convert images to binary with different gray level backgroung

Hi all !

I have the following code:

clc;
OriginalImage = imread('te3.jpg');
subplot(2,2,1);
imshow(OriginalImage);
g = rgb2gray(OriginalImage);
subplot(2,2,2);
imshow(g);
BW = imbinarize(g);
subplot(2,2,3);
imshow(BW);

% level = multithresh(g);
% I = imquantize(g,level);
% subplot(2,2,4);
% imshow(I);

I used two images, see the RESULT below:

the second image:

..

I wonder why this worked in images with dark gray background and didn't work with light gray background !

Can you please explain to me why ! And how can I make it works in the second image ..

Thank you !

Answer 1

[Image Analyst]

It may not know what you consider to be the background. For your computer graphics images, you can just take the upper left pixel and then threshold.

backgroundGrayLevel = OriginalImage (1,1);
brightStuff = OriginalImage > backgroundGrayLevel;
darkStuff = OriginalImage < backgroundGrayLevel;
nonBackgroundStuff = OriginalImage ~= backgroundGrayLevel;