matlab对一幅图像，采用半径值为20、80、200的截止频率进行理想低通滤波，观察其结果。要求给出原图像及相应的付利叶频谱图和滤波结果-For an image, using the radius value of the cutoff frequency 20,80,200 ideal low-pass filter to observe the results. Required to give the original image and the corresponding Fouri

二维图像的投影和图像重建，直接傅里叶反变换法，先反投影后滤波，先滤波后反投影，卷积反投影(Two dimensional image projection and image reconstruction, direct Fourier inverse method, first backprojection, after filtering, first filter, back projection, convolution back projection)