OpenCL & volume rendering

-implemented a simple volume renderer by OpenCL, which can load the mouse dataset from the project.
In WinXP system on my laptop (bootcamp, nVidia 9400M),
it takes around 250ms(4fps) to render a skull ( 85x96x134 voxels) and
. 500 ms(2fps) with 169x192x268 voxels.
The screen size is 500x500.

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GPU info:

 CL_DEVICE_NAME:    GeForce 9400M
 CL_DEVICE_VENDOR:    NVIDIA Corporation
 CL_DRIVER_VERSION:    195.62
 CL_DEVICE_TYPE:   CL_DEVICE_TYPE_GPU
 CL_DEVICE_MAX_COMPUTE_UNITS:  2
 CL_DEVICE_MAX_WORK_ITEM_DIMENSIONS: 3
 CL_DEVICE_MAX_WORK_ITEM_SIZES: 512 / 512 / 64
 CL_DEVICE_MAX_WORK_GROUP_SIZE: 512
 CL_DEVICE_MAX_CLOCK_FREQUENCY: 1100 MHz
 CL_DEVICE_ADDRESS_BITS:  32
 CL_DEVICE_MAX_MEM_ALLOC_SIZE:  128 MByte
 CL_DEVICE_GLOBAL_MEM_SIZE:  253 MByte
 CL_DEVICE_ERROR_CORRECTION_SUPPORT: no
 CL_DEVICE_LOCAL_MEM_TYPE:  local
 CL_DEVICE_LOCAL_MEM_SIZE:  16 KByte
 CL_DEVICE_MAX_CONSTANT_BUFFER_SIZE: 64 KByte
 CL_DEVICE_QUEUE_PROPERTIES:  CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE
 CL_DEVICE_QUEUE_PROPERTIES:  CL_QUEUE_PROFILING_ENABLE
 CL_DEVICE_IMAGE_SUPPORT:  1
 CL_DEVICE_MAX_READ_IMAGE_ARGS: 128
 CL_DEVICE_MAX_WRITE_IMAGE_ARGS: 8
 CL_DEVICE_SINGLE_FP_CONFIG:  INF-quietNaNs round-to-nearest round-to-zero round-to-inf fma

 CL_DEVICE_IMAGE    2D_MAX_WIDTH  8192
     2D_MAX_HEIGHT  8192
     3D_MAX_WIDTH  2048
     3D_MAX_HEIGHT  2048
     3D_MAX_DEPTH  2048

 CL_DEVICE_EXTENSIONS:   cl_khr_byte_addressable_store
     cl_khr_gl_sharing
     cl_nv_compiler_options
     cl_nv_device_attribute_query
     cl_khr_global_int32_base_atomics
     cl_khr_global_int32_extended_atomics


 CL_DEVICE_COMPUTE_CAPABILITY_NV: 1.1
 CL_DEVICE_REGISTERS_PER_BLOCK_NV: 8192
 CL_DEVICE_WARP_SIZE_NV:  32
 CL_DEVICE_GPU_OVERLAP_NV:  CL_FALSE
 CL_DEVICE_KERNEL_EXEC_TIMEOUT_NV: CL_TRUE
 CL_DEVICE_INTEGRATED_MEMORY_NV: CL_TRUE
 CL_DEVICE_PREFERRED_VECTOR_WIDTH_ CHAR 1, SHORT 1, INT 1, FLOAT 1, DOUBLE 1