optimize_rft_speed:
Optimize the runtime of the real-valued FFT.
加速效果受图片尺寸影响，可能就是做的类似金字塔补0的操作
“mode”:standard,patient,exhaustive
优化结果受电脑硬件影响
gen_gauss_filter
Generate a Gaussian filter in a frequency domian.(possibly anisotropic)
sigma: standard deviation
output: gauss filter as image in frequency domian.
Sigma1 := 10.0
Sigma2 := 3.0
gen_gauss_filter (GaussFilter1, Sigma1, Sigma1, 0.0, ‘none’, ‘rft’, Width, Height)
gen_gauss_filter (GaussFilter2, Sigma2, Sigma2, 0.0, ‘none’, ‘rft’, Width, Height)
sub_image (GaussFilter1, GaussFilter2, Filter, 1, 0)
只取波动较大的，可以封装一个算子。太大是噪声，图像噪声频域如下：
rft_generic:
derection space-freq;freq-space