Function render

pub fn render(
    ffmpeg: &str,
    audio: &Path,
    picture: &Path,
    size: Size,
    caption: Option<&str>,
) -> Result<(), String>

Draws the picture for one file.

Two attempts: with the caption, then without. drawtext needs a font, and an installation that has none fails the whole filter graph — losing the picture over a line of text would be a poor trade, and the legend still carries the axis that matters.

Both attempts write inside a private temporary directory beside the destination. Only a completed picture replaces the old one; destination links and special files are refused before rendering and publication.