Why Does My Image Jump When Zooming from the Mouse Cursor Using C# Transformations?

Zooming an Image from the Mouse Cursor Using Transformations

In this scenario, an attempt is made to zoom (scale) an Image from the mouse location using transformations in the Paint event to translate the bitmap origin to the mouse location and then scale the Image and translate its origin back.

Issue:

• The Image jumps and fails to scale from the relocated origin when translating the mouse location.
• Rotate, scale, and pan function correctly without translating to the mouse location.

Platform:

• .Net 4.7.2
• Visual Studio in Windows 10 1909 (18363.778)

Relevant Code Blocks:

private void trackBar1_Scroll(object sender, EventArgs e) {
    // Get rotation angle
    ang = trackBar1.Value;
    pnl1.Invalidate();
}

private void pnl1_MouseWheel(object sender, MouseEventArgs e) {
    // Get mouse location
    mouse = e.location;

    // Get new scale (zoom) factor
    zoom = (float)(e.Delta > 0 ? zoom * 1.05 : zoom / 1.05);
    pnl1.Invalidate();
}

private void pnl1_MouseDown(object sender, MouseEventArgs e) {
    if (e.Button != MouseButtons.Left) return;
    pan = true;
    mouX = e.X;
    mouY = e.Y;
    oldX = imgX;
    oldY = imgY;
}

private void pnl1_MouseMove(object sender, MouseEventArgs e) {
    if (e.Button != MouseButtons.Left || !pan) return;

    // Coordinates of panned image
    imgX = oldX + e.X - mouX;
    imgY = oldY + e.Y - mouY;
    pnl1.Invalidate();
}

private void pnl1_MouseUp(object sender, MouseEventArgs e) {
    pan = false;
}

private void pnl1_Paint(object sender, PaintEventArgs e) {
    // Apply rotation angle @ center of bitmap
    e.Graphics.TranslateTransform(img.Width / 2, img.Height / 2);
    e.Graphics.RotateTransform(ang);
    e.Graphics.TranslateTransform(-img.Width / 2, -img.Height / 2);

    // Apply scaling factor - focused @ mouse location
    e.Graphics.TranslateTransform(mouse.X, mouse.Y, MatrixOrder.Append);
    e.Graphics.ScaleTransform(zoom, zoom, MatrixOrder.Append);
    e.Graphics.TranslateTransform(-mouse.X, -mouse.Y, MatrixOrder.Append);

    // Apply drag (pan) location
    e.Graphics.TranslateTransform(imgX, imgY, MatrixOrder.Append);

    // Draw "bmp" @ location
    e.Graphics.DrawImage(img, 0, 0);
}

Possible Solutions:

To address this issue and achieve smooth zooming from the mouse location, consider the following suggestions and tricks:

1. Divide and Conquer: Break down the different graphics effects and transformations into separate, specialized methods that perform specific tasks. Then, design these methods to work together seamlessly when needed.

2. Keep It Simple: When applying multiple graphic transformations, the order in which Matrices are stacked can lead to confusion and unexpected results. It's more straightforward to calculate certain transformations (translation and scaling, primarily) beforehand and let GDI handle the rendering of pre-processed objects and shapes.

3. Use the Right Tools: A Panel as a "canvas" is not recommended for scenarios like this. It lacks double buffering, although it can be enabled. However, a PictureBox (or a non-System flat Label) offers double-buffering out of the box and is designed for drawing rather than containing child Controls.

4. Implement Zoom Modes: Instead of blindly scaling from the mouse location, provide different methods to control the zoom behavior. Implement zoom modes such as ImageLocation, CenterCanvas, CenterMouse, and MouseOffset to offer flexibility and meet various use cases.

By following these guidelines and implementing tailored zoom modes, you can achieve smooth and effective zooming from the mouse location while maintaining the desired image position and scale factor.

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