OpenGL—ES 1.0 2d rounded rectangle

Question

How to make rounded rectangle in OpenGL, or any polygon with rounded corners?

Answer 1

I came across this fixing a crash in some open-source software - the non-GL version worked fine but basically the intention was to implement a rounded rectangle but the developer was too lazy for that and decided to force a crash instead :-(

Although I think @vime's answer is succinct and complete, I have seen lots of similar examples, none of which gave me any confidence and that I thought were non-obvious, so here's mine for the record... the calling function implements the 4-corners ( code snippet )...

glBegin(GL_POLYGON);

// top-left corner
DrawGLRoundedCorner(x, y + radius, 3 * PI / 2, PI / 2, radius);

// top-right
DrawGLRoundedCorner(x + size_x - radius, y, 0.0, PI / 2, radius);

// bottom-right
DrawGLRoundedCorner(x + size_x, y + size_y - radius, PI / 2, PI / 2, radius);

// bottom-left
DrawGLRoundedCorner(x + radius, y + size_y, PI, PI / 2, radius);

glEnd();

... and the arc-section function DrawGLRoundedCorner(). Note that this assumes that glBegin() has already been called and plots both the start and the end of the arc - which is why you don't need to explicitly add the vertices at the end of the sides.

void DrawGLRoundedCorner(int x, int y, double sa, double arc, float r) {
    // centre of the arc, for clockwise sense
    float cent_x = x + r * cos(sa + PI / 2);
    float cent_y = y + r * sin(sa + PI / 2);

    // build up piecemeal including end of the arc
    int n = ceil(N_ROUNDING_PIECES * arc / PI * 2);
    for (int i = 0; i <= n; i++) {
        double ang = sa + arc * (double)i  / (double)n;

        // compute the next point
        float next_x = cent_x + r * sin(ang);
        float next_y = cent_y - r * cos(ang);
        glVertex2f(next_x, next_y);
    }
}

By using a different glBegin such as with GL_LINE_LOOP I think you would end up with a non-filled rounded rectangle. For larger corner radii there might be a need to use various anti-aliasing hints or the like to make it look prettier, but there are other posts regarding that.

Hope that helps someone.