Am 08.11.2013 12:02, schrieb Bram Vaessen:
Also, Do you setup a blendfunction and activate it for the stateset?
No I didn't. The thing is, I made some other shaders for a volume light, and
there I just enabled blending and it worked as I expected. And to clarify what
I mean with that: I thought that normal 'standard' blending involved mixing the
colors something like this:
mix(source.rgb, dest.rgb, dest.a). so in other words, it would use the rgb in
the framebuffer and mix the color of the new pixels based on the alpha of the
new pixel (ignoring the alpha value from the source pixel).
You're mixing up source and destination. Source is the incoming color
(i.e. the color written in the shader), destination the color in the
buffer (i,e, what was already in the target buffer).
Simple blendigng works with two parameters:
First parameter is the source factor, the second the destination factor,
So for instance glBlendFunc(GL_ONE, GL_ZERO) would take the source * 1.0
and the target * 0.0. Effectively overwritting the buffer.
(mix(source.rgb, target, 0.0))
glBlendFunc(GL_ONE, GL_ONE) therefore would give you mix(source,
target, 0.5)
There are more parameters which don't use a simple factor, but the
source and destination colors/alphas to scale contribution of the
written result. The OpenGL specification shows how to use them to get an
appropriate result. So you'll have to dig into this.
Anyways, some combinations are not valid. For instance you cannot scale
your src with GL_SRC_COLOR.
I don't understand what you mean? Can you point to the blendfunction here?
There is no simple way to blend onyl gl_FragData[0] without touching the
other buffers.
I guess, now I looked at the blend functions of opengl better (sorry I'm kinda
new with opengl), that I would like a glBlendFunc(GL_ONE_MINUS_DST_ALPHA,
GL_DST_ALPHA);
Does that make sense? that matched with I said above right?
Somehow. Frankly I have to look up the correct blending function every
time I do something out of the ordinary.
If I'm not mistaken the above equation will effectively write.
vec4(1 - source.a) * source.rgba + vec4(dst.a) * dst.rgba
Another thing to mention is
glBlendFuncSeparate. In OSG its simply the blendfunc with four parameters.
It allows to set different factors for rgb and alpha values. So this
sounds what you're after.
My suggestion here is to start with a simple task to get used to the
different blending modes. I found them counter-intuitive in the
beginning, but actually they are designed pretty smart ;-)
As said before: Blending function is applied to all bound
buffers.
what about glBlendFunci, which enabled different blendings per buffer... or is
that not supported?
I asked this a while ago. There is no function prototype, so it has to
be added to the extensions somehow I guess....
So could try to post your buffer setup and your passes or at least
create a minimalistic example showing your problem?
A little more context on what you are trying to achieve would be helpful too
I will try to post some code tonight if needed, don't have access to it right
now.
For context: I am rendering terrain with water. the surface of the water is
partly transparant. for the part underwater I want to use some form of fog
effect and for the part above water I use some other 3d fog effect. So if the
camera is above water and look at the water it need to do the following for
each pixel:
-start with the terrain color (from rendered terrain)
-apply underwater effect
-apply the semi-transparent water-surface pixel (from rendered water)
-apply above water fog effect
So the thing is: I need both the original rendered terrain pixel and also the
original rendered water-surface pixel.
so that is why I want to render my terrain and my water-surface to two separate
buffers, so that in the 'final' shader (the deffered one or however you call
it) I have access to both the rendered water and the rendered terrain under
that water and calculate the final pixel in a way that I find logical. I
understand that there might be tons of other ways to do this, but as I'm pretty
new I hope to do things first in a way which seems logical to me, before doing
it some other way that is harder to understand for me. But still if you have a
better idea on how to do what I want I'm open for it :)
If I got this correctly, you have your terrain-buffer in one texture and
your water result in another. Then you don't need hw-blending to combine
them. The pass would simply take both inputs and you can mix them freely
and write it to your destination buffer. The destination however cannot
be one of the bound textures. So if you want to reuse one of the buffers
as final one. you'll have to go with blending.
But seriously, this seems a lot to swallow given your just starting to
use OSG/OpenGL. Start off with a simpler example to get the blending
things right ;-)
hth,
cheers
Sebastian
Hope I explained this clear enough!
------------------
Read this topic online here:
http://forum.openscenegraph.org/viewtopic.php?p=57134#57134
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