Make matrix numbers below 'X' equal to zero and help with overlaying images

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Alex Mason 2011-5-4

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https://ww2.mathworks.cn/matlabcentral/answers/6785-make-matrix-numbers-below-x-equal-to-zero-and-help-with-overlaying-images

Hey guys

I am doing some image processing from HD video. I am reading the video in frame by frame, converting each frame to grey and then subtracting a template image from them.

What I am doing then is overlaying the resulting images. Or trying to anyway.

The first thing I need help with, is that really I am only interested in a certain event within the images. The event is an electrical discharge and this event shows up as pure white (255). So I want to perform a matrix operation on each image whereby I cull all the numbers less than say 250 and make them zero (but still keeping the original matrix dimensions).

The second thing is, how do I overlay the images ontop of each other so as to build up a plot showing all the locations of this event at once? (ie so I can check that the event occured evenly across an area). is imadd(X,Y) sufficient?

Thanks for any help offered

Alex