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Billboarding

Started by ATronic Nov 20, 2001 at 10:59 PM 15 replies 900+ views

Original Post

Nov 20, 2001 at 10:59 PM

Hey, I''ve been using the following function for billboarding.

  
void teBillboard()
{
	float temp[16]={1.0f, 0.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f, 1.0f}, m[16];
	glGetFloatv(GL_MODELVIEW_MATRIX, m);
	temp[0]=m[0];
	temp[1]=m[4];
	temp[2]=m[8];
	temp[4]=m[1];
	temp[5]=m[5];
	temp[6]=m[9];
	temp[8]=m[2];
	temp[9]=m[6];
	temp[10]=m[10];
	glMultMatrixf(temp);
}

It works fine asside from one problem. It makes the particles face the camera AND eliminates all effects of rotation. It should make them face the camera, but is there a way to do this with the particles being in the resulting rotated position?

XaOs

Nov 21, 2001 at 10:05 AM

I think you are missing the whole point with billboarding here!
You use billboarding to make the particles always face the camera, and that will make it a little hard to rotate them!

- -- ---[XaOs]--- -- -

[ project fy ]

ATronic

Author

ATronic

Nov 21, 2001 at 4:45 PM

You don''t see what I''m saying. When I rotate the camera, everything moves exept the particles. I WANT them to face the camera, but to have their center position moved. Understand what I''m saying? I just want to know if that is possible.

Alex Broadwin
A-Tronic Software & Design
-----
"if you fail in life, you were destined to fail. If you suceed in life, call me."
"The answer is out there."
"Please help, I''m using Windows!"

Prosper/LOADED

Nov 21, 2001 at 5:31 PM

When you multiply the current matrix by your "temp" matrix, you lose all translation informations.

Try this one instead :

  
float temp[16]={ 1.0f, 0.0f, 0.0f, 1.0f, 
                 0.0f, 1.0f, 0.0f, 1.0f, 
                 0.0f, 0.0f, 1.0f, 1.0f, 
                 0.0f, 0.0f, 0.0f, 1.0f}

ATronic

Author

ATronic

Nov 21, 2001 at 5:43 PM

I''m afraid that one just kills everything. When I use your temp it makes the particles all stretch from the center of the screen out to the lower left corner of the screen. I think you messed up somewhere.

Alex Broadwin
A-Tronic Software & Design
-----
"if you fail in life, you were destined to fail. If you suceed in life, call me."
"The answer is out there."
"Please help, I''m using Windows!"

miles vignol

Nov 21, 2001 at 6:21 PM

I have found this works fine for me:

float m[16];

glGetFloatv(GL_MODELVIEW_MATRIX, m);
glPushMatrix();
glLoadIdentity();
glTranslatef(m[12],m[13],m[14]);

... draw particle ...

glPopMatrix();

I suppose you could glLoadMatrix(m) at the end instead of the push/pop but I think the card probably does it faster than sending 16 floats...

ATronic

Author

ATronic

Nov 21, 2001 at 6:27 PM

you do that for EVERY particle? Yikes! Overhead must be INSANE.

Alex Broadwin
A-Tronic Software & Design
-----
"if you fail in life, you were destined to fail. If you suceed in life, call me."
"The answer is out there."
"Please help, I''m using Windows!"

feagle814

Nov 21, 2001 at 8:50 PM

OK what you need to do is NOT invert the 4th column of the matrix. I think how to do that is get the transformation matrix, then set everything to identity except the fourth column. This will reset all rotation and scaling to none (hopefully) giving you a quick billboard.

I read that somewhere on the Internet but I can''t find it now.

miles vignol

Nov 21, 2001 at 8:52 PM

Yeah, it kinda seems that way, eh? Is there a better technique? If you''ve got a bunch of seperate items that all need to point back at the camera, I can''t think any method will get around having to do a bunch of work each particle.

It''s not that bad, really. It''s all internal to the card ''cept for the get matrix call and the translate.

ATronic

Author

ATronic

Nov 21, 2001 at 9:39 PM

My way you just call once, then draw all particles, then pop the matrix. Simple enough. But I think I''m inverting part I should not be.... I''ll figure it out soon enough.

Alex Broadwin
A-Tronic Software & Design
-----
"if you fail in life, you were destined to fail. If you suceed in life, call me."
"The answer is out there."
"Please help, I''m using Windows!"

miles vignol

Nov 22, 2001 at 1:18 AM

How does that work? Do you calculate your particle positions in camera space or something? I''d love to learn a faster technique.

miles vignol

Nov 22, 2001 at 1:20 AM

Oh wait. I see, you''re the one asking the question. Lemme know if you get it to work. What matrix are you grabbing in your glGetFloat call if not the matrix at each particle?

ATronic

Author

ATronic

Nov 22, 2001 at 7:28 AM

Try the function I posted, just to see what I''m saying. Do this.

  
glPushMatrix();
teBillboard();
   for(int p=0; p<particleCount; p++)
   {
      particle[p].Draw();
   }
glPopMatrix();

And it will work. The position of the particle is done by the following

glVertex3f(particle[p].x, particle[p].y, particle[p].z);
.
.
.

Make sense? So my particle system and billboarding technically work, it just has one thing that is bothering me. Try it though.

Alex Broadwin
A-Tronic Software & Design
-----
"if you fail in life, you were destined to fail. If you suceed in life, call me."
"The answer is out there."
"Please help, I''m using Windows!"

miles vignol

Nov 22, 2001 at 1:00 PM

I take it your glTranslate calls happen in your draw routine...

I think I understand... This will only work with Translations and Scales, I think (which was your initial problem). I guess you only need to billboard at the point of the last rotation... I''ll see what speed improvements I get, as my particles for the most part are only translated.

miles vignol

Nov 23, 2001 at 6:44 PM

I call glTranslate per particle, then draw a textured quad. Is there some means to accomplish this using some other method? I know I can certainly draw each particle as a gl primitive (like a point or something) using vertex calls, but I don't see how to do this with geometry at each particle...

or is it actually faster to have the processor create a few thousand quads based on the particle positions?

Edited by - miles vignol on November 23, 2001 7:46:36 PM

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