max will set the Black- and Whitepoint for the key. The Despill would be needed to be adjusted in situations where the green is reflected onto the clothing of the talent for example. As already mentioned this can be prevented by proper lighting conditions for your environment. See below the Result of an properly set Keyer:

The before and after of a Preview: Key. Left side shows the key with the default settings... also notice the bit of noise. Right side shows the final result - take care to not set the white- and blackpoint (KeyMin/Max) to tight, this will cut too much from hairs.

The settings applied for the shown example are as following, also play around with the Softness and Choke / EdgeSoftness settings, to create a perfect results for the keyed edges:

Color Difference Keyer with Internal Keying

You can use the Ventuz Configuration Editor to use the Color Difference Keyer directly in combination with your SDI hardware. For example you would chose to use a Deltacast hd-e-key 22 and select for the output mode internal software chroma keying:

You would have for the outputs configuration, as well as for the live options, the same properties available as if you would use the color difference keyer IPP effect node.

To adjust the keying you would use the same procedure as with the color difference keyer node.

If you are working with the Live Options, remeber to save the changes to the configurations. The following images illustrate the properties for the outputs configuration on the AV Config side and the Live Options which are accessible trough the configuration editor.

above the AV Config

above the Live Options

Using the Color Difference Keyer with Material

For certain situations you would need the color difference keyer to work with a geometry that contains a keyed image or video. The best idea would be to use the Green Screen Material in a 2D-Layer Movie Clip, apply the Color Difference Keyer IPP effect and have the whole layer render as an Offscreen Render Target. This way you could use the Render Target as a material source and bind it to wherever you need it, on top it is possible to use the alpha informations (Key) to generate a shadow out of the fill signal.

Conclusion

The Ventuz Color Difference Keyer can be a powerful and good solution for your needs. It is easy to setup and if you follow the "top to bottom" adjustment procedure you will get fast a quit good result compared to other color difference keyer. You can use either the color difference keyer as an IPP effect or chosing it with an sdi board as an internal software keyer solution. The keyer offers you to key from Green, Red and Blue backgrounds and has also a despiller available.

Change the Preview property from Live to Key will make your life easier since you can see the resulting alpha/key output. This way you do not need to estimate the output, you can clearly see the alpha/key output and fine tune it this way.