why threholded image didn't show objects

Hi, I have a binary image (multithreholded; 3472 * 3256), and now I want to get objects larger than 50 pixels in area, but every time the output is nothing (pure white or pure black when i changed 50 to others). Does it mean it regards the whole as one object after threholding? Mine is like grain distributed image. Could anyone help me? Thank you.

[Adam]

It would help if you would show the image, otherwise it is just guesswork.

[Tian Tian]

The attached is the figure outputted by matlab after threholding (I used multithrehold, then converted to binary). I want to get the grain size distribution (e.g.how many objects have area of 50 pixels), shape (how many objects have cicurlarity of some degree), etc. Could you help me？ Many thanks.

[Image Analyst]

See my Image Segmentation Tutorial

Answer 1

[Image Analyst]

Use either bwareafilt() or bwareaopen()

binaryImage = bwareaopen(binaryImage, 50);

or

binaryImage = bwareafilt(binaryImage, [50, inf]);

If you have a multilevel image, like 3 gray levels, then extract just one of the numbers to form a binary image:

binaryImage = grayImage == 1; % or whatever number you want.

[Tian Tian]

Many thanks! Just a quick question, after multithresholding, the image is in black, gray and white (I only want white), then I found binaryImage = grayImage == 3 can help to extract white out, does this make sense?

[Image Analyst]

Yes.