Matlab - displaying a background image - image

Question

I'm trying to find the median values for the R,G & B channels of each pixel for each 10th image in a set of 100, to find the background image. My values all seem correct but when i try to display the background at the end of my code it's always white, please help
%// list all the files in some folder
folder = '~/V&R/1/';
filelist = dir(folder);
images = zeros(480,640,3,100);
% images = [];
%// the first two in filelist are . and ..
count = 1;
for i=3:size(filelist,1)
if filelist(i).isdir ~= true
fname = filelist(i).name;
%// if file extension is jpg
if strcmp( fname(size(fname,2)-3:size(fname,2)) ,'.jpg' ) == 1
tmp = imread([folder fname]);
images(:,:,:,count) = tmp;
count = count +1;
end
end
end
background = zeros(480,640,3);
for j=1:480
for i=1:640
tmpR = zeros(1,10);
tmpG = zeros(1,10);
tmpB = zeros(1,10);
for k=1:10
tmpR(k) = images(j,i,1,k*10);
tmpG(k) = images(j,i,2,k*10);
tmpB(k) = images(j,i,3,k*10);
end
background(j,i,1) = floor(median(tmpR));
background(j,i,2) = floor(median(tmpG));
background(j,i,3) = floor(median(tmpB));
end
end
imshow(background)
thanks

Answer 1

The first step is to vectorize your code. Instead of the following block of code:
background = zeros(480,640,3);
for j=1:480
for i=1:640
tmpR = zeros(1,10);
tmpG = zeros(1,10);
tmpB = zeros(1,10);
for k=1:10
tmpR(k) = images(j,i,1,k*10);
tmpG(k) = images(j,i,2,k*10);
tmpB(k) = images(j,i,3,k*10);
end
background(j,i,1) = floor(median(tmpR));
background(j,i,2) = floor(median(tmpG));
background(j,i,3) = floor(median(tmpB));
end
end
write:
subimages = images(:, :, :, 1:10:end);
background = median(subimages, 4);
now as said before, use imshow with the [] option to show your image:
imshow(background, []);
if you still see a white image, then it's possible that you are dealing with a matrix of double values that are not between [0, 1]. Images in Matlab are usually of class double or single with values between 0 and 1, or of class uint8 or uint16 with values between 0, 255 or 0, 65535 respectively. If your values are between 0 and 255 but class(subimages) returns double or single, do the following before using imshow():
subimages = uint8(subimages);

Answer 2

Try
imshow(background,[])
When using imshow, MATLAB needs to set a display range. For single or double grayscale images, the default display range is [0 1]. This means that any value larger than 1 will be represented as white. You can fix this by setting your own display range manually, say
imshow(background,[0 100],
or you can let MATLAB calculate a new display range by doing
imshow(background,[])
which is the same as
imshow(background,[min(background(:)) max(background(:))])

Answer 3

You can rewrite your code as:
%# get filenames of all JPG images in some folder
folder = '~/V&R/1/';
filelist = dir( fullfile(folder,'*.jpg') );
filelist = strcat(folder, filesep, {filelist.name});
%# read files, and store as 'double' images in a 4D matrix
images = zeros(480,640,3, numel(filelist), 'double');
for i=1:numel(filelist)
images(:,:,:,i) = im2double( imread(filelist{i}) );
end
%# estimate background using median
subimages = images(:,:,:,1:10:end);
background = median(subimages, 4);
imshow(background)