Should I translate first or rotate first? - c

Question

I'm trying to create a simple scene where I can walk around, with the criteria of being able to pan around and walk around with the keys. However, in my draw scene function, when I translate my scene than rotate, the panning around doesn't work properly as the entire scene just rotates around me, causing objects to go through me. When I rotate than translate my scene, I can pan around properly, however, I can move only in a certain direction, so if I pan around to my right 90 degrees, I'll move left instead of going forward. Is there anyway where I can put these 2 effects together?
This is the code that I use to draw my view:
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glPushMatrix();
glTranslated(xposition, 0, zposition); //This is where I translate my views
glRotated(yrot, 0, 1, 0); //
glEnableClientState(GL_NORMAL_ARRAY);
glEnableClientState(GL_COLOR_ARRAY);
glEnableClientState(GL_VERTEX_ARRAY);
glBindBuffer(GL_ARRAY_BUFFER_ARB, quadVBO);
glNormalPointer(GL_FLOAT, 0, (void*)sizeof(sideArray));
glColorPointer(3, GL_FLOAT, 0, (void*)sizeof(sideArray)+sizeof(normals));
glVertexPointer(3, GL_FLOAT, 0, 0);
glDrawArrays(GL_QUADS, 0, sizeof(sideArray)/sizeof(GLfloat)/3);
glPopMatrix();
glFlush();
glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_COLOR_ARRAY);
glDisableClientState(GL_NORMAL_ARRAY);
glBindBufferARB(GL_ARRAY_BUFFER_ARB, 0);
Here are some pictures that illustrate my problem right now:
Rotate then translate:
Pic1
http://dl.dropbox.com/u/2747708/Screen%20Shot%202012-04-03%20at%2010.17.39%20PM.PNG
Pic2
I can imitate the turn of the camera
http://dl.dropbox.com/u/2747708/Screen%20Shot%202012-04-03%20at%2010.17.48%20PM.PNG
Pic3
However, when I walk forward, it only walks in one direction, and not the direction I'm looking at.
http://dl.dropbox.com/u/2747708/Screen%20Shot%202012-04-03%20at%2010.18.30%20PM.PNG
http://dl.dropbox.com/u/2747708/Screen%20Shot%202012-04-03%20at%2010.18.39%20PM.PNG
Translate then Rotate:
Pic1
http://dl.dropbox.com/u/2747708/Screen%20Shot%202012-04-03%20at%2010.19.44%20PM.PNG
Pic2
I can move around freely, walking straight to any direction I'm looking at.
http://dl.dropbox.com/u/2747708/Screen%20Shot%202012-04-03%20at%2010.19.52%20PM.PNG
Pic3
However, when I rotate the scene, the entire thing rotates, which causes objects to clip through me and doesn't "pan" through the view anymore like when I rotate then translate my view.
http://dl.dropbox.com/u/2747708/Screen%20Shot%202012-04-03%20at%2010.20.01%20PM.PNG

Answer 1

Your problem isn't with the order of transformations. You should be rotating then translating.
The problem is that you aren't taking rotation into account when moving. The formula for movement:
movementX = sin(direction);
movementY = cos(direction);
where direction is the number of radians turned clockwise from north, and positive X is east, and positive Y is north.

Answer 2

I'm not sure how to comment on or add to Kendall's answer so I'll just add this as a new answer.
I think you may be using Y for up, so you would use this if direction is in radians:
movementX = sin(direction);
movementZ = cos(direction);
if direction is in degrees you will have to convert it to radians:
radians = degrees*(PI/180);
Just multiply the movement by 1 or -1 based on whether you are moving forwards or backwards:
movementX = sin(direction)*forwardsBackwards;
movementZ = cos(direction)*forwardsBackwards;
If you need strafing as well you can do:
movementX = sin(direction)*forwardsBackwards+sin(direction+1.5707)*sideToSide;
movementZ = cos(direction)*forwardsBackwards+cos(direction+1.5707)*sideToSide;
Where sideToSide is 1 or -1 based on if you are strafing left or right. The 1.5707 is 90 degrees in radians(PI/2), which means no matter what direction you are facing it takes the 90 degree angle to the right of it. You can also add 90 degrees to the degree rotation before converting if you want to.
Multiply the entire thing by your desired movement speed:
movementX = (sin...eToSide)*speed;
movementZ = (cos...eToSide)*speed;
This will however create a straferunning effect where you will move faster if you move in 2 directions at once. If you want to make it so that doesn't happen add this before calculating movement:
if (!forwardsBackwards && !sideToSide )
{
forwardsBackwards *= 0.7071;
sideToSide *= 0.7071;
}
You could also replace .7071 with cos(45) if you need it to be extremely accurate.
Alternatively you could:
float diagonalMod = 1;
if (!forwardsBackwards && !sideToSide)
diagonalMod = 0.7071; // or cos(45)
movementX = (sin...eToSide)*speed*diagonalMod;
movementZ = (cos...eToSide)*speed*diagonalMod;
You will want to rotate first and then translate. The other way, if I am thinking about it right, is basically like moving the point at which your camera will rotate around, the pivot point. This is why you would clip through objects when rotating.
Also, you should perform "camera" rotation and translation immediately following a glLoadIdentity(); as you want everything else in your scene to move and rotate, as that is how an OpenGL "camera" functions. This is how it would be setup:
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glRotated(yrot, 0, 1, 0);
glTranslated(xposition, 0, zposition);
glPushMatrix();
And for future reference in case you expand on it:
glRotatef(Pitch, 1, 0, 0); // Up and down look
glRotatef(Yaw, 0, 1, 0); // Left and right look
glRotatef(Roll, 0, 0, 1); // Like a barrel roll in a jet
glTranslatef(X,Y,Z);