CUDA OpenGL interop, resource mapping messes up buffer

Question

I recently started coding CUDA OpenGL interoperability. Most of the time I've been developing on a Linux environment. Trying to transfer my code to my MacBook Pro running OS X I came across the following problem:

Whenever I map a vertex buffer object from OpenGL as a resource to CUDA it messes up the data stored in the respective buffer.

This is my code:

// OpenGL and window handler includes
#include <GL/glew.h>
#include <SDL/SDL.h>
#include <OpenGL/GL.h>

// OpenGL/CUDA interop
#include <cuda_runtime.h>
#include <cuda_gl_interop.h>

// C libraries
#include <iostream>

int main(int argc, char* argv[])
{
    GLuint vbo;
    struct cudaGraphicsResource *cuda_vbo_resource;

    // create OpenGL context with SDL
    SDL_Init(SDL_INIT_VIDEO);
    SDL_GL_SetAttribute( SDL_GL_RED_SIZE, 5 );
    SDL_GL_SetAttribute( SDL_GL_GREEN_SIZE, 5 );
    SDL_GL_SetAttribute( SDL_GL_BLUE_SIZE, 5 );
    SDL_GL_SetAttribute( SDL_GL_DEPTH_SIZE, 16 );
    SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );
    SDL_SetVideoMode( 400, 200, 16, SDL_OPENGL);

    // initialize extension wrangler
    glewInit();

    // set cuda/GL device
    cudaGLSetGLDevice(0);

    // generate buffer
    glGenBuffers(1, &vbo);

    // fill buffer with data (1, 2, 3)
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    int inData[] = {1,2,3};
    glBufferData(GL_ARRAY_BUFFER, 3*sizeof(int), inData, GL_DYNAMIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    // Register buffer with cuda
    cudaGraphicsGLRegisterBuffer(&cuda_vbo_resource, vbo,             
                                        cudaGraphicsMapFlagsWriteDiscard);

    // let's have a look at the buffer
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    int* mappedBuffer = (int *) glMapBuffer(GL_ARRAY_BUFFER,GL_READ_ONLY);
    printf("\tbefore mapping: %d, %d, %d\n",mappedBuffer[0], mappedBuffer[1], 
            mappedBuffer[2]);
    glUnmapBuffer(GL_ARRAY_BUFFER);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    // map and unmap the cuda resource
    if (cudaGraphicsMapResources(1, &cuda_vbo_resource, 0) != cudaSuccess)
        printf("Resource mapping failed...\n");
    if (cudaGraphicsUnmapResources(1, &cuda_vbo_resource, 0) != cudaSuccess)
        printf("Resource unmapping failed...\n");

    // let's have a look at the buffer again
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    mappedBuffer = (int *) glMapBuffer(GL_ARRAY_BUFFER,GL_READ_ONLY);
    printf("\tafter mapping: %d, %d, %d\n",mappedBuffer[0], mappedBuffer[1], 
            mappedBuffer[2]);
    glUnmapBuffer(GL_ARRAY_BUFFER);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    // unregister the cuda resource
    cudaGraphicsUnregisterResource(cuda_vbo_resource);

    // delete the buffer
    glBindBuffer(1, vbo);
    glDeleteBuffers(1, &vbo);

    return 0;
}

After compiling with:

g++ -o resourceMapping resourceMapping.cpp `sdl-config --cflags --libs` -lglew 
         -framework OpenGL -I /usr/local/cuda/include  -L /usr/local/cuda/lib -lcudart

I get

before mapping: 1, 2, 3
after mapping:  0, 33554432, 32

Does anyone know why the mapping changes the buffer data? It's especially confusing to me since this doesn't happen on the Linux machine.

Thanks for any comments or hints!

Roger Allen Roger Allen · Accepted Answer · 2013-04-20T23:05:17

I notice you register the vbo with cudaGraphicsMapFlagsWriteDiscard.

From here that "Specifies that CUDA will not read from this resource and will write over the entire contents of the resource, so none of the data previously stored in the resource will be preserved."

Perhaps try cudaGraphicsMapFlagsNone or cudaGraphicsMapFlagsReadOnly?

CUDA OpenGL interop, resource mapping messes up buffer

1 Answers