OpenGL draw circle with Mouse Move

Question

I am trying to use the function mouseMove(int x, int y) to draw a circle centered at my mouse as I click and drag it across the screen. Circles will be drawn on the moving mouse like a spray paint. So far, this is what I have

void mouseMove(int x, int y) {
    glBegin(GL_POLYGON);
        for (int i = 0; i <= 360; i++)
        {
            float theta = (2 * 3.14 * i) / 360;
            glVertex2f((size/2 + x) * cos(theta), (size/2 + y) * sin(theta));
        }
    glEnd();
    glutPostRedisplay();
}

But when using this, it draws very large circles that aren't centered around my mouse. How would I alter this to make the program draw circles centered at my mouse?

To describe the project, I am creating a painting program that changes shapes, colors, sizes, and rotations of the drawing done in mouseMove. For now, the size is an int set to 32. When the user selects the shape using the 'b' key in a keyboard function, he/she can switch the shapes that are drawn around the mouse as the user clicks and drags the mouse around. Like a spray paint. All the other shapes work shaped around the mouse except for the circle shape spray.

That's probably because your camera is off. Disambiguation: in terms of position. — Ivan Rubinson
Inside this function also have if statements for other shapes such as squares and triangles. All of those draw correctly. This is the only one that is off. — JMV12
I think it's relevant to include in the question how your projection is made (ortho or perspective? Dimensions? Fov?) — Ivan Rubinson

Xirema Xirema · Accepted Answer · 2016-09-07T16:00:43

This answer assumes that things like your viewport and projection matrices are set up correctly, and that the input to this function is taking into account the fact that "screen coordinates" (what the mouse uses) are not the same thing as "OpenGL Coordinate Space" (this usually implies reversing the direction of the y-axis for one or the other).

The math you're using for setting your vertex coordinates is wrong. The mouse's x and y coordinates should not be multiplied by the sine/cosine functions.

The correct way to write it is

glVertex2f((size/2) * cos(theta) + x, (size/2) * sin(theta) + y);

I would also add that you appear to still be using OpenGL's Immediate Mode rendering, which is Deprecated and will offer extremely poor training for a professional setting. I highly advise you learn Modern OpenGL (3.x+) and reapply those concepts to whatever projects you're already working on. This is a very good tutorial.

OpenGL draw circle with Mouse Move

1 Answers