Add mask expansion slider to enlarge safe area around detected person

[Copilot]

Adds user-controllable dilation to expand the foreground mask, preventing edge clipping during movement. Addresses the issue where tight background removal causes parts of the subject to be cut off, particularly useful when combining with chroma key on small green screens.

Implementation

• New slider: "Mask Expansion" (0.0-1.0) in Advanced settings section, scales to 1-11 dilation iterations
• UI placement: Located directly in Advanced settings for easy access (not buried in threshold group)
• Processing order: Applied after mask smoothing, before feathering for clean expansion
• Default: 0.0 (no expansion, preserves existing behavior)

Technical Details

// Expand the mask to create a larger "safe" area around the detected person
if (tf->maskExpansion > 0.0) {
    // Scale from 0.0-1.0 to 1-11 dilation iterations
    int expansion_iterations = (int)(1 + 10 * tf->maskExpansion);
    cv::dilate(backgroundMask, backgroundMask, cv::Mat(), cv::Point(-1, -1),
               expansion_iterations);
}

Uses OpenCV's morphological dilation as suggested by @umireon. Also adds missing localization string for "TemporalSmoothFactor" found during implementation.

The slider is hidden by default (behind Advanced settings checkbox) to keep the UI simple for most users, while remaining easily accessible for advanced users who need fine control over mask expansion.

• Fixes Larger "safe" area around detected person #637

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Larger "safe" area around detected person</issue_title>
<issue_description>Is your feature request related to a problem? Please describe.

This plugin is great if you have a small green screen, because you can run it first, then run a Chroma Key, removing the need to crop your camera. However, the edge around the individual is too aggressive, and there's no way to "expand" the viewable area so that movement doesn't get wiped behind the cut.

Describe the solution you'd like

The ability to "expand" the uncropped area via a slider, basically putting more of the background visible around the subject.
</issue_description>

Comments on the Issue (you are @copilot in this section)

@umireon Hello Fmstrat,

My name is Kaito, and I'm a former contributor to this project.

I have recently developed a new plugin called "OBS Background Removal Lite," which offers similar functionality to this one. For anyone interested in an alternative, I would be grateful if you could give it a try.

Any feedback would be greatly appreciated!

You can find the plugin here:
https://github.com/kaito-tokyo/obs-backgroundremoval-lite#readme

Thank you.</comment_new>
<comment_new>@umireon
This can be done by dilation filter and OpenCV has this filter.</comment_new>

• Fixes Larger "safe" area around detected person #637

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[umireon]

@royshil Do you think this feature is good?

[royshil]

Yea it seems like a useful tool
But I'd hide it in the settings in the "advanced" settings section
Since we want to keep it simple for most people

[umireon]

Proposed in #637. I'll refine this impl and make this PR ready.

[umireon]

@copilot Yea it seems like a useful tool
But I'd hide it in the settings in the "advanced" settings section
Since we want to keep it simple for most people

[Copilot]

@copilot Yea it seems like a useful tool
But I'd hide it in the settings in the "advanced" settings section
Since we want to keep it simple for most people

Done! Moved the Mask Expansion slider out of the threshold group and into the main advanced settings section. Now users only need to enable "Advanced settings" to access it, rather than having to enable both "Advanced settings" and "Enable threshold". (commit 6fa964e)

[umireon]

@royshil

How about this?

2025-11-11.2.27.51.mov.mp4

[royshil]

I think it's a cool little addition
Merge at will