Function collect_input_frames 

pub fn collect_input_frames(
    input_folder: &Path,
    file_pattern: Option<String>,
) -> Result<Vec<PathBuf>, Box<dyn Error>>

Collect files from input_folder matching the optional pattern, in natural order (frame_2.png before frame_10.png). Defaults to *.png.

Besides glob patterns (*.png, scene1_*.png) this accepts ffmpeg-style sequence patterns such as frame_%04d.png.