Скачать или смотреть How to Effectively Rotate a Scene in OpenGL with Qt

Learn how to rotate your OpenGL scene in Qt applications effectively. We'll walk you through the necessary steps and code adjustments to ensure smooth rotations.
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This video is based on the question https://stackoverflow.com/q/73165184/ asked by the user 'Anton Ivanushkin' ( https://stackoverflow.com/u/19646924/ ) and on the answer https://stackoverflow.com/a/73167705/ provided by the user 'Yasser Malaika' ( https://stackoverflow.com/u/2775347/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Effectively Rotate a Scene in OpenGL with Qt

Creating a dynamic scene in OpenGL using Qt often involves manipulating objects with transformations like rotation. However, many developers struggle with implementing these transformations correctly, especially when it comes to using the right OpenGL functions in conjunction with Qt’s event handling. In this post, we will break down how to rotate your OpenGL scene effectively and troubleshoot common issues encountered in the process.

Understanding the Problem

In your current implementation, you want to rotate points drawn on a 2D plane scene when a mouse wheel event occurs. However, the rotation isn't taking effect as anticipated. Here's a brief overview of the code snippets involved and what they do:

initializeGL(): Sets up the OpenGL environment.

resizeGL(): Adjusts the viewport and projection matrix.

paintGL(): Contains the drawing logic for rendering points.

mouseDoubleClickEvent(): Adds points to your scene on a mouse double-click.

wheelEvent(): Here, you attempt to adjust the rotation angle on mouse wheel scrolling.

The underlying issue is the improper sequence and placement of OpenGL commands related to matrix transformations, specifically rotation.

The Solution

Proper Use of Matrix Stack

The OpenGL matrix stack is crucial for applying transformations like rotation. Here’s the correct approach to ensure your scene rotates as intended based on user input.

1. Update the Rotation Logic in the wheelEvent()

Instead of trying to draw within your event handler, which is not its intended purpose, modify the wheelEvent() as follows:

[[See Video to Reveal this Text or Code Snippet]]

This change ensures that you are updating the angle and requesting a redraw on the OpenGL widget.

2. Corrected paintGL() Implementation

Now you need to incorporate the rotation into your drawing logic within the paintGL() function:

[[See Video to Reveal this Text or Code Snippet]]

3. Maintain Matrix Stack Integrity

It's essential to maintain the correct order of glPushMatrix() and glPopMatrix(). Always push before transformations and pop after you are done with your drawing routines. This way, the transformations affect only the desired objects, without changing subsequent drawings.

4. Manage the Matrix Mode Correctly

Ensure that you're using the right matrix mode during transformations. OpenGL allows you to switch between different matrices (e.g., projection vs. model-view):

[[See Video to Reveal this Text or Code Snippet]]

By ensuring you are in the correct mode, your transformations will apply properly.

Conclusion

Following these guidelines will help you successfully rotate a scene in OpenGL using Qt. Remember to respect the stack and manage the sequence of matrix operations correctly. By separating your drawing code from your event handling code, you'll maintain clarity and prevent common pitfalls in 3D graphics programming. Happy coding!