在Matlab中操作没有for循环的矩阵

Question

I'm working in Matlab. I have a colored image in a matrix called im1. I need to make all black pixels as white without altering other pixels. How can I do this without for loop? This code takes a good 10s to execute on a large image.

for i=1:h
for j=1:w
    if im1(i,j,:)==0
        im1(i,j,:)=255;
    end
end
end

Answer 1

I can think of two approaches using logical indexing.

Approach 1 -

im1(repmat(all(im1==0,3),[1 1 size(im1,3)]))=255

Approach 2 -

im1(bsxfun(@and,all(im1==0,3),im1==0))=255

It would be interesting to see which one is more efficient.

Answer 2

try this

[height, width, dim_3] = size(im1);
[row col] = find(im1(:,:,1) == 0 & im1(:,:,2) == 0 & im1(:,:,3) == 0);
im1(sub2ind([height, width, dim_3],repmat(row,dim_3,1),repmat(col,dim_3,1),repmat((1:dim_3)',numel(row),1))) = 255;