How to Get the Mouse Position When Using Ortho in OpenGL

Описание: Learn how to correctly calculate mouse positions in OpenGL when changing the orthographic projection range from `0-1` to `-1 to 1`.
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This video is based on the question https://stackoverflow.com/q/64274911/ asked by the user 'Surya K' ( https://stackoverflow.com/u/14206778/ ) and on the answer https://stackoverflow.com/a/64274999/ provided by the user 'paddy' ( https://stackoverflow.com/u/1553090/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Mouse Positioning in OpenGL with Different Ortho Ranges

As an OpenGL newcomer, it's common to run into challenges while setting up the mouse interactions within your graphical application. You may be dealing with an issue where your program successfully registers mouse clicks when using an orthographic projection range of 0.0 to 1.0, but fails to do so when you switch to -1.0 to 1.0. Let's take a closer look at the problem and explore a simple solution.

The Problem

In your current setup, you have created a simple program that draws a dot at the mouse's position when clicked. Your function myMouseButton gets the mouse coordinates in the existing viewport and adjusts them according to your orthographic projection.

Here's the essence of the issue:

Original Ortho range: gluOrtho2D(0.0, 1.0, 0.0, 1.0)

New Ortho range: gluOrtho2D(-1.0, 1.0, -1.0, 1.0)

When the orthographic projection changes, the math used to calculate the corresponding mouse coordinates also needs to change to reflect that new range.

The Solution

To rectify this issue, you need to update the calculations in your myMouseButton function for transforming the mouse coordinates. Specifically, you will take your existing mouseX and mouseY computations and adapt them for the new orthographic projection.

Steps to Update Mouse Coordinates

Scaling Update: Since the new orthographic range can be visualized as stretching the previous range (0-1) by 2 and centering it at 0, you can directly modify the existing calculations.

Update Code Snippet: You will change your mouse coordinate calculations to look like this:

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

Implementing the Changes

Update your myMouseButton function like so:

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

Conclusion

Changing the orthographic projection in your OpenGL programming requires careful calculation of mouse coordinates. By adjusting the scaling to coordinate adjustments as shown above, you can maintain the expected behavior regardless of the projection limits.

Now that you have a clear understanding of how to handle mouse clicks with different orthographic projections, you can confidently expand your OpenGL projects. Happy coding!