OBS Studio Pre-multiply Alpha Setting Explained

When importing transparent PNGs or graphics into OBS Studio, you may encounter the “Pre-multiply Alpha” setting. This article explains what this setting does, why it matters for your stream’s visual quality, and how to determine whether you should enable or disable it to avoid ugly, distorted borders around your transparent overlays.

What is Pre-multiply Alpha?

To understand this setting, you first need to understand how transparency (the alpha channel) works in digital images. Graphic assets handle transparency in one of two ways:

  1. Straight Alpha (Unmultiplied): The color channels (Red, Green, Blue) and the transparency channel (Alpha) are kept completely separate. The color of a pixel remains the same regardless of how transparent it is.
  2. Premultiplied Alpha: The color values of the pixels are pre-multiplied by their transparency value against a background color (usually black). This is often done by design software to speed up rendering times.

The Purpose of the Setting in OBS Studio

The “Pre-multiply Alpha” checkbox in OBS Studio tells the software how to interpret the color data of your transparent source.

If you import a graphic that was saved with premultiplied alpha, but OBS treats it as straight alpha, the math used to render the transparency will be incorrect. This mismatch results in a noticeable, ugly “halo” or dark pixelated border around the semi-transparent edges of your graphic.

By checking the Pre-multiply Alpha box, you instruct OBS to apply the correct mathematical formula to blend the pre-multiplied pixels with your stream background. This eliminates the dark fringes and ensures smooth, anti-aliased, and professional-looking edges.

When Should You Enable It?