MATLAB blockproc函数中的'BorderSize'和'TrimBorder' [英] 'BorderSize' and 'TrimBorder' in MATLAB blockproc function

问题描述

注意网格线？在这种特殊情况下，您只需在完整图像上调用imfilter即可。但是blockproc允许您处理大于物理内存的图像。所以对于这个讨论，想象一下我是一个巨大的tiff文件。

对于这个工作流程 - 如果您只是使用BorderSize在每个20x20块周围包含3个像素边框和 不修剪输出边框：

  h = fspecial（'gaussian' ）; 
 im = imread（'peppers.png'）; 
 B1 = blockproc（im，[20 20]，@（bs）imfilter（bs.data，h），'BorderSize'，[3 3]，'TrimBorder'，false）; 
 imshowpair（im，B1，'montage'）; 
  

所以 - 你真的需要修剪边框（默认）

  h = fspecial（'gaussian'） ; 
 im = imread（'peppers.png'）; 
 B1 = blockproc（im，[20 20]，@（bs）imfilter（bs.data，h），'BorderSize'，[3 3]，'TrimBorder'，true）; 
 imshowpair（im，B1，'montage'）; 
  

注意 - 我以IMFILTER为例。对于小图像，可以直接使用IMFITLER。只有大图像才会考虑在BLOCPROC中使用IMFITLER。

Blockproc is a really useful function for "gridding" an image in MATLAB. It's pretty well documented, and it even comes complete with a tutorial page. However, when you want some sort of overlap between blocks, things get trickier. The Mathworks forums have some explanations, including this one and this one, and there's an attempt at an explanation here (Question #1), but nobody really explains anywhere why certain flags need to be set with other ones. Can someone please explain what the purpose of the 'BorderSize' parameter is? It seems that when 'Trim Borders' is set to false, the 'BorderSize' parameter does exactly what the documentation says (and what you'd expect):

'BorderSize': A two-element vector, [V H], specifying the amount of border pixels to add to each block. The function adds V rows above and below each block and H columns left and right of each block. The size of each resulting block will be: [M + 2*V, N + 2*H]

By default, the function automatically removes the border from the result of fun. See the 'TrimBorder' parameter for more information. The function pads blocks with borders extending beyond the image edges with zeros.

But when you read the 'TrimBorder' details, it doesn't clear up much:

'TrimBorder': A logical scalar. When set to true, the blockproc function trims off border pixels from the output of the user function, fun. The function removes V rows from the top and bottom of the output of fun, and H columns from the left and right edges. The 'BorderSize' parameter defines V and H. The default is true, meaning that the blockproc function automatically removes borders from the fun output.

Why would I want to include a 'BorderSize' (i.e. overlap tiles) but not apply it to the output? Is this just a poorly explained flag: 'TrimBorder' must be off in order to use 'BorderSize', or is there something bigger I'm missing? I guess the gist of my confusion is: when would I want 'TrimBorder' set to false?

Examples:

% Non-overlapping
A = ones(10);
B = blockproc(A, [2,2], @(x)sum(sum(x.data)));
% B = 
% [ 4 4 4 4 4 ]
% [ 4 4 4 4 4 ]
% [ 4 4 4 4 4 ]
% [ 4 4 4 4 4 ]
% [ 4 4 4 4 4 ]

% GOOD Overlapping--one-pixel border
B = blockproc(A, [2,2], @(x)sum(sum(x.data)), 'BorderSize', [1,1], 'TrimBorder', false);
% B =
% [ 9  12 12 12 9  ]
% [ 12 16 16 16 12 ]
% [ 12 16 16 16 12 ]
% [ 12 16 16 16 12 ]
% [ 9  12 12 12 9  ]

% BAD Overlapping--one-pixel border
B = blockproc(A, [2,2], @(x)sum(sum(x.data)), 'BorderSize', [1,1]);
% B = []

解决方案

Why would I want to include a 'BorderSize' (i.e. overlap tiles) but not apply it to the output?

Consider all the workflows where you want to apply a function fun on each block of size MxN in an image, but for the result to be valid you actually need the border pixels around the MxN block. (filtering, morphology, any function where a single output pixel value depends on a mxn surrounding neighborhood). i.e you need (M+m, N+n) block of input to compute one MxN output block.

Simple (aka madeup) example:

h = fspecial('gaussian', 3);
im = imread('peppers.png');
B1 = blockproc(im, [20 20], @(bs)imfilter(bs.data,h));
imshowpair(im, B1,'montage');

Notice the grid lines? In this particular case, you would just call imfilter on the full image. But blockproc lets you work with images which are larger than your physical memory. So for this discussion, imagine im is a huge tiff file.

For this workflow - if you just used BorderSize to include the 3 pixels border around each 20x20 block and did not trim the output border:

h = fspecial('gaussian');
im = imread('peppers.png');
B1 = blockproc(im, [20 20], @(bs)imfilter(bs.data,h), 'BorderSize', [3 3], 'TrimBorder', false);
imshowpair(im, B1,'montage');

So - you really need to trim the border (the default)

h = fspecial('gaussian');
im = imread('peppers.png');
B1 = blockproc(im, [20 20], @(bs)imfilter(bs.data,h), 'BorderSize', [3 3], 'TrimBorder', true);
imshowpair(im, B1,'montage');

Note - I used IMFILTER as an example. For small images, one would use IMFITLER directly. Only for images that are large would one consider using IMFITLER in BLOCPROC.

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