title: How can I transform XY coordinates and height/width on a scaled image to an original sized image?
body: <p><a href="https://stackoverflow.com/questions/2988467/how-to-know-coordinates-in-a-real-image-from-a-scaled-image">Related Question</a></p>
  
  <p>I am trying to do the same thing as in the linked question, but with C#. I am showing a scaled image and am allowing a user to select an area to crop. However, I can't just take the x1y1, x2y2 coordinates from the scaled image selection and crop that from the original. I've tried doing some basic math like in the other question, but that's obviously not the right approach either (it's definitely closer). </p>
  
  <p><strong>Edit</strong></p>
  
  <p>Original Image Dimensions: <code>w = 1024 h = 768</code></p>
  
  <p>Scaled Image Dimensions: <code>w = 550  h = 412</code></p>
  
  <p>I start with an image, say 1024x768. I want it to fit as large as possible in a 550x550 box. I'm using the following method to get the scaled image size (while maintaining aspect ratio). Then I do a basic resize to those new dimensions.</p>
  
  <p>As for a selection area, it can be anything (0,0) to (100,100).</p>
  
  <pre><code>private static Rectangle MaintainAspectRatio(Image imgPhoto, Rectangle thumbRect)
  {
      int sourceWidth = imgPhoto.Width; int sourceHeight = imgPhoto.Height; int sourceX = 0; int sourceY = 0; int destX = 0; int destY = 0;
  
      float nPercent = 0;
      float nPercentW = 0;
      float nPercentH = 0;
  
      nPercentW = ((float)thumbRect.Width / (float)sourceWidth);
      nPercentH = ((float)thumbRect.Height / (float)sourceHeight);
  
      //if we have to pad the height pad both the top and the bottom
      //with the difference between the scaled height and the desired height
      if (nPercentH &lt; nPercentW)
      {
          nPercent = nPercentH;
          destX = (int)((thumbRect.Width - (sourceWidth * nPercent)) / 2);
      }
      else
      {
          nPercent = nPercentW;
          destY = (int)((thumbRect.Height - (sourceHeight * nPercent)) / 2);
      }
  
      int destWidth = (int)(sourceWidth * nPercent);
      int destHeight = (int)(sourceHeight * nPercent);
  
      Rectangle retRect = new Rectangle(thumbRect.X, thumbRect.Y, destWidth, destHeight);
      return retRect;
  }
  </code></pre>