how to detect only object without background?

[Guillaume]

You'll get more chance of getting help if you actually write words to explain what you want.

As it is,we just have a question title with no definition of what's an object (the foot? the white dots?) or what's background (the black bit).

[wongsathorn pinwihok]

the object is the foot and the background is the black bits.

Answer 1

[Image Analyst]

Find out where all color channels are darker than some threshold, then and the masks and take the largest blob

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
% Threshold to create mask for each channel.
threshold = 20; % Whatever...
mask = (redChannel < threshold) & (greenChannel < threshold) & (blueChannel < threshold);
backgroundMask = bwareafilt(mask, 1); % Take largest blob
footMask = ~backgroundMask; % The inverse of the background mask.

[wongsathorn pinwihok]

I = imread('test.jpg');

rotI = imrotate(I,0,'crop');

% Extract the individual red, green, and blue color channels.

redChannel = rgbImage(:, :, 1);

greenChannel = rgbImage(:, :, 2);

blueChannel = rgbImage(:, :, 3);

% Threshold to create mask for each channel.

threshold = 20; % Whatever...

mask = (redChannel < threshold) & (greenChannel < threshold) & (blueChannel < threshold);

backgroundMask = bwareafilt(mask, 1); % Take largest blob

footMask = ~backgroundMask; % The inverse of the background mask.

imshow(rotI);

Am I right?