FrameIndex

public int FrameIndex { get; }

[C#]
using (Presentation pres = new Presentation("pres.pptx"))
{
    using (var animationsGenerator = new PresentationAnimationsGenerator(pres))
    using (var player = new PresentationPlayer(animationsGenerator, 33))
    {
        player.FrameTick += (sender, args) =>
        {
            args.GetFrame().Save($"frame_{sender.FrameIndex}.png");
        };
        
        animationsGenerator.Run(pres.Slides);
    }
}