matlab - Get matrix statistics ignoring internal matrix -

i have image. have sliding window going across image , sliding window has "frame". both window , frame defined rectangles [x y w h]. know window contained within frame (may share edge), can't assume location. need get, example, standard deviation of in frame only.

for sake of visualization, assume have window (alpha characters) , frame (numeric characters). have [x y w h] defining each of them. know window in frame. how stdev of numeric characters?

1 8 3 8 2 0

a b c 5 2 8

c b 3 9 9

a c b 0 5 2

9 6 8 3 4 1

note: in reality piece of image numeric. used alpha characters distinguish window frame.

if know row , column range of window within frame, logical indexing 1 approach. self-contained example:

framesize = [5, 6];     % outer [width, height] of frame windowsize = [3, 3];    % [width, height] of window windowlocation = [2, 1];% [row, column] of top left element of window windowrowrange = windowlocation(1):(windowlocation(1)+windowsize(1)-1); windowcolrange = windowlocation(2):(windowlocation(2)+windowsize(2)-1);  % generate example data: make 'frame' numbers negative frame = unifrnd(0,1,framesize); frame(windowrowrange, windowcolrange) = unifrnd(-1,0,windowsize); disp('frame:'); disp(frame);  % generate logical indexes 'inside window' insidewindow = zeros(framesize); insidewindow(windowrowrange, windowcolrange) = 1;  % take standard deviation of outer frame std(frame(~insidewindow)); 

outputs:

frame:     0.9961    0.0046    0.3998    0.1818    0.5797    0.3510    -0.8767   -0.5827   -0.0552    0.2638    0.5499    0.5132    -0.8161   -0.9503   -0.5091    0.1455    0.1450    0.4018    -0.7600   -0.0973   -0.5107    0.1361    0.8530    0.0760     0.9619    0.0844    0.9106    0.8693    0.6221    0.2399  std:     0.3234 

the actual numbers different, have not seeded random number generation.