Scale video to fit window and an overlay to fit video

Scale_video_overlay.js

/**
 * algorithm to center fit overlay image 
 * over an arbitrary sized video or image
 */

function fit(video_width, video_height) {
  
  const video_aspect = video_width / video_height;
  
  // Overlay image - png with transparency
  const over_width = 1280;
  const over_height = 720;
  const over_aspect = over_width / over_height;
  
  // Container to display preview
  const window_width = 600;
  const window_height = 420;
  const window_aspect = window_width / window_height;
  
  let video_k;
  
  // Fit video to container
  if( window_aspect < video_aspect) {
    // common top+bottom, equal heights
    video_k = window_height / video_height;
    
  } else {
    // common sides, equal width
    video_k = window_width / video_width;
    
  }
  
  // Video scale to display within the window
  const new_video_height = video_height * video_k;
  const new_video_width = video_width * video_k;
  
  // Video positioning within display window
  const video_x = (window_width - new_video_width) / 2;
  const video_y = (window_height - new_video_height) / 2;
  
  // TODO: scale and place video
  
  
  let over_k;

  // Overlay scale to place on top of scaled video
  if( over_aspect < video_aspect) {
    // equal heights
    over_k = video_height / over_height;
    
  } else {
    // equal widths
    over_k = video_width / over_width;
    
  }
  
  // Scale overlay to fit displayed video
  const new_over_width = over_width * over_k;
  const new_over_height = over_height * over_k;
  
  // Position overlay centered with the video
  const over_x = video_x + (new_video_width - new_over_width) / 2;
  const over_y = video_y + (new_video_height - new_over_height) / 2
  
  
  // TODO: scale and place overlay.
  
}