功能：对任意类型数组或多维图像进行滤波。
B = imfilter(A,H,option1,option2,...)
或写作g = imfilter(f, w, filtering_mode, boundary_options, size_options)
其中，f为输入图像，w为滤波掩模，g为滤波后图像。filtering_mode用于指定在滤波过程中是使用“相关”还是“卷积”。boundary_options用于处理边界充零问题，边界的大小由滤波器的大小确定。具体参数选项见下表：
| 选项 | 描述 | |
| filtering_mode | ‘corr’ | 通过使用相关来完成，该值为默认。 |
| ‘conv’ | 通过使用卷积来完成 | |
| boundary_options | ‘X’ | 输入图像的边界通过用值X（无引号）来填充扩展 其默认值为0 |
| ‘replicate’ | 图像大小通过复制外边界的值来扩展 | |
| ‘symmetric’ | 图像大小通过镜像反射其边界来扩展 | |
| ‘circular’ | 图像大小通过将图像看成是一个二维周期函数的一个周期来扩展 | |
| size_options | ‘full’ | 输出图像的大小与被扩展图像的大小相同 |
| ‘same’ | 输出图像的大小与输入图像的大小相同。这可通过将滤波掩模的中心点的偏移限制到原图像中包含的点来实现，该值为默认值。 |
举例：
originalRGB = imread('peppers.png');
imshow(originalRGB)
h = fspecial('motion', 50, 45);%创建一个滤波器
filteredRGB = imfilter(originalRGB, h);
figure, imshow(filteredRGB)
Matlab 的fspecial函数用法
fspecial函数用于建立预定义的滤波算子，其语法格式为：
h = fspecial(type)
h = fspecial(type，para)
其中type指定算子的类型，para指定相应的参数；
type的类型有：
1、'average'
averaging filter
为均值滤波，参数为hsize代表模板尺寸，默认值为【3，3】。
H = FSPECIAL('average',HSIZE) returns an averaging filter H of size
HSIZE. HSIZE can be a vector specifying the number of rows and columns in
H or a scalar, in which case H is a square matrix.
The default HSIZE is [3 3].
2、 'disk'
circular averaging filter
为圆形区域均值滤波，参数为radius代表区域半径，默认值为5.
H = FSPECIAL('disk',RADIUS) returns a circular averaging filter
(pillbox) within the square matrix of side 2*RADIUS+1.
The default RADIUS is 5.
3、'gaussian'
Gaussian lowpass filter
为高斯低通滤波，有两个参数，hsize表示模板尺寸，默认值为【3 3】，sigma为滤波器的标准值，单位为像素，默认值为0.5.
H = FSPECIAL('gaussian',HSIZE,SIGMA) returns a rotationally
symmetric Gaussian lowpass filter
of size HSIZE with standard
deviation SIGMA (positive). HSIZE can be a vector specifying the
number of rows and columns in H or a scalar, in which case H is a
square matrix.
The default HSIZE is [3 3], the default SIGMA is 0.5.
4、'laplacian' filter approximating the 2-D Laplacian operator
为拉普拉斯算子，参数alpha用于控制算子形状，取值范围为【0，1】，默认值为0.2.
H = FSPECIAL('laplacian',ALPHA) returns a 3-by-3 filter
approximating the shape of the two-dimensional Laplacian
operator. The parameter ALPHA controls the shape of the
Laplacian and must be in the range 0.0 to 1.0.
The default ALPHA is 0.2.
5、'log'
Laplacian of Gaussian filter
为拉普拉斯高斯算子，有两个参数，hsize表示模板尺寸，默认值为【3 3】，sigma为滤波器的标准差，单位为像素，默认值为0.5.
H = FSPECIAL('log',HSIZE,SIGMA) returns a rotationally symmetric
Laplacian of Gaussian filter of size HSIZE with standard deviation
SIGMA (positive). HSIZE can be a vector specifying the number of rows
and columns in H or a scalar, in which case H is a square matrix.
The default HSIZE is [5 5], the default SIGMA is 0.5.
6、'motion'
motion filter
为运动模糊算子，有两个参数，表示摄像物体逆时针方向以theta角度运动了len个像素，len的默认值为9，theta的默认值为0；
H = FSPECIAL('motion',LEN,THETA) returns a filter to approximate, once
convolved with an image, the linear motion of a camera by LEN pixels,
with an angle of THETA degrees in a counter-clockwise direction. The
filter becomes a vector for horizontal and vertical motions.
The
default LEN is 9, the default THETA is 0, which corresponds to a
horizontal motion of 9 pixels.
7、'prewitt'
Prewitt horizontal edge-emphasizing filter
用于边缘增强，大小为【3 3】，无参数
H = FSPECIAL('prewitt') returns 3-by-3 filter that emphasizes
horizontal edges by approximating a vertical gradient. If you need to
emphasize vertical edges, transpose the filter H: H'.
[1 1 1;0 0 0;-1 -1 -1].
8、'sobel'
Sobel horizontal edge-emphasizing filter
用于边缘提取，无参数
H = FSPECIAL('sobel') returns 3-by-3 filter that emphasizes
horizontal edges utilizing the smoothing effect by approximating a
vertical gradient. If you need to emphasize vertical edges, transpose
the filter H: H'.
[1 2 1;0 0 0;-1 -2 -1].
9、'unsharp'
unsharp contrast enhancement filter
为对比度增强滤波器。参数alpha用于控制滤波器的形状，范围为【0，1】，默认值为0.2.
H = FSPECIAL('unsharp',ALPHA) returns a 3-by-3 unsharp contrast
enhancement filter. FSPECIAL creates the unsharp filter from the
negative of the Laplacian filter with parameter ALPHA. ALPHA controls
the shape of the Laplacian and must be in the range 0.0 to 1.0.
The default ALPHA is 0.2