urgent exam help pls!.what is the current matrix?

OpenGL OpenGL: Basic Coding
1

in opengl i know that there is a projection matrix and a modelview matrix.but then what is the current matrix?
in the code below,glLoadIdentity() is used to load identity matrix into what matrix?why is it needed?

void display()
{
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
tetrahedron(n);
glFlush();
}

without the glLoadIdentity() function the object i draw gets smaller and smaller each time i minimize and maximize.

2

The loadidentity call is similar to clearing a variable. Its is setting the current modelview matrix to identity.

Explanation:
Suppose you have a program which does this in the display function;


int x=0;
int dx=1;
void display(){
   x+= dx; //similar to translate call
}

In the above code, the x variable will increment each time and then the object will move in the x direction 1 units. What we wanted was to shift the object to pos x=1; So the solution we do is this,


void display(){
   x=0; //similar to loadidentity
   x+= dx; //similar to translate call
}

NOw x will have a value of 1 and it will remain 1. I hope this helps u in understanding what is going on.

3

Which is the “current matrix”? The first line of this will tell you the answer: http://www.opengl.org/sdk/docs/man/xhtml/glMatrixMode.xml

The last line of the “description” section also contains more useful info.

Shouldn’t you already know this if you’re getting to exam time?