[vtkusers] Please Help

[Mayuresh Dalal]

Hi Lisa,

Thank you for your response. Things are a little clearer now because of your 
input about harware renderering.

I have gone over all the vtkOpenGL* .cxx files, and especially 
vtkOpenGLRenderWindow.cxx . But I still cannot see the snippet of code where 
z-values of pixels are compared. I have been looking for the <= function, 
like you said, but could not find it. Can you please be a little more 
specific?

Thank You.


>From: Lisa Avila <lisa.avila at kitware.com>
>To: "Mayuresh Dalal" <mayur_vtk at hotmail.com>, vtkusers at vtk.org
>Subject: Re: [vtkusers] Please Help
>Date: Sun, 01 Feb 2004 01:48:25 -0500
>
>Hello,
>
>VTK does use OpenGL (or Mesa) for the hardware rendering. You can find all 
>the code in the Rendering directory in the vtkOpenGL* files. The hardware z 
>buffer is used and you can see in vtkOpenGLRenderWindow.cxx that a less 
>than or equal function is used (a pixel will be overwritten if the new z 
>value is less than or equal to the one currently stored in the z buffer).
>
>What you are trying to do does not sound straight-forward. The frame buffer 
>only stores one result per pixel - you want more than one. You could 
>consult the z buffer after every polygon is drawn and save the results 
>(adding any changes into your own modified z buffer that can store multiple 
>results per pixel) but that would be very very slow. There are some 
>specialized OpenGL extensions supported by some graphics cards that can 
>help you do this - but again this is not a simple change to the VTK code 
>base.
>
>Lisa
>
>
>
>
>
>
>At 12:52 AM 2/1/2004, Mayuresh Dalal wrote:
>>Hi People,
>>
>>Please help me......Can someone please tell me which is the method where 
>>Z-buffering is actually
>>performed for each pixel?
>>
>>I need to modify the way z buffering is performed and also the way Z 
>>buffer is used for displaying objects/props
>>
>>Does vtk rely on OpenGL z-buffering or does it have its own explicit 
>>z-buffering ?   If, can someone point out the C++ method where z-buffering 
>>is performed in VTK ?
>>
>>Ok, in the terminology that I am used to(might differ with some of you), 
>>frame buffer is the array in which all pixel info( R,G,B,A,Z) is stored 
>>and is used to plot indivisual pixels on the screen.
>>
>>I want to render 2 objects. Areas of object 2 are occluded behind object 
>>1.I want to preserve the data corresponding to the OCCLUDED areas, which 
>>are traditionally eliminated / overwritten in the frame buffer. I want to 
>>preserve the data so that I can use it when looking at the same 2 objects
>>from a different camera position.
>>
>>how should I go about backing up that info before it is lost/overwritten 
>>in the framebuffer?
>>
>>I need to stop the removal of FRONT FACING , but HIDDEN surfaces.
>>
>>Is there a way of doing this?
>>
>>Please help me.
>>
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