function image do't work!

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dear all:
I'm a new comer of octave. Here is my computer spec:
PowerPC G4 1.33MHz 1G RAM
OSX 10.4.11

I use octave 3.0.0 with gnuplot 4.0 & aquaterm1.0.1

My problem is, when I try to use imshow() or image, the output figure is
whole blank, exepct the grid line..
here is an example:

img=zeros([20 20]);
[sizeY sizeX]=size(img);

offset=2;
img((sizeY/2-offset):(sizeY/2+1+offset),
(sizeX/2-offset):(sizeX/2+1+offset))=1;
figure; imshow(img, [min(min(img)) max(max(img))]);


Does anyone have the same problem ?

Please give me some advices, thanks a lot!
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