void *mapped_pbo = glMapBufferARB(GL_PIXEL_UNPACK_BUFFER_ARB, GL_WRITE_ONLY);
memset(mapped_pbo, 0, width * height * bytes_per_pixel);
glUnmapBufferARB(GL_PIXEL_UNPACK_BUFFER_ARB);
void *mapped_pbo = glMapBufferARB(GL_PIXEL_UNPACK_BUFFER_ARB, GL_WRITE_ONLY);
memset(mapped_pbo, 0, width * height * bytes_per_pixel);
glUnmapBufferARB(GL_PIXEL_UNPACK_BUFFER_ARB);