How to Configure Pre-multiply Alpha in OBS Studio

This guide provides a straightforward tutorial on how to configure the “Pre-multiply Alpha” setting for transparent media sources in OBS Studio. You will learn what this setting does, why it is important for eliminating unsightly borders on transparent videos, and the exact step-by-step process to configure it within your sources.

What is Pre-multiply Alpha?

When working with transparent media—such as WebM or MOV video overlays—transparent edges can sometimes display a strange halo, or a faint white or black outline. This artifact occurs due to how color pixels are blended with the transparency (alpha) channel:

• Straight Alpha: Color channels and transparency channels are kept entirely separate.
• Pre-multiplied Alpha: Color channels are pre-blended with the alpha channel, usually against a black background, during export.

Configuring the “Pre-multiply Alpha” setting in OBS Studio tells the software how to interpret these pixels, ensuring smooth, clean edges without unwanted borders.

Steps to Configure Pre-multiply Alpha in OBS Studio

Follow these steps to apply the setting to your transparent media source:

1. Open OBS Studio and locate the Sources dock.
2. Open the Source Properties:
   • If you are adding a new source, click the + icon, select Media Source, name it, and click OK.
   • If the source is already in your scene, double-click it, or right-click it and select Properties.
3. Locate the Setting: Scroll down the properties window until you find the checkbox labeled Pre-multiply alpha.
4. Toggle the Setting:
   • Check the box if your video asset was exported using a pre-multiplied alpha channel (this is common for assets rendered out of Adobe After Effects).
   • Uncheck the box if your video was exported using straight alpha.
5. Click OK to save your changes and close the properties window.

Troubleshooting the Setting

If you do not know which alpha type your video file uses, you can determine the correct setting through a quick visual test:

• If your graphic has a dark or light outline/halo around its semi-transparent edges, enable Pre-multiply alpha.
• If the edges of your graphic look too dark, thin, or unnaturally eaten away, disable Pre-multiply alpha.