Stencil with CUDA

Hi everyone

Is there some way to process stencil values stored inside the kernel? I mean, I’m rendering some GL_LINE_STRIP and I store the stencil values for the render. I want to process these stencil values in a kernel in order to compute an algorithm of mine. Does anybody knows if CUDA allows to get the values stored in the stencil buffer.

I know that the solution could be glReadPixels and then process them inside the kernel but I would like to avoid CPU->GPU->CPU->GPU.

Thanks everyone for your time

No, there is currently no way to read directly from the frame buffer in CUDA (color or stencil). However, you can do a glReadPixels() from the frame buffer to a pixel buffer object, which can be mapped and read from CUDA. The postProcessGL sample in the SDK shows how to do this. The copy happens in GPU memory, so there is no transfer back to the CPU.

BTW, if you want to read stencil values I would recommend using this extension to avoid format conversions in the driver:
http://www.nvidia.com/dev_content/nvopengl…pth_stencil.txt

I’ve already found this example in this forum and I didn’t understand it at all. But I’ll check it again.

Actually I’m using this extension.

Thank you very much for your help Simon.

I check the example in the SDK postProcessGL to use inside a CUDA kernel a PBO from openGL. I think I understood the code but with my code (which I made some changes) I get an execution error of segmentation fault. I think all is clear but I can’t find the error.

void init()

{

	void *data=malloc(SCREEN_RES_X*SCREEN_RES_Y*sizeof(GLfloat));

	//create the PBO of 1 float per pixel

	glGenBuffers(1,&pboSource);

	glBindBuffer(GL_ARRAY_BUFFER,pboSource);

	glBufferData(GL_ARRAY_BUFFER,sizeof(GLfloat)*SCREEN_RES_X*SCREEN_RES_Y,data,GL_DYNAMIC_DRAW);

	glBindBuffer(GL_ARRAY_BUFFER,0);

	//register the PBO in CUDA

	cutilSafeCall(cudaGLRegisterBufferObject(pboSource));

	//alloc mem to final values

	cutilSafeCall(cudaMalloc((void**)&finalStencilDevice, sizeof(int)*SCREEN_RES_X*SCREEN_RES_Y));

}

void run()

{

	init();

	//render my scene

   render();

callCUDAFunction();

}

void callCUDAFunction()

{

	   // tell cuda we are going to get into these buffers

		cutilSafeCall(cudaGLUnregisterBufferObject(pboSource));

		// activate destination buffer

	glBindBuffer(GL_ARRAY_BUFFER, pboSource);

	   //read

	glReadPixels(0,0,SCREEN_RES_X,SCREEN_RES_Y,GL_STENCIL_INDEX,GL_FLOAT,

NULL);

	 // Done : re-register for Cuda

	cutilSafeCall(cudaGLRegisterBufferObject(pboSource));

	float *stencil;

		//map values from PBO

		cutilSafeCall(cudaGLMapBufferObject((void**)&stencil, pboSource));

	dim3 dimBlock(BLOCK_DIM_X,BLOCK_DIM_Y);

	dim3 dimGrid(SCREEN_RES_X/dimBlock.x,SCREEN_RES_Y/dimBlock.y);

	  

	   //call kernel

	stencilJoin<<<dimGrid,dimBlock>>>(finalStencilDevice,stencil);

	cutilSafeCall(cudaGLUnmapBufferObject(pboSource));

	   // detach from CUDA

		cutilSafeCall(cudaGLUnregisterBufferObject(pboSource));

}

The error is produced in this line glBindBuffer(GL_ARRAY_BUFFER, pboSource).

Does anybody can help me?

Thank you very much