[vtkusers] [Try Again] Multiple Volumes with one Renderer

["Johannes Käfer"]

Okay answering my own question.

I had to modify two files in order to get the desired result. 
1) vtkWin32OpenGLRenderWindow.cxx
   There I had to enable the stencil buffer (which was disabled). In the
function void vtkWin32OpenGLRenderWindow::SetupPixelFormat(...) I had to put
"1" instead of "0" into the line with the comment /* no stencil buffer */
(parameter 12).

2) VtkRenderer.cxx 
  There I made modifications to the "int vtkRenderer::UpdateGeometry()"
function. The changes were made before and in the second for loop (rendering
translucent geometry). See the code below.

	// enable some opengl commands, so we achieve the effect of
	// translucent volumes but not beeing able to see a volume lying behind
	// another volume

	glEnable(GL_DEPTH_TEST);
	// the stencil test basically writes to a different buffer than the frame
buffer
	// and based on the values in the stencil buffer we write a pixel to the
frame buffer or we don't
	glEnable(GL_STENCIL_TEST);
	// we only write to the frame/stencil buffer if the value in the stencil
buffer is 0x0 (zero)
	glStencilFunc(GL_NOTEQUAL, 0x1, 0xff);
	// when drawing, replace the value in the stencil buffer
	glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);
	// reset the stencil buffer to all zeros (we can draw everywhere)
	glClearStencil(0x0);

	// needed
	glClear(GL_STENCIL_BUFFER_BIT);

	// loop through props and give them a chance to 
	// render themselves as translucent geometry
  	for ( i = 0; i < this->PropArrayCount; i++ ){
		// we need to do this in here because after the RenderTranslucentGeometry
the alpha test is 
		// not enabled any more
		// we need the alpha test too, so we can "blend out" the black area
surrounding our object
		glEnable(GL_ALPHA_TEST);
		// draw only pixels with a alpha value greater than 0.00001, this should
also work with zero
		// as the background should have zero alpha but it doesn't (maybe float
inaccurancy)
		glAlphaFunc(GL_GREATER, 0.00001);
	
		this->NumberOfPropsRendered +=
this->PropArray[i]->RenderTranslucentGeometry(this);
	}

	// disable the stencil and alpha test
	glDisable(GL_STENCIL_TEST);
	glDisable(GL_ALPHA_TEST);


Hope this helps someone.

Regards
  Johannes







> Hi,
> 
> I'm trying to render two or more Volumes with one Renderer. I don't want
> to
> be able to see a volume lying behind the volume in front. Can someone give
> me a hint?
> 
> More detailed. I'm rendering two volumes in one renderer as said above.
> This
> works quite nice. I'm using the vtkVolumeRayCastMapper with the
> vtkVolumeRayCastCompositeFunction.
> The Volumes I render are quite translucent so when I have two volumes I
> can
> see the volume lying behind the first one. I don't want this to happen. I
> want to stop the raycasting when I enter a new Volume. Is there any way
> this
> can be done/ has already been done? If not were to start modifying vtk in
> order to get it done.
> 
> Thank you for your help.
> 
> best regards
>   Johannes
> Regards
>   Johannes
> 
> 
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