Configure Frame Count and Weights in FFmpeg tmix

This article provides a straightforward guide on how to configure the frame count and weight factors using the FFmpeg tmix filter. You will learn the exact syntax for the frames and weights parameters, understand how they interact to blend successive video frames, and see practical command-line examples to achieve smooth motion blur or temporal mixing effects.

Understanding the tmix Filter Parameters

The tmix (temporal mix) filter in FFmpeg blends successive video frames together. To control how many frames are blended and how much influence each frame has on the final output, you must configure two primary parameters: frames and weights.

• frames: This parameter defines the number of consecutive frames to mix. The default value is 3.
• weights: This parameter defines the weight factor for each frame in the mix, separated by spaces. The default is 1 for all frames, which results in an equal average.

An optional but important parameter is scale. By default, scale is set to 0, which automatically scales the output by the sum of the weights to prevent the video from becoming oversaturated or blown out.

Configuration Syntax and Rules

The basic syntax for the tmix filter is:

tmix=frames=N:weights="w1 w2 w3 ... wN":scale=S

Rule 1: Match Weights to Frame Count

If you specify a certain number of frames, you should ideally provide the same number of space-separated values in the weights string. For example, to mix 5 frames with a fade-in/fade-out weight distribution:

tmix=frames=5:weights="1 2 3 2 1"

Rule 2: Handling Fewer Weight Values

If you specify fewer weight values than the number of frames, the filter will automatically use the last specified weight value for all the remaining frames. For example:

tmix=frames=5:weights="1 2"

In this scenario, FFmpeg interprets the weights as "1 2 2 2 2".

Rule 3: Enclosing Weights in Quotes

Because the weight values are separated by spaces, you must enclose the weight sequence in quotes (either single or double depending on your command-line shell) so FFmpeg does not parse the spaces as command delimiters.

Practical Command Examples

Example 1: Equal Blend (Simple Motion Blur)

To blend 7 successive frames equally to create a smooth motion blur, you can set the frame count to 7. Since the default weight is 1 for all frames, you do not need to specify the weights manually:

ffmpeg -i input.mp4 -vf "tmix=frames=7" output.mp4

Example 2: Weighted Blend (Emphasizing the Current Frame)

If you want to blend 5 frames but keep the center (current) frame sharper than the surrounding temporal frames, you can assign a higher weight to the middle frame:

ffmpeg -i input.mp4 -vf "tmix=frames=5:weights='1 2 4 2 1'" output.mp4

In this command, the third frame (index 2) has a weight of 4, making it twice as prominent as the adjacent frames and four times as prominent as the outer frames.

Example 3: Temporal Fade-Out

To create an effect where past frames trail off progressively, you can decrease the weights from the newest frame to the oldest frame:

ffmpeg -i input.mp4 -vf "tmix=frames=4:weights='4 3 2 1'" output.mp4

Example 4: Manual Scaling

If you want to manually adjust the brightness of the mixed frames rather than relying on automatic normalization, set the scale factor. For example, to scale the output to 80% of the sum of weights:

ffmpeg -i input.mp4 -vf "tmix=frames=3:weights='1 2 1':scale=0.2" output.mp4