Calculate zoom level to fit an image into a panel

Question

How could I calculate the zoom level (graphics scale) to fit any image to any panel?

The image size and the picture size could be any size.

The method signature I need is the following:

  public float CalculateZoomToFit(Image image, Panel targetPanel)
  {
     // I need to calculate the zoom level to make the picture fit into the panel
     return ???
  }

Answer 1

The ratio of width over height for both the panel and the image is the key for the answer.

var panel_ratio = targetPanel.Width / targetPanel.Height;
var image_ratio = image.Width / image.Height;

return panel_ratio > image_ratio
     ? targetPanel.Height / image.Height
     : targetPanel.Width / image.Width
     ;

Add checks for divide-by-zero errors, if you want.