[osg-users] transparent object sorting and back face culling

Paul Speed pspeed at progeeks.com
Thu Aug 13 23:58:44 PDT 2009

P.S.: I don't pretend to understand any of this.  Render bins and render 
bin details are one of the more confusing parts of OSG... I generally 
just keep monkeying with them until something sticks.

I will point out that in my understanding, the render bin number and the 
render bin details are not tightly related like one might think.  It's 
not like there's a fixed array of bins somewhere.  As I recall, bin 
number controls rendering order locally to that part of the scene graph. 
  Render bin details define how to sort/render things within that local 
bin.  So if child 1 is render bin 1 it will be rendered before child 2 
at render bin 2.  The parent node might be render bin 25 and it's 
irrelevant.  I think.

Though as soon as I pretend I understand that I've had it bite me.  So I 
could be somewhat wrong somewhere.


Paul Speed wrote:
> Even within the transparent bin you can enforce specific ordering with 
> the bin number.  Lower numbers get rendered before higher numbers.
> I'd have to dig to find any of my own example code but I hoped perhaps 
> the hint would be enough in short time.
> We did this all the time to render double-side transparent geometry. 
> Draw the inner faces in a lower bin number than the outer faces.  Even 
> with different bin numbers you can still depth sort them.
> We had semi-transparent glass boxes with text on the sides.  I think we 
> ultimately had three different bin numbers we used for that associated 
> with the inner, outer, and text geometries.
> -Paul
> Jason Daly wrote:
>> Rabbi Robinson wrote:
>>> Hi,
>>> Is there anyway that I can enforce drawing the objects inside first 
>>> before drawing the outside membrane?
>> Sure, just use setRenderBinDetails() to set a render bin for the 
>> membrane that has a higher number than the render bin used by the 
>> interior objects.  For the other parameter, you can set either 
>> "DefaultBin" for a state-sorted bin, or "DepthSortedBin" for a 
>> depth-sorted bin.
>> interiorStateSet->setRenderBinDetails(0, "DefaultBin");
>> membraneStateSet->setRenderBinDetails(10, "DepthSortedBin");
>> However, this is equivalent to this code:
>> interiorStateSet->setRenderingHint(StateSet::OPAQUE_BIN);
>> membraneStateSet->setRenderingHint(StateSet::TRANSPARENT_BIN);
>> Which is probably close to what you're already doing.  The problem is 
>> that the transparent geometry still has to obey the depth (z-buffer) 
>> test, even if it's drawn last.  You can try disabling the depth test, 
>> but this is likely to cause other problems (if you ever have anything 
>> move in front of the membrane, for example):
>> depth = new Depth();
>> depth->setFunction(Depth::ALWAYS);
>> membraneStateSet->setAttributeAndModes(depth, StateAttribute::ON);
>> This is the main reason why the alpha to coverage method is sometimes 
>> used, even though it's a bit expensive in draw time.
>> --"J"
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