triangle.cpp

        float cx = (x + x2 + x3) / 3;
        float cy = (y + y2 + y3) / 3;
        float l1 = std::sqrt((x - cx) * (x - cx) + (y - cy) * (y - cy));
        float l2 = std::sqrt((x2 - cx) * (x2 - cx) + (y2 - cy) * (y2 - cy));
        float l3 = std::sqrt((x3 - cx) * (x3 - cx) + (y2 - cy) * (y2 - cy));
        float dx = 2 * (x - cx) / l1 * (width / (float)glutGet(GLUT_WINDOW_WIDTH));
        float dy = 2 * (y - cy) / l1 * (width / (float)glutGet(GLUT_WINDOW_HEIGHT));
        float dx2 = 2 * (x2 - cx) / l2 * (width / (float)glutGet(GLUT_WINDOW_WIDTH));
        float dy2 = 2 * (y2 - cy) / l2 * (width / (float)glutGet(GLUT_WINDOW_HEIGHT));
        float dx3 = 2 * (x3 - cx) / l3 * (width / (float)glutGet(GLUT_WINDOW_WIDTH));
        float dy3 = 2 * (y3 - cy) / l3 * (width / (float)glutGet(GLUT_WINDOW_HEIGHT));
        glBegin(GL_TRIANGLE_STRIP);
            glVertex2f(x + dx, y + dy);
            glVertex2f(x - dx, y - dy);
            glVertex2f(x2 + dx2, y2 + dy2);
            glVertex2f(x2 - dx2, y2 - dy2);
            glVertex2f(x3 + dx3, y3 + dy3);
            glVertex2f(x3 - dx3, y3 - dy3);
            glVertex2f(x + dx, y + dy);
            glVertex2f(x - dx, y - dy);
        glEnd();