如何使用Matlab制作零和一的对角线分隔符？

Question

这是我想要的结果。 位数分辨率为256 x 256。

// assign default background to white.
img = ones(256, 256);

示例结果：

0 1 1 1
0 0 1 1
0 0 0 1
0 0 0 0

有没有办法可以在 MATLAB 中使用 zeros() 和 ones() 函数来实现这个结果？我应该如何进行循环？

结果是eye() 函数可以做的事情，但它只做对角线。我想要一条分隔零和一的对角线。

Answer 1

您正在寻找triu 函数

img = triu( ones( 256 ), 1 );

Answer 2

如果您关心性能，可以尝试基于bsxfun 的方法-

n = 256; %// resolution of img would be nxn
img = bsxfun(@le,[1:n]',0:n-1);

---

比较 BSXFUN 和 TRIU 的基准 -

num_runs = 50000; %// Number of iterations to run benchmarks
n =  256; %// nxn would be the resolution of image

%// Warm up tic/toc.
for k = 1:50000
    tic(); elapsed = toc();
end

disp(['For n = ' num2str(n) ' :'])
disp('---------------------- With bsxfun')
tic
for iter = 1:num_runs
    out1 = bsxfun(@le,[1:n]',0:n-1); %//'
end
time1 = toc;
disp(['Avg. elapsed time = ' num2str(time1/num_runs) ' sec(s)']),clear out1

disp('---------------------- With triu')
tic
for iter = 1:num_runs
    out2 = triu( true( n ), 1 );
end
time2 = toc;
disp(['Avg. elapsed time = ' num2str(time2/num_runs) ' sec(s)']),clear out2

结果

For n = 256 :
---------------------- With bsxfun
Avg. elapsed time = 0.0001506 sec(s)
---------------------- With triu
Avg. elapsed time = 4.3082e-05 sec(s)

For n = 512 :
---------------------- With bsxfun
Avg. elapsed time = 0.00035545 sec(s)
---------------------- With triu
Avg. elapsed time = 0.00015582 sec(s)
 
For n = 1000 :
---------------------- With bsxfun
Avg. elapsed time = 0.0015711 sec(s)
---------------------- With triu
Avg. elapsed time = 0.0019307 sec(s)

For n = 2000 :
---------------------- With bsxfun
Avg. elapsed time = 0.0058759 sec(s)
---------------------- With triu
Avg. elapsed time = 0.0083544 sec(s)

For n = 3000 :
---------------------- With bsxfun
Avg. elapsed time = 0.01321 sec(s)
---------------------- With triu
Avg. elapsed time = 0.018275 sec(s)

基准测试的结论

对于256x256 大小的问题，triu 可能是首选方法，但对于足够大的数据量，可以查看bsxfun 以获得高达 50% 的性能提升。