Is it possible to access vertex values of an object created using display lists?

Question

I wanted to learn to develop games so started with tetris. I encountered a problem right in the beginning. My game is 20% ready. There are five types of shapes viz YELL, BOX, LINE, TEE, YEL which are shapes of different kinds. I created five display lists like this

#define YELL 1
#define BOX 2
#define LINE 3
#define TEE 4
#define YEL 5
int bs=20;
void defineshapes()
{
glNewList(YELL,GL_COMPILE);
    glPushMatrix();
    glTranslated(-bs,0,0);
    glColor3fv(color[color1]);
    glBegin(GL_POLYGON);
    glVertex2i(0,0);
    glVertex2i(bs,0);
    glVertex2i(bs,-bs);
    glVertex2i(0,-bs);
    glEnd();
    glTranslated(bs,0,0);
    glColor3fv(color[color2]);
    glBegin(GL_POLYGON);
    glVertex2i(0,0);
    glVertex2i(bs,0);
    glVertex2i(bs,-bs);
    glVertex2i(0,-bs);
    glEnd();
    glTranslated(bs,0,0);
    glColor3fv(color[color3]);
    glBegin(GL_POLYGON);
    glVertex2i(0,0);
    glVertex2i(bs,0);
    glVertex2i(bs,-bs);
    glVertex2i(0,-bs);
    glEnd();
    glTranslated(-2*bs,-bs,0);
    glColor3fv(color[color4]);
    glBegin(GL_POLYGON);
    glVertex2i(0,0);
    glVertex2i(bs,0);
    glVertex2i(bs,-bs);
    glVertex2i(0,-bs);
    glEnd();
    glPopMatrix();
glEndList();
glNewList(BOX,GL_COMPILE);
    glPushMatrix();
      .....

and randomly picking one like this

srand(time(NULL));
color1=rand()%6;
color2=rand()%6;
color3=rand()%6;
color4=rand()%6;
defineshapes();
randshape=rand()%5+1;

and drawing in display function like this

void display()
{
    glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
    glPushMatrix();
    glTranslated(x2,y2,0);
    glCallList(randshape);
    glPopMatrix();
    glFlush();
}

I expected problem in the beginning itself when i used the display lists because i had to access the vertex values(x,y) of the shapes to check if they might overlap with another block below before moving them down. Is there a way of knowing the vertex values of objects drawn using display lists.

I encountered the same problem once before when i needed to create a tyre of a car. I wanted the tyre to be painted with colors so created a mesh by rotating a circle again using display lists. Then i needed to know the values of the intersection points of longitudes and latitudes which i didnt know how to get so i abandoned that task.

A YELL looks like this

What is the easiest method of getting around this problem. I am sure OpenGL has a way of getting this done but i am new to it.

Answer 1

No, there's no way to look at the vertex data inside a display list. Even using more modern technology like VBOs or whatever, while you can look at the vertex data after you set it up, it's not necessarily a good idea to do so (it could cause performance problems if you try to look at the data while the GPU is using it).

In any case, it's not necessary. Just keep your own copy of the data in your own structures and you can look at it whenever you want. In fact, for collision detection for Tetris you don't even really want vertex data; it's much simpler if you just keep track of which grid squares are occupied or not.