写真とサムネイルの相対的なサイズに基づいて、元の画像の切り取り四角形を自分で計算する必要があるように思えます。
public static class CoordinateTransformationHelper
{
public static Point ThumbToOriginal(this Point point, Size thumb, Size source)
{
Point rc = new Point();
rc.X = (int)((double)point.X / thumb.Width * source.Width);
rc.Y = (int)((double)point.Y / thumb.Height * source.Height);
return rc;
}
public static Size ThumbToOriginal(this Size size, Size thumb, Size source)
{
Point pt = new Point(size);
Size rc = new Size(pt.ThumbToOriginal(thumb, source));
return rc;
}
public static Rectangle ThumbToOriginal(this Rectangle rect, Size thumb, Size source)
{
Rectangle rc = new Rectangle();
rc.Location = rect.Location.ThumbToOriginal(thumb, source);
rc.Size = rect.Size.ThumbToOriginal(thumb, source);
return rc;
}
}
使用例:
Size thumb = new Size(10, 10);
Size source = new Size(100, 100);
Console.WriteLine(new Point(4, 4).ThumbToOriginal(thumb, source));
Console.WriteLine(new Rectangle(4, 4, 5, 5).ThumbToOriginal(thumb, source));