OpenGL着色器vs CUDA

我在我的OpenGL程序中使用此FXAA Shader进行抗锯齿。现在，我在CUDA中重新实现了此代码并对其进行了测试。生成的图像是相同的，但是CUDA版本要慢得多。（着色器通过vsync以60 FPS运行，而CUDA降至〜40 FPS）

这是CUDA代码：

__device__ uchar4 readChar(int x, int y){
    return surf2Dread<uchar4>( surfaceRead, (x)*sizeof(uchar4), (y),cudaBoundaryModeClamp);
}

__device__ uchar4 readFloatBilin2(float x, float y){
    int x1 = floor(x);
    int y1 = floor(y);

    uchar4 z11 = readChar(x1,y1);
    uchar4 z12 = readChar(x1,y1+1);
    uchar4 z21 = readChar(x1+1,y1);
    uchar4 z22 = readChar(x1+1,y1+1);

    float u_ratio = x - x1;
    float v_ratio = y - y1;
    float u_opposite = 1 - u_ratio;
    float v_opposite = 1 - v_ratio;
    uchar4 result = (z11   * u_opposite  + z21   * u_ratio) * v_opposite +
                    (z12 * u_opposite  + z22 * u_ratio) * v_ratio;


    return result;
}
__device__ float fluma(const uchar4 &c){
    return c.x*0.299 * (1.0/255) + c.y *0.587 * (1.0/255) + c.z*0.114 * (1.0/255);
}
__global__ void filter_fxaa_opt(TextureData data)
{

    int x = blockIdx.x*blockDim.x + threadIdx.x;
    int y = blockIdx.y*blockDim.y + threadIdx.y;

    if(x >= data.w || y >= data.h)
    {
        return;
    }

    uchar4 out_color;

    const float FXAA_SPAN_MAX = 8.0;
    const float FXAA_REDUCE_MUL = 1.0/8.0;
    const float FXAA_REDUCE_MIN = (1.0/128.0);



    float lumaNW = fluma(readChar(x-1,y-1));

    float lumaNE = fluma(readChar(x+1,y-1));

    float lumaSW = fluma(readChar(x-1,y+1));

     float lumaSE = fluma(readChar(x+1,y+1));

    float lumaM = fluma(readChar(x,y));

    float lumaMin = min(lumaM, min(min(lumaNW, lumaNE), min(lumaSW, lumaSE)));
    float lumaMax = max(lumaM, max(max(lumaNW, lumaNE), max(lumaSW, lumaSE)));

    float2 dir;
    dir.x = -((lumaNW + lumaNE) - (lumaSW + lumaSE));
    dir.y = ((lumaNW + lumaSW) - (lumaNE + lumaSE));

    float dirReduce = max((lumaNW + lumaNE + lumaSW + lumaSE) * (0.25 * FXAA_REDUCE_MUL), FXAA_REDUCE_MIN);

    float rcpDirMin = 1.0/(min(abs(dir.x), abs(dir.y)) + dirReduce);


//    float2 test = dir * rcpDirMin;
    dir = clamp(dir * rcpDirMin,-FXAA_SPAN_MAX,FXAA_SPAN_MAX);


    uchar4 rgbA = (
                readFloatBilin2(x+ dir.x * (1.0/3.0 - 0.5),y+ dir.y * (1.0/3.0 - 0.5))*0.5f+
                readFloatBilin2(x+ dir.x * (2.0/3.0 - 0.5),y+ dir.y * (2.0/3.0 - 0.5))*0.5f);
    uchar4 rgbB = rgbA * (1.0/2.0) +  (
                readFloatBilin2(x+ dir.x * (0.0/3.0 - 0.5),y+ dir.y * (0.0/3.0 - 0.5))*0.25f+
                readFloatBilin2(x+ dir.x * (3.0/3.0 - 0.5),y+ dir.y * (3.0/3.0 - 0.5))*0.25f);
    float lumaB = fluma(rgbB);


    if((lumaB < lumaMin) || (lumaB > lumaMax)){
        out_color=rgbA;
    } else {
        out_color=rgbB;
    }

    surf2Dwrite<uchar4>(out_color, surfaceWrite, x*sizeof(uchar4), y);
}

建立：

//called for the 'src' and 'dst' texture once at the beginning
checked_cuda( cudaGraphicsGLRegisterImage(&res, gl_buffer,gl_target, cudaGraphicsRegisterFlagsSurfaceLoadStore));

//called for the 'src' and 'dst' texture every frame
checked_cuda( cudaGraphicsMapResources(1, &res, 0));
checked_cuda( cudaGraphicsSubResourceGetMappedArray(&array, res, 0,0));

//kernel call every frame
dim3 block_size(8, 8);
dim3 grid_size;
grid_size.x = (src->w) / (block_size.x) ;
grid_size.y = (src->h) / (block_size.y) ;
checked_cuda(cudaBindSurfaceToArray(surfaceRead, (cudaArray *)src->d_data));
checked_cuda(cudaBindSurfaceToArray(surfaceWrite, (cudaArray *)dst->d_data));
filter_fxaa_opt<<<grid_size, block_size>>>(*src);

系统：

Ubuntu 14.04
Opengl version: 4.4.0 NVIDIA 331.113
Renderer version: GeForce GTX 760M/PCIe/SSE2
CUDA 5.5

问题：OpenGL Shader有什么更好的表现，为什么它要快得多？