Translate Rectangle Position in a Picturebox in Zoom Mode

Question

I'm determining the rectangular area in an image and showing it to the user in a PictureBox.
Since the image can sometimes be very large, I'm using a PictureBox with its SizeMode set to Zoom.

I'm using the following code to translate the Rectangle (X, Y) coordinates:

public Point TranslateZoomMousePosition(Point coordinates)
{
    // test to make sure our image is not null
    if (pictureBox5.Image == null) return coordinates;
    // Make sure our control width and height are not 0 and our 
    // image width and height are not 0
    if (pictureBox5.Width == 0 || pictureBox5.Height == 0 || pictureBox5.Image.Width == 0 || pictureBox5.Image.Height == 0) return coordinates;
    // This is the one that gets a little tricky. Essentially, need to check 
    // the aspect ratio of the image to the aspect ratio of the control
    // to determine how it is being rendered
    float imageAspect = (float)pictureBox5.Image.Width / pictureBox5.Image.Height;
    float controlAspect = (float)pictureBox5.Width / pictureBox5.Height;
    float newX = coordinates.X;
    float newY = coordinates.Y;
    if (imageAspect > controlAspect)
    {
        // This means that we are limited by width, 
        // meaning the image fills up the entire control from left to right
        float ratioWidth = (float)pictureBox5.Image.Width / pictureBox5.Width;
        newX *= ratioWidth;
        float scale = (float)pictureBox5.Width / pictureBox5.Image.Width;
        float displayHeight = scale * pictureBox5.Image.Height;
        float diffHeight = pictureBox5.Height - displayHeight;
        diffHeight /= 2;
        newY -= diffHeight;
        newY /= scale;
    }
    else
    {
        // This means that we are limited by height, 
        // meaning the image fills up the entire control from top to bottom
        float ratioHeight = (float)pictureBox5.Image.Height / pictureBox5.Height;
        newY *= ratioHeight;
        float scale = (float)pictureBox5.Height / pictureBox5.Image.Height;
        float displayWidth = scale * pictureBox5.Image.Width;
        float diffWidth = pictureBox5.Width - displayWidth;
        diffWidth /= 2;
        newX -= diffWidth;
        newX /= scale;
    }
    return new Point((int)newX, (int)newY);
}

Adding a frame control at the determined position:

pictureBox5.Controls.Clear();
var c = new FrameControl();
c.Size = new Size(myrect.Width, myrect.Height);
c.Location=TranslateZoomMousePosition(newPoint(myrect.Location.X,myrect.Location.Y));
pictureBox5.Controls.Add(c);

But the determined frame/rectangle location is not correct.
What am I i doing wrong?

Update: I'm trying to translate a Rectangle on an image to a Frame Control on a PictureBox using similar code

public Rectangle GetRectangeOnPictureBox(PictureBox p, Rectangle selectionRect,Bitmap bit)
    {
        var method = typeof(PictureBox).GetMethod("ImageRectangleFromSizeMode",
            System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
        var imageRect = (Rectangle)method.Invoke(p, new object[] { p.SizeMode });
        if (p.Image == null)
            return selectionRect;
        int cx = bit.Width / imageRect.Width;
        int cy = bit.Height / imageRect.Height;
        Rectangle trsRectangle = new Rectangle(selectionRect.X * cx, selectionRect.Y * cy, selectionRect.Width * cx, selectionRect.Height * cy);

        trsRectangle.Offset(imageRect.X, imageRect.Y);
        return trsRectangle;
    }

This produces invalid result.Please advice

Answer 1

You can translate selected rectangle on the picture box to the rectangle on image this way:

public RectangleF GetRectangeOnImage(PictureBox p, Rectangle selectionRect)
{
    var method = typeof(PictureBox).GetMethod("ImageRectangleFromSizeMode",
        System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
    var imageRect = (Rectangle)method.Invoke(p, new object[] { p.SizeMode });
    if (p.Image == null)
        return selectionRect;
    var cx = (float)p.Image.Width / (float)imageRect.Width;
    var cy = (float)p.Image.Height / (float)imageRect.Height;
    var r2 = Rectangle.Intersect(imageRect, selectionRect);
    r2.Offset(-imageRect.X, -imageRect.Y);
    return new RectangleF(r2.X * cx, r2.Y * cy, r2.Width * cx, r2.Height * cy);
}

Note: You can find ImageRectangleFromSizeMode method source code here and use it as write such method as part of your application code.

Example - Crop Image of PictureBox having SizeMode = Zoom

As an example, the following code will crop the given rectangle of the picture box 1 and will set the result as image of picture box 2:

var selectedRectangle = new Rectangle(7, 30, 50, 40);
var result = GetRectangeOnImage(pictureBox1, selectedRectangle);
using (var bm = new Bitmap((int)result.Width, (int)result.Height))
{
    using (var g = Graphics.FromImage(bm))
        g.DrawImage(pictureBox1.Image, 0, 0, result, GraphicsUnit.Pixel);
    pictureBox2.Image = (Image)bm.Clone();
}

Here is the input image:

And this is the result:

Answer 2

A specialized class that provides some helper tools to determine the scaling factor of a selection and translates the selection coordinates to the scaled Bitmap coordinates.
This version is for zoomed images only.

