Why does the image after subtracting the original image from average image displays a black screen

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karishma koshy on 14 Jul 2019
Commented: KALYAN ACHARJYA on 14 Jul 2019
Why does the image after subtracting the original image from average image displays a black screen?

KALYAN ACHARJYA on 14 Jul 2019
Edited: KALYAN ACHARJYA on 14 Jul 2019
There may be two issues-
1. Suppose the image as following-
image=[3 4 5;5 6 7;1 2 0];
If you calulate the average of the image,you will get the almost same image (near values).Therefore original image - average image = Near about zero matrix or lower values, hence difference may show as black image (image having lower pixels values).
2. If the difference is smaller range, Matlab auto set to display, therefore use full scale range
imshow(difference_image,[]);
Better to check the pixels on difference image, you may easily find out the reason.
Still, not clear let me know.

karishma koshy on 14 Jul 2019
Thank you so much. I was able to get the image.
Image Analyst on 14 Jul 2019
Unless your image1 was always brighter than every pixel in image 2, then you'll still have the problem I described below in my answer, and that is, negative values will be clipped. So simply using [] and not casting to double will ONLY show you positive differences, not negative differences. That's why it's safer to always cast to double when you're doing a subtraction, like I showed you in my answer. But if your first image is always brighter, or you only care about the pixels that are brighter, then doing a subtraction without casting to double would be OK (though it's not a good practice in general).
KALYAN ACHARJYA on 14 Jul 2019
@Image Analyst yes sir, thanks

Image Analyst on 14 Jul 2019
Some or all of your values may be small (too dark to see) or would be negative (and so they will be clipped to zero if you have a uint8 image). Cast to double before subtracting, so negative numbers will be allowed (not clipped), then use [] in imshow() so that negative numbers will show up:
diffImage = double(image1) - double(image2);
imshow(diffImage, []);