OpenGL - GlVertex relative/absolute position

Question

Imagen I have a list of 2D points (x,y) that describe a 2D terrain in my simple game. I have then glVertex() to draw all those points in GL_POINTS mode.

Then I have a Ball that also has it's (x,y) coordinates.

I want the ball to have a definite size in relation to everything else (such as the terrain). How should I set the values of the (x,y) coordinates to draw everything the size I want it? Having a 600x400 screen.

I am troubled also because glVertex2f(1,1) will draw a primitive point on the upper right corner. So 1 represents to go 100% to the right or top. But the screen is 600x400 so I can't have dimensions of equal length on x and y axis.

Answer 1

Since 0 is 0% (absolute left/bottom) and 1 is 100% (absolute right/top), you just have to find a point in between that will line up with the pixels.

For example. Say your ball is 20x20 pixels. This means that it is 5% of the screen tall and 3.33% of the screen wide. Therefore, the square surrounding your ball would have the following vertices:

void drawBall()
{
   glVertex2f(ball.x - (20/600)/2, ball.y - (20/400)/2);
   glVertex2f(ball.x - (20/600)/2, ball.y + (20/400)/2);
   glVertex2f(ball.x + (20/600)/2, ball.y + (20/400)/2);
   glVertex2f(ball.x + (20/600)/2, ball.y - (20/400)/2);
}

See how I'm dividing the width of the ball by the width of the screen to get a floating point value that works with glVertex2f? Also, ball.x and ball.y should be a floating point value between 0 and 1.

I divide these numbers by 2 because I'm assuming that (ball.x, ball.y) is the coordinate of the center of the ball, so half of the addition goes on either side of the center.

Answer 2

You can write your own function that draws the vertices and that takes pixels in arguments:

#define WINDOW_WIDTH 600
#define WINDOW_HEIGHT 400
void glVertex_pixels(const double x,const double y){
    glVertex2d(x * 2.0 / (double)WINDOW_WIDTH - 1.0, 1.0 - y * 2.0 / (double)WINDOW_HEIGHT);
}

You can also use a macro:

#define WINDOW_WIDTH 600
#define WINDOW_HEIGHT 400
#define glVertex_pixels(x,y) glVertex2d((double)(x) * 2.0 / (double)WINDOW_WIDTH - 1.0, 1.0 - (double)(y) * 2.0 / (double)WINDOW_HEIGHT);

No matter which of the above codes you use, the use of this function is simple. For example, the following code draws a vertex 10 pixels from the left side and 20 pixels from the top side:

glVertex_pixels(10,20);