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Additive blending. How to do it?

Started by ZadrraS Sep 24, 2005 at 4:00 AM 8 replies 25.9k views

Original Post

Sep 24, 2005 at 4:00 AM

Hello. I would like to make a fire effect using my particle engine. Additive blending seems to be the perfect thing for me, but i have no idea how should i do it. I've tried searching for additive blending tutorials, but found nothing. So, could someone explain how to do it or point me in the right direction to learn about it? I'm using OpenGL btw.

coelurus

Sep 24, 2005 at 4:27 AM

Lesson 8 at NeHe

ZadrraS

Sep 24, 2005 at 11:57 AM

I do know how to make things transparent, but making something transparent isn't all there is to additive blending, right?

Cypher19

Sep 24, 2005 at 12:04 PM

Additive blending is done very much in the same way as alpha blending. I haven't worked in OGL, but in D3D, you set the source blend to one and the dest blend to one* to get additive blending.

*i.e.

mD3DDevice->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_ONE);mD3DDevice->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_ONE);

Kalidor

Sep 24, 2005 at 12:31 PM

As Cypher19 said, all you need to do is switch to a proper blend function. In OpenGL you do this...

glEnable(GL_BLEND);glBlendFunc(GL_ONE, GL_ONE);

This will make the blend equation

C<sub>o</sub> = 1*C<sub>s</sub> + 1*C<sub>d</sub>

where C_o is the output color, C_s is the source color, and C_d is the destination color.

ZadrraS

Sep 24, 2005 at 12:41 PM

Ahh... It's that simple then :)
Rating++ for all of you.
And can i just use glBlendFunc(GL_SRC_ALPHA, GL_ONE); instead of glBlendFunc(GL_ONE, GL_ONE);? I seem to be getting the same results.

Kalidor

Sep 24, 2005 at 1:00 PM

Quote:
Original post by ZadrraS
And can i just use glBlendFunc(GL_SRC_ALPHA, GL_ONE); instead of glBlendFunc(GL_ONE, GL_ONE);? I seem to be getting the same results.

Yep, that is still additive, but the blend equation then becomes

C<sub>o</sub> = A<sub>s</sub>*C<sub>s</sub> + 1*C<sub>d</sub>

so that the source color is first scaled by source alpha, then added to destination color. So if your source alpha equals 1.0, it will be equivalent to glBlendFunc(GL_ONE, GL_ONE).

SimmerD

Sep 24, 2005 at 11:45 PM

There are other additive type blend modes. One of my favorite is

srccolor * one + destcolor * invsrccolor.

This scales down the dest by 1-src, then adds in the src. This keeps things from oversaturating, and can give a foggy feel.

ZadrraS

Sep 25, 2005 at 1:24 AM

Is the invsrccolor - Inversed Source Color? What flag do I need to get it?

superpig

Sep 25, 2005 at 8:46 AM

GL_ONE_MINUS_SRC_COLOR.

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