Fill Background Type

Sets the background type used to pad the border when fit=fill or fit=fillmax is used. The fill covers the area between the scaled video and the exact output dimensions.

fill accepts solid (the default), blur, or image.

• solid — fills the border with a flat color. Default color is black (000000); change it with fill-color.
• blur — fills the border with a cover-scaled, blurred copy of the video frame — the popular “blurred background” look. Blur strength is controlled by fill-blur; background dimming by fill-darken.
• image — fills the border with a fetched image (a branded backdrop, gradient, or any external image), cover-scaled to fill the canvas without distorting. Supply the image URL with fill-image, or its Base64-encoded twin fill-image64. Use fill-darken to dim the backdrop. If the image fails to fetch, fill-color is used as a silent fallback.

fill only takes effect with fit=fill or fit=fillmax. It is ignored for other fit modes.