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equally blended problem

Started by prunkdump Feb 23, 2009 at 3:32 PM 5 replies 1.1k views

Original Post

Feb 23, 2009 at 3:32 PM

My problem is very simple. I have a scene composed by multiple textured GL_QUADS and I want to equally blend them. 1)the sequence: glBlendEquation(GL_FUNC_ADD) glBlendFunc(GL_ONE,GL_ZERO); draw(obj_A); glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA); draw(obj_B) doesn't work because obj_B is blended with the background. 2)the sequence: glBlendColor(0.0,0.0,0.0,0.5); glBlendEquationSeparate(GL_FUNC_ADD,GL_MAX); glBlendFuncSeparate(GL_ONE_MINUS_DST_ALPHA,GL_DST_ALPHA,GL_CONSTANT_ALPHA,GL_DST_ALPHA); draw(obj_A); draw(obj_B); may work but is not implemented on my graphic card (need 2.0) Is there exist a simple way to achieve this? Thanks.

dpadam450

Feb 23, 2009 at 3:38 PM

I dont understand what your asking. and if obj_B is rendered on top of obj_A, then how is it blending with the background?

If you want 2 quads on top of each other to be not blending with the background, but blended with each other (.5 of each), then just do

glDisable(GL_BLEND);
Draw obj_A

glEnable( GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glColor4f(1,1,1,.5);
Draw obj_B

which is basically what you have, excpet you didnt setup SRC_ALPHA but again I dont really know what your problem is.

prunkdump

Feb 23, 2009 at 3:48 PM

The obj_B is not on top of obj_A, one part of it is over the background!

I don't want to obj_A and obj_B seems translucent, I want melt them like a single object. ( as Enblend http://enblend.sourceforge.net/ )

dpadam450

Feb 23, 2009 at 8:19 PM

I really cant think of a good way to do that. If using shaders I could bust something out. I really dont know, try GL_ONE, GL_ONE

I dont remember how its done in hardware but im thinking:

1*black + 1*color = color //no background problems

1*old_image_color + 1*new_image_color = blended

only problem is I think the first one works, but the second equation, you have addative blending, rather than mixed blending (.5 of each).

prunkdump

Feb 24, 2009 at 3:44 AM

You have understood my problem!

I need to thread the background differently: no blending over the background, mixed blending over obj_A.

But I don't know if I can do this without generating alpha mask or texture shading programming.

Lord_Evil

Feb 24, 2009 at 6:25 AM

You could try to set the background alpha to 0 and then use

glBlendEquation(GL_FUNC_ADD)
glBlendFunc(GL_ONE,GL_ZERO);
draw(obj_A);
glBlendFunc(GL_ONE_MINUS_DST_ALPHA,GL_DST_ALPHA);
draw(obj_B)

i.e. first have object A fill the front buffer's alpha channel with a value you want (like 0.5) where object A is drawn, 0 elsewhere. Then draw object B and use the alpha values of the front buffer for blending -> fb * 0.5 + B * (1-0.5) where object A was drawn and fb * 0 + B * (1-0) elsewhere.

prunkdump

Feb 24, 2009 at 7:30 AM

Yes!!! it works!!

Doesn't need glBlendFunc(GL_ONE,GL_ZERO), simply:

glBlendEquation(GL_FUNC_ADD);
glBlendFunc(GL_ONE_MINUS_DST_ALPHA,GL_DST_ALPHA);
glColor4f(1.0, 1.0, 1.0, 0.5);
draw(obj_A);
draw(obj_B);
draw(obj_C);
...

Thanks a lot!!

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