How to Unpremultiply Alpha in FFmpeg

This article explains how to use the unpremultiply filter in FFmpeg to reverse channel premultiplication. You will learn what premultiplied alpha is, why you need to reverse it, and how to use the FFmpeg command-line syntax with practical examples to restore your video’s color channels to their straight (unpremultiplied) state.

Understanding Premultiplied Alpha

In digital video and imagery, transparent assets can represent alpha (transparency) in two ways: * Straight Alpha: The RGB color channels and the Alpha channel are kept completely independent. * Premultiplied Alpha: The RGB color values are already multiplied by the Alpha value. This is often used to render semi-transparent edges more efficiently during compositing.

If you perform color correction or resizing on a premultiplied video, you can introduce dark halos or color fringing around transparent edges. Reversing this multiplication—known as “unpremultiplying”—restores the original color values so you can process the image accurately.

The FFmpeg `unpremultiply` Filter Syntax

The unpremultiply filter in FFmpeg divides the color channels (RGB) of a video by its alpha channel.

The basic filter syntax is:

unpremultiply[=inplace]

inplace: A boolean flag (0 or 1). Setting this to 1 enables in-place processing, which is faster and uses less memory. The default value is 0.

Practical Command Examples

Example 1: Basic Unpremultiplication

To reverse premultiplication on an input video or image sequence and output the result, use the following command:

ffmpeg -i input_premultiplied.mov -vf "unpremultiply=inplace=1" output_straight.mov

In this command: * -i input_premultiplied.mov specifies the source video that contains premultiplied alpha. * -vf "unpremultiply=inplace=1" applies the video filter, processing the pixel division in place for optimal performance.

Example 2: Unpremultiplying and Applying Other Filters

If you need to color-correct or scale your video, you should unpremultiply first, apply your filters, and then premultiply again (if your delivery format requires it).

Here is how to unpremultiply, adjust the gamma/contrast using the eq filter, and output the video:

ffmpeg -i input_premultiplied.mov -vf "unpremultiply=inplace=1,eq=gamma=1.2:contrast=1.1" output_corrected.mov

By placing unpremultiply at the beginning of the filterchain, you ensure that the color adjustments do not corrupt the semi-transparent edge pixels.