Starting in ProPresenter 7.14, ProPresenter has the option to output an alpha channel with either a Premultiplied key or a Straight key. This new option in ProPresenter will be beneficial to have an optimal key for the video equipment and signal flow you are using. This article does not discuss how to set up an Alpha key within ProPresenter, but rather discusses the differences between Premultiplied keying and Straight keying, and what these settings look like in common video switchers.
If you’re looking to learn more about setting up an alpha key in ProPresenter, please visit this article.
What is Premultiplied Keying and Straight Keying?
"Straight" and "Premultiplied" merely describe whether the alpha (transparency) values of an image are applied to that image prior to being output by ProPresenter. If you aren't doing key/fill outputs then your output is always "premultiplied". Let's use an example to make this clearer.
When using a Premultiplied key (also called a "shaped key"), content on the fill output will look exactly like it does in the ProPresenter editor. ProPresenter outputs have been premultiplied in all versions prior to 7.14. Premultiplied keying 'premultiplies' the alpha transparency layer with the color values to provide the premultiplied value.
Here is an example of what the output would look like of a blue box with a gradient to transparent. Notice that the fill channel looks like a gradient to black. This is because the key signal has been "pre multiplied" on the blue box to the background color in ProPresenter… black in this case.
When using a Straight key (also called an "unshaped key") in your setup, the Fill within your linear key is going to be the full color regardless of the amount of transparency. The Key signal is going to handle all of the transparency, and therefore when mixed together in a switcher, looks correct.
When using a straight key, the fill image always shows the full color saturation, tint, and shade of the graphic elements without any alpha channel applied. Many times, when you look at the fill signal from a straight key, it will appear to look a bit odd, particularly when one image is being transitioned to another, as in an A-> B transition because parts of A will remain on the fill signal output throughout the transition. This isn’t a mistake… the alpha key is specifying the transparency of the fully saturated fill channel, in anticipation of the video switcher combining the key and fill channels together. Using this “straight keyer” method, which is brand new in ProPresenter 7.14, you can achieve a transparency of every color as they become fully transparent, instead of a muddied gray color.
Because straight-keying offers so much more flexibility, it is a superior choice whenever possible. However, using a straight keyer may require making some adjustments to your video switcher if it is already optimized for a pre-multiplied key. Often, video switchers will do some manipulating of the image when it knows that a key has been premultiplied with the fill signal to get the results the user was intending. Further, if you are using your fill channel signal for other purposes without the key signal, then you’ll likely prefer to stick with a pre-multiplied keying output. To set up a Premultiplied alpha or Straight alpha output, you can select “Premultiplied” or “Straight” from the Alpha Key tab of the Screen Configuration window.
Setting up screen outputs for an Alpha key
When outputting from ProPresenter using an Alpha Key channel, you choose the Alpha Type in the Screen Configuration. Just select "Straight" or "Premultiplied" under the "Alpha Type" menu.
Setting up a BlackMagic ATEM Switcher Inputs for Alpha Keying
By default, BlackMagic ATEM Switchers are set up for Premultiplied Keys, so no changes are necessary if using premultiplied keying outputs; however, if setting up a Straight Key, you will need to make some adjustments. First, toggle the "Premultiplied Key" off by clicking on it, and per BlackMagic's spec, set the Clip value to 50%, and Gain value to 11%
Setting up a Ross Switcher
When setting up a Ross Switcher, in Ross Dashboard, selecting the "Normal" Keyer Mode is sets the switcher up as a Straight keying, and selecting the "Additive" Keyer Mode is a Premultiplied key. No changes are needed in the Key Parameters on either mode.
Setting the type of alpha-enabled graphics and inputs
You also have the ability to specify the type of key used on input signals and imported media as Premultiplied or Straight. This will impact the way that ProPresenter renders these channels internally. NDI Video Inputs can be configured in the the Inputs tab of ProPresenter Preferences, and media files from within the Media Inspector. By default, all media (with alpha channels) and NDI video inputs will be rendered without further manipulation (as a straight key). You will not see this option if you inspect a media file that does not contain an alpha channel.
There is no way to definitively tell what type of alpha mode is on the content you have, unless the content provider has stated what type of alpha mode with which it was produced. Should you encounter an error, simply make sure that you have set the correct alpha mode on your output, and on the content you are showing, whether that is a video input (set in the inputs tab of ProPresenter Preferences), or media content (set from the media inspector).
If bringing in an NDI video signal, you can uncheck "Premultiplied" in the Inputs tab of ProPresenter Preferences for that specific input, or uncheck "Premultiplied" in the Properties tab of the Media Inspector, if that content has an alpha channel.