Question

I’m developing an application to manipulate images scanned on a wide-image scanner. These images are shown as a ImageBrush on a Canvas.
On this Canvas they can a make Rectangle with the mouse, to define an area to be cropped.

My problem here is to resize the Rectangle according to the original image size, so that it crops the exact area on the original image.

I’ve tried many things so far and it’s just sqeezing my brain, to figure out the right solution.
I know that I need to get the percent that the original image is bigger than the image shown on the canvas.

The dimentions of the original image are:

h: 5606
w: 7677

And when I show the image, they are:

h: 1058,04
w: 1910

Which gives these numbers:

float percentWidth = ((originalWidth - resizedWidth) / originalWidth) * 100;
float percentHeight = ((originalHeight - resizedHeight) / originalHeight) * 100;

percentWidth = 75,12049
percentHeight = 81,12665

From here I can’t figure how to resize the Rectangle correctly, to fit the original image.

My last approach was this:

int newRectWidth = (int)((originalWidth * percentWidth) / 100);
int newRectHeight = (int)((originalHeight * percentHeight) / 100);
int newRectX = (int)(rectX + ((rectX * percentWidth) / 100));
int newRectY = (int)(rectY + ((rectY * percentHeight) / 100));

Hopefully someone can lead me in the right direction, because i’m off track here and I can’t see what i’m missing.

Solution

    private System.Drawing.Rectangle FitRectangleToOriginal(
        float resizedWidth,
        float resizedHeight,
        float originalWidth,
        float originalHeight,
        float rectWidth,
        float rectHeight,
        double rectX,
        double rectY)
    {
        // Calculate the ratio between original and resized image
        float ratioWidth = originalWidth / resizedWidth;
        float ratioHeight = originalHeight / resizedHeight;

        // create a new rectagle, by resizing the old values
        // by the ratio calculated above
        int newRectWidth = (int)(rectWidth * ratioWidth);
        int newRectHeight = (int)(rectHeight * ratioHeight);
        int newRectX = (int)(rectX * ratioWidth);
        int newRectY = (int)(rectY * ratioHeight);

        return new System.Drawing.Rectangle(newRectX, newRectY, newRectWidth, newRectHeight);
    }

Answer 1

You are actually doing a form of projection. Don’t use percentages, just use the ratio between 5606 and 1058,4 = ~5.30. When the user drags the rectangle, reproject it which is selectedWidth * 5606/1058.4.