similarity of binary images

There are two results (binary vessel images) obtained from two different methods.

They appear very close (almost the same). So, I expect that their similarity index with Dice, which is calculate by the equation given in http://www.mathworks.com/matlabcentral/answers/68720-dice-similarity-index-between-segmented-image-and-ground-truth

However, their similarity index is very different (0.80 and 0.15)

Why ?

[S C.Carl]

I used other results (binary vessel images, background is black, vessels are white) from two methods.

These two images visually almost the same. So, their dice similarity index must be close to each other. However, the dice similarity index is VERY DIFFERENT (0.55 and 0.15 for these images)

Please, is there anybody who can explain this. Why the index values are different although images seem very similar ?

[Image Analyst]

What is your reference image? If you just have two images, then one is the reference and one is the "test" image, and you get one dice number out. So I guess you have three images? If so, of course there could be a big difference between the images because one might match the reference image much better than the other.

[S C.Carl]

My Dear Image Analyst, I really very happy to see you and need your help. I find the problem but I dont know its solution

This is the first time that I met to the strange case. There is no any problem related to reference image or dice equation. When I examine my matlab codes, I saw that

clear all; im = imread('resultVessel.png'); imshow(im);

the above codes show segmented grayscale liver image instead of binary vessel image. However, when I open the resultVessel.png image by ImageJ, I can see the binary vessel image. I am surprised. How is this possible ?

When I open the resultVessel.png image by Bitmap, I can see the binary vessel image. How can I solve this problem ? Why does the above codes show grayscale liver image ? Pls, help

[Image Analyst]

You didn't answer my questions. So post your code and images.

[S C.Carl]

Thank you ImageAnalyst. I could not get permission to upload images, so I could not share it with you. After opening all binary vessel images with bitmap, they are saved as png again. Then I can used the above matlab codes. the problem has been solved by this way Thanks again for your interest