OpenGL+GLUT Not filling Topmost-Leftmost Polygon? - c++

Question

I'm encountering a strange OpenGL Bug. OpenGL is pretty new to me, but we're required to use it in my AI class (because the teacher is really a Graphics professor).
Either way, this is happening: http://img818.imageshack.us/img818/422/reversiothello.png
It happens to only the topmost, leftmost polygon. In other words, it finds the furthest polygon left, and then the furthest up and it does that to it. (There is not currently anything erasing polygons from the board).
My display function is this:
void display_func(void)
{
glClearColor(0.0, 0.45, 0.0, 1.0); // Background Color (Forest Green :3)
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 0.0, 0.0);
draw_board();
glFlush();
glutSwapBuffers();
};
My draw_board function is this:
void draw_board()
{
int size = 8;
int stepX = WINDOW_XS / size;
int stepY = WINDOW_YS / size;
glColor3f(0.0,0.0,0.0); // line color black
glBegin(GL_LINES);
// Draw Columns
for(int i = 0;i <= WINDOW_XS;i += stepX)
{
glVertex2i(i,0);
glVertex2i(i, WINDOW_YS);
}
// Draw Rows
for(int j = 0;j <= WINDOW_YS;j += stepY)
{
glVertex2i(0, j);
glVertex2i(WINDOW_XS, j);
}
// Draw Circles
for(int i = 0;i < 8;++i)
{
for(int j = 0;j < 8;++j)
{
if(engine->getOnBoard(i,j) == Reversi::PIECE_NONE) continue;
if(engine->getOnBoard(i,j) == Reversi::PIECE_WHITE)
glColor3f(1.0,1.0,1.0);
if(engine->getOnBoard(i,j) == Reversi::PIECE_BLACK)
glColor3f(0.0,0.0,0.0);
int drawX = ((i+1)*64)-32;
int drawY = 512-((j+1)*64)+32;
gl_drawCircle(drawX,drawY,30);
}
}
glEnd();
};
My mouse function is this:
void mouse_func(int button, int state, int x, int y)
{
if(button == GLUT_LEFT_BUTTON && state == GLUT_DOWN && x < WINDOW_XS)
{
// row and column index
x = (int)( x / (WINDOW_XS/8) );
y = (int)( y / (WINDOW_YS/8) );
std::cout << "Attempting to make a move at " << x << "," << y << std::endl;
if(engine->makeMove(x,y))
{
glutPostRedisplay();
}
}
};
and my gl_drawCircle function is this:
void gl_drawCircle(float x, float y, float r)
{
// http://stackoverflow.com/questions/5094992/c-drawing-a-2d-circle-in-opengl/5095188#5095188
glBegin( GL_POLYGON );
float t;
int n;
for(t = 0,n = 0; n <= 90; ++n, t = float(n)/90.f ) // increment by a fraction of the maximum
{
glVertex2f( x + sin( t * 2 * PI ) * r, y + cos( t * 2 * PI ) * r );
}
glEnd();
}
Can anyone please help me?

Answer 1

The only bug worth giving an answer that I can find is that your draw_board function doesn't use properly the glBegin and glEnd statements. You have to use a glEnd statement before calling gl_drawCircle, otherwise you'll get a nasty behavior.
Edit: you first circle is drawn using lines because the glBegin is ignored (since you are in a glBegin context). All other circles are done ok because you do a glEnd before calling glBegin again. The first drawn circle is the leftmost, topmost circle.

Answer 2

You need a call to glEnd after drawing the rows.
When you do not call glEnd, OpenGL ignores your call glBegin( GL_POLYGON ); and assumes you still want to draw lines.
So just adding
glEnd ();
after drawing the rows should solve it.