0
101 Aug 04, 2003 at 04:15

hey, can someone explain ot me how todo world oriented billboarding, i read a tutorial on it, done everyhtign it said, but still didnt work… :wtf:

So i was wondering if someone could quicly explain the math of it all, or if oyu have to some sample code…. (explaining the math behind it all is prefereable)

#### 11 Replies

0
101 Aug 04, 2003 at 13:17

uhm.. just from brain, for some ideas..

your billboard has a point, where it rotates around, and a “prefered up vector”..

now, you want the point-to-cam vector, simply to_cam = normalized(cam - midpoint).
next, you want a “right” vector, wich simply is perpendicular to your to_cam and the up vector, so you use a crossproduct.. to_right = normalized(cross(to_cam,up)) (or the other way around.. don’t know from brain).

finally you want to adjust your up vector (but you let the original up vector still exist..), by crossing your current to_right vector with the to_cam vector..

 to_up = cross(to_right,to_cam) (again, or the other way around.. : )

with those three, you can transform all your billboard points of that specific billboard by these 3 vectors (they build a rotational 3x3 matrix.. look up orthogonal bases.. or so).

i hope that helps a bit..

0
101 Aug 04, 2003 at 13:45

I bet that within the next few hours at least one person comes in here and says something like “Hey cool davepermen, why don’t you write an article about it?”… :P

0
101 Aug 04, 2003 at 13:52

because i only write snippets..

now, the question will be “why no snippet?”

answer: because right now i could not get arsed to write one..

0
101 Aug 04, 2003 at 15:46

check out the billboarding tutorial at nate.scuzzy.net. it’s opengl, but the theory and math carries over.

0
101 Aug 04, 2003 at 23:53

ok, well thats umm *checks code* (which i seem have to deleted sience it didnt work) *checks the tutorial he leaarned from* ok, i was doing exactly what you said, except your cross products are around hte wrong way (accordign to the billboarding tutorial on flipcode)

So either my matrix transformations are wrong (which they arent, i checked that) or my computer hates me. :lol: …oh or i’m nto constructing the matrix correctly.

Anyway, i’ll start again and if i have no luck i’ll post here again with big capitol letters saying ‘argh’ (but alot longer ;))

P.S. i’m doing world oriented billboarding, so you use the up vector of the world, or in my case (due to the architecture of my engine) the particle system up vector; not the camera up vector

0
101 Aug 05, 2003 at 04:32

as long as you don’t overwrite your original up vector..

0
101 Aug 05, 2003 at 04:35

possibly you’ll have to transform by the transpose rotation matrix in the end (inverse rotation..)

i’ve set up some code for my raytracing cam yesterday and had to do exactly that..

0
101 Aug 05, 2003 at 06:21

would be helpful btw to see whats wrong.. demo, or pics, or what ever..

0
101 Aug 18, 2003 at 00:08

I have been doing this so far…is there anything wrong with it??
It seems to work fine.

float modelview[16];
glGetFloatv(GL_MODELVIEW_MATRIX ,modelview);

modelview[0] = 1;
modelview[5] = 1;
modelview[10] = 1;

glPushMatrix();
// -> all transformations here
// -> must do scaling here
// -> render  here
glPopMatrix();

0
101 Aug 19, 2003 at 19:08

if it works, why complain?

off-topic: this is my 666th post!

0
101 Aug 19, 2003 at 20:45

it depends on what you do.. it essencially sets the billboarding point, the one you rotate around, to the current 0,0,0 point of your world matrix. thats about it.. if you can life with that, fine