Re: [osg-users] about drawing only in the front buffer

Tue, 22 Feb 2011 07:14:02 -0800

Hi, Gianluca Natale
 
In order to get correct results you cant just go and draw changed object on top of previous frame (there are one exception that is if object in question dont change size and position and is completely opaque). You will get depth buffer issues and parts of old object position can still remain visible. To get it work you need to redraw all region where old object was + draw changed object on top of that.
 
If this exception applies - you can draw each frame to textures (storing color and depth), then in next frame if you need to redraw only some objects, then, considering you know what objects need redraw, you first draw previous frame textures with screen aligned quads to color and depth, and after that you draw objects on top of that with depth func set to equal. With transparent geometry you still need to redraw background as in previous case.
 
So in general case you can do the following:
Draw from previous frame textures with screen aligned quads to color and depth like in previous case.
Get screen coords of regions that needs to be redrawed with transforming changed objects bboxes to screen coords.
Draw quads with depth set to 1.0 and depth test set to always covering regions where changed objects were.
Search you graph for objects that after transforming to screen coords will overlap with that regions.
Draw them as usual along with changed objects.
 
I dont think that you will be able to gain sufficient speedup with that though.
 
Cheers, Sergey.
 
 
 
22.02.2011, 16:47, "Gianluca Natale" <[email protected]>:

Hi there,
I hope someone of you can help me on this issue.
I have a very complicated scene graph, that takes a very long time to be completely redrawn (>200 ms).

But the most frequent use case I have is to redraw only a very small part of the model at a time
(suppose for example that I would need to redraw only the object under the mouse).

So, I could increase performances a lot by simply redrawing that object in the front buffer without clearing the depth buffer,
in order to keep it in the right position WRT the other object of the model. And without swapping front and
back buffers at the end of drawing.

Is this possible?

 

If I’m not wrong, sometimes ago I received an email from you saying that the drawing traversal of OSG

always clean the depth and back buffers, redraws the complete scene graph and then swap the buffers.
This works pretty well when the scene changes dynamically, but in my case where there is a static drawing for

a long time, I would need a way to tell the drawing traversal to draw in the front buffer, and not to perform the swapping
of buffers at the end.

Is there a way to do that?

 

Thanks in advance

Gianluca Natale

 

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