## onionman

### Brandon joined Sep 30, 2010

May 23, 2013
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Desert Combat Level

18

Game Engineer at Filament Games

Billboarding with OpenGL

0 comments by onionman on Dec 12th, 2011

Standard Billboards

So billboarding in games development was something that always used to frighten me. I understood you were supposed to make an object always face the screen, but I didn't know an easy way to do that. I mean, we're you supposed to somehow calculate all the rotations needed by glRotatef() to make a picture always face you?

Anyway, eventually I found a solution with some online help that made the whole thing really easy, and I couldn't find it anywhere else, so I just thought I'd share.

Basically, you just take the current rotation matrix and invert it. Pretty simple! So here's some sample code to do that.

cpp code:
GLfloat modelview[16];
GLfloat tempModelView[16];
glGetFloatv( GL_MODELVIEW_MATRIX, modelview );
glGetFloatv( GL_MODELVIEW_MATRIX, tempModelView );

modelview[0] = tempModelView[0];
modelview[1] = tempModelView[4];
modelview[2] = tempModelView[8];
modelview[3] = 0.f;

modelview[4] = tempModelView[1];
modelview[5] = tempModelView[5];
modelview[6] = tempModelView[9];
modelview[7] = 0.f;

modelview[8] = tempModelView[2];
modelview[9] = tempModelView[6];
modelview[10] = tempModelView[10];
modelview[11] = 0.f;

//if you'd like, plug the position of the object into the transformation matrix, here I'm drawing a sun
modelview[12] = sunPosition.position[0];
modelview[13] = sunPosition.position[1];
modelview[14] = sunPosition.position[2];
modelview[15] = 1.f;

glPushMatrix();
glMultMatrixf(modelview);

renderFlare(0.8f,0.8f,0.6f,1.f,5.f); //render the billboard

glPopMatrix();

Semi-Billboards: Lasers

So a friend also asked me how I drew the lasers in our upcoming game Ensign 1. These really aren't anything special, and still need some work. Still, the basic idea is that these objects should be drawn facing the direction they are traveling, but their up vector should be pointing into the line of site of the viewer. Iunno if I explained that right, but anyway, here's the code I used to do it.
Basically you generate the forward, up, and right vectors for this object. And you need to ensure that the up vector is completely orthogonal to the forward vector.

cpp code:
Vector up,forward,right;

forward = lasers[index].velocity;
forward = forward*6.f;

up = lasers[index].position - parentModel->position;
up -= forward * forward.dot(forward,up); //ensure up vector is orthogonal to forward vector
up.normalize();

right = up.cross(forward,up);
right.normalize();
right = right*1.f;

GLfloat modelView[16];

modelView[0] = right[0];
modelView[1] = right[1];
modelView[2] = right[2];
modelView[3] = 0.f;

modelView[4] = up[0];
modelView[5] = up[1];
modelView[6] = up[2];
modelView[7] = 0.f;

modelView[8] = forward[0];
modelView[9] = forward[1];
modelView[10] = forward[2];
modelView[11] = 0.f;

modelView[12] = lasers[index].position[0];
modelView[13] = lasers[index].position[1];
modelView[14] = lasers[index].position[2];
modelView[15] = 1.f;

glPushMatrix();
glMultMatrixf(modelView);

glEnable(GL_BLEND);
lasers[index].Draw(viewerPosition);
glDisable(GL_BLEND);
glPopMatrix();

Some more tricks would might be that, when the up vector here is very small, the laser might appear flat, you might actually purposely add some forward vector to it to give it some depth. I haven't tried this myself yet though.

Anyway, enjoy!

OnlySolus Dec 2 2010, 3:03pm said:

If you want a proper Zombie(ish) game you might want to try out Killing Floor, I was hoping RR to be more of a Survival (scavange/barricade/survive) game then a coop game and I think thats more what they are going for...
Afterall they are making a freeroam open world zombie survival :'D

onionman Jun 7 2011, 7:55pm replied:

Hey just curious, was this zombie game comment meant to be for me? How'd you know I love zombie games!!?

onionman Jun 25 2011, 1:36pm replied:

For zombie games I have project zomboid now :)

cork279 Dec 8 2010, 5:28pm replied:

Yep, that's exactly how were planning RR to be :D
And there'll be a large map!

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