Question

I am working on a simple mesh viewer implementation in C++ with basic functionality such as translation, rotation, scaling.
I’m stuck with with implementing the rotation of the object along z-axis using the mouse. What I want to implement is the following:

1. Click and drag the mouse vertically (almost vertical will do, as I use a simple threshold to filter slight deviations along the horizontal axis) to rotate the object along y-axis (this part is done).
2. Click and drag the mouse horizontally just as described above to rotate the object along x-axis (this part is done too).
3. For z-axis rotation, I want to detect a circular (or along an arc) mouse movement. I’m stuck with this part, and don’t know how to implement this.

For the above two, i just use atan2() to determine the angle of movement. But how do I detect circular movements?

Answer 1

The only way to deal with this is to have a delay between the user starting to make the motion and the object rotating:

When user clicks and begins to move the mouse you need to determine if its going to become a straight line movement, or a circular one. This will require a certain amount of data to be collected before that judgement can be made.

The most extreme case would be requiring the user to make one complete circle first, then the rotation begins (in reality you could do much better than this). Just how small you are able to cut this period down to will depend on a) how precise you dictate your users actions must be, and b) how good you are with pattern recognition algorithms.

To get you started heres an outline of an extremely poor algorithm:

On user click store the x and y coordinates.
Every 1/10 of a second store the new coordinates and process_for_pattern.

in process_for_pattern you’re looking for:

A period where the x coordinates and the y coordinates regularly both increase, both decrease, or one increases and one decreases. Over time if this pattern changes such that either the x or the y begins to reverse whilst the other continues as it was, then at that moment you can be fairly sure you’ve got a circle.

This algorithm would require the user to draw a quarter circle before it was detected, and it does not account for size, direction, or largely irregular movements.

If you really want to continue with this method you can get a much better algorithm, but you might want to reconsider your control method.