Crop matrix based on entries

2021 5 29
1 个回答
回答已采纳
更新时间：2021 5 30
10 次查看（30 天）

0 个投票

I am working with matrix that contains only zeros and ones. I am trying to find a way to crop the matrix to include only the parts of the image where there are ones.
[0,0,0,0,0,0,0,0,0,0;
0,0,0,0,0,0,0,0,0,0;
0,0,0,0,0,0,0,1,0,0;
0,0,1,0,0,0,0,0,0,0;
0,1,1,1,0,0,0,0,0,0;
0,0,0,1,0,0,0,1,0,0;
0,0,1,1,0,0,1,1,0,0;
0,0,0,1,0,0,1,1,0,0;
0,0,1,0,0,0,0,0,0,0;
0,0,0,0,0,0,0,0,0,0]
This is just an example of the matrix but I am really working with a 10000x10000 array. The ones are largely near the center of that image so I am trying to basically filter out all of the extraneous zeros. Any ideas on how I would accomplish this?

请先登录，再回答此问题。

 采纳的回答

Matt J
Matt J 2021-5-29
编辑：Matt J 2021-5-29
Ran in:

0 个投票

Ran in:
A=[0,0,0,0,0,0,0,0,0,0;
0,0,0,0,0,0,0,0,0,0;
0,0,0,0,0,0,0,1,0,0;
0,0,1,0,0,0,0,0,0,0;
0,1,1,1,0,0,0,0,0,0;
0,0,0,1,0,0,0,1,0,0;
0,0,1,1,0,0,1,1,0,0;
0,0,0,1,0,0,1,1,0,0;
0,0,1,0,0,0,0,0,0,0;
0,0,0,0,0,0,0,0,0,0];
I=find(any(A,2));
J=find(any(A,1));
A=A(min(I):max(I) , min(J):max(J))
A = 7×7
0 0 0 0 0 0 1 0 1 0 0 0 0 0 1 1 1 0 0 0 0 0 0 1 0 0 0 1 0 1 1 0 0 1 1 0 0 1 0 0 1 1 0 1 0 0 0 0 0

3 个评论
显示 1更早的评论 隐藏 1更早的评论

Alec Bledsoe
Alec Bledsoe 2021-5-29
Got it. It took me a second to figure out what was going on there. Thanks for the help.
Image Analyst
Image Analyst 2021-5-29
You could also skip the any() calls:
[r, c] = find(A); % Find all non-zero rows and columns.
ACropped = A(min(r):max(r), min(c):max(c));
Not sure which way is faster - you'd have to try both with tic() and toc() to see.
Matt J
Matt J 2021-5-30
编辑：Matt J 2021-5-30
Ran in:
Ran in:
Using any() is going to be much faster (EDIT: when there is a thin boundary of zeros) because it requires finding only the first 1 in the columns and rows of A.
A=randi([0,1],6000);
A([1:10,end-10:end],:)=0;
A(:,[1:10,end-10:end])=0;
timeTest(A)
Elapsed time is 0.004106 seconds. Elapsed time is 0.694781 seconds.
When the boundary is thick (as seems to be the case for the OP), using any() is slower, but only because we are not given the matrix in sparse form.
A=randi([0,1],6000);
A([1:2500,end-2500:end],:)=0;
A(:,[1:2500,end-2500:end])=0;
timeTest(A)
Elapsed time is 0.526749 seconds. Elapsed time is 0.063143 seconds.
timeTest(sparse(A))
Elapsed time is 0.001937 seconds. Elapsed time is 0.006975 seconds.
function timeTest(A)
tic;
I=find(any(A,2));
J=find(any(A,1));
[a,b,c,d]=deal(min(I), max(I), min(J), max(J));
toc;
tic;
[r, c] = find(A); % Find all non-zero rows and columns.
[a,b,c,d]=deal(min(r), max(r), min(c), max(c));
toc
disp ' '
end

请先登录，再进行评论。

更多回答（0 个）

类别

在 帮助中心 和 File Exchange 中查找有关 Image Processing Toolbox 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by