The ZoomFactor class provides these methods:

PointF TranslateZoomPosition(PointF Coordinates, SizeF ContainerSize, SizeF ImageSize):
returns the PointF translated coordinates of a Point location inside a Container to the Point location inside a Bitmap, zoomed in the container.

RectangleF TranslateZoomSelection(RectangleF Selection, SizeF ContainerSize, SizeF ImageSize):
returns a RectangleF representing a selection created inside a Container, translated to the Bitmap coordinates.

RectangleF TranslateSelectionToZoomedSel(RectangleF SelectionRect, SizeF ContainerSize, SizeF ImageSize):
returns a RectangleF representing a pre-selected area of the original Bitmap translated to the zoomed selection Image inside a Container.

PointF GetImageScaledOrigin(SizeF ContainerSize, SizeF ImageSize):
returns the PointF reference of the zoomed Image origin coordinates inside the Container.

SizeF GetImageScaledSize(SizeF ContainerSize, SizeF ImageSize):
returns the SizeF reference of the Image when scaled inside the Container.

Sample usage, showing how to crop a Bitmap using a selection Rectangle created inside a Container control. The TranslateZoomSelection method returns the Bitmap section corresponding to a selection area:

ZoomFactor zoomHelper = new ZoomFactor()
Bitmap originalBitmap;

RectangleF currentSelection = [Current Selection Rectangle];
RectangleF bitmapRect = zoomHelper.TranslateZoomSelection(currentSelection, [Container].Size, originalBitmap.Size);

var croppedBitmap = new Bitmap((int)bitmapRect.Width, (int)bitmapRect.Height, originalBitmap.PixelFormat))
using (var g = Graphics.FromImage(croppedBitmap))
{
    g.DrawImage(originalBitmap, new Rectangle(Point.Empty, Size.Round(bitmapRect.Size)), 
                bitmapRect, GraphicsUnit.Pixel);
    [Container].Image = croppedBitmap;
}

A Sample of the behaviour described above:

Note: In the example, the pre-selection of the image in Portrait inverts Width and Height

The ZoomFactor class:

public class ZoomFactor
{
    public ZoomFactor() { }

    public PointF TranslateZoomPosition(PointF coordinates, SizeF containerSize, SizeF imageSize)
    {
        PointF imageOrigin = TranslateCoordinatesOrigin(coordinates, containerSize, imageSize);
        float scaleFactor = GetScaleFactor(containerSize, imageSize);
        return new PointF(imageOrigin.X / scaleFactor, imageOrigin.Y / scaleFactor);
    }

    public RectangleF TranslateZoomSelection(RectangleF selectionRect, SizeF containerSize, SizeF imageSize)
    {
        PointF selectionTrueOrigin = TranslateZoomPosition(selectionRect.Location, containerSize, imageSize);
        float scaleFactor = GetScaleFactor(containerSize, imageSize);

        SizeF selectionTrueSize = new SizeF(selectionRect.Width / scaleFactor, selectionRect.Height / scaleFactor);
        return new RectangleF(selectionTrueOrigin, selectionTrueSize);
    }

    public RectangleF TranslateSelectionToZoomedSel(RectangleF selectionRect, SizeF containerSize, SizeF imageSize)
    {
        float scaleFactor = GetScaleFactor(containerSize, imageSize);
        RectangleF zoomedSelectionRect = new
            RectangleF(selectionRect.X * scaleFactor, selectionRect.Y * scaleFactor,
                       selectionRect.Width * scaleFactor, selectionRect.Height * scaleFactor);

        PointF imageScaledOrigin = GetImageScaledOrigin(containerSize, imageSize);
        zoomedSelectionRect.Location = new PointF(zoomedSelectionRect.Location.X + imageScaledOrigin.X,
                                                  zoomedSelectionRect.Location.Y + imageScaledOrigin.Y);
        return zoomedSelectionRect;
    }

    public PointF TranslateCoordinatesOrigin(PointF coordinates, SizeF containerSize, SizeF imageSize)
    {
        PointF imageOrigin = GetImageScaledOrigin(containerSize, imageSize);
        return new PointF(coordinates.X - imageOrigin.X, coordinates.Y - imageOrigin.Y);
    }

    public PointF GetImageScaledOrigin(SizeF containerSize, SizeF imageSize)
    {
        SizeF imageScaleSize = GetImageScaledSize(containerSize, imageSize);
        return new PointF((containerSize.Width - imageScaleSize.Width) / 2,
                          (containerSize.Height - imageScaleSize.Height) / 2);
    }

    public SizeF GetImageScaledSize(SizeF containerSize, SizeF imageSize)
    {
        float scaleFactor = GetScaleFactor(containerSize, imageSize);
        return new SizeF(imageSize.Width * scaleFactor, imageSize.Height * scaleFactor);

    }
    internal float GetScaleFactor(SizeF scaled, SizeF original)
    {
        return (original.Width > original.Height) ? (scaled.Width / original.Width)
                                                  : (scaled.Height / original.Height);
    }
}