How to draw square on the button at the center using OpenGL? - c++

Question

How to draw a square on the button at the center (like stop button)?
In this code that I try it be like a rectangle and full of the button:
void ButtonDraw(Button *b)
{
if(b)
{
/*
* We will indicate that the mouse cursor is over the button by changing its
* colour.
*/
if (b->highlighted)
glColor3f(0.7f,0.7f,0.8f);
else
glColor3f(0.6f,0.6f,0.6f);
/*
* draw background for the button.
*/
glBegin(GL_QUADS);
glVertex2i( b->x , b->y );
glVertex2i( b->x , b->y+b->h );
glVertex2i( b->x+b->w, b->y+b->h );
glVertex2i( b->x+b->w, b->y );
glEnd();
/*draw red square on the button*/
glBegin(GL_QUADS);
glColor3f(1.0f, 0.0f, 0.0f);
glVertex2i(b->x, b->y);
glVertex2i(b->x+b->w, b->y);
glVertex2i(b->x+b->w, b->y + b->h);
glVertex2i(b->x, b->y + b->h);
glEnd();
/*
* Draw an outline around the button with width 3
*/
glLineWidth(3);
}
}

Answer 1

The obvious way is to not draw a red rectangle the same size as the button, but instead draw a square. Here is the most basic way to do this:
/*draw red square on the button*/
glBegin(GL_QUADS);
glColor3f(1.0f, 0.0f, 0.0f);
const int SQUARE_SIDE = 6; // Or however long you want a side
// Calculate the centre of the button rectangle
const int xMid = b->x + b->w / 2;
const int yMid = b->y + b->h / 2;
// Trace a square around the centre
glVertex2i(b->xMid - SQUARE_SIDE / 2, b->yMid - SQUARE_SIDE / 2);
glVertex2i(b->xMid - SQUARE_SIDE / 2, b->yMid + SQUARE_SIDE / 2);
glVertex2i(b->xMid + SQUARE_SIDE / 2, b->yMid + SQUARE_SIDE / 2);
glVertex2i(b->xMid + SQUARE_SIDE / 2, b->yMid - SQUARE_SIDE / 2);
glEnd();
The above code calculates the middle of the button then draws a square around it. For production code you'd of course check that you didn't overlap the edges of the button, and use floats to deal with rounding issues, etc.