MATLAB: Subtract average image from a series of images

Image Processing Toolboxsubtract images

I'm having some difficulty subtracting an averaged image from a series of images from which the average was obtained. The images have been obtained from a MP4 video file.
This is what i have done:
Ven23 = VideoReader('MyVideo23.MP4'); %Read in the video file
Ven23frame1 = read(Ven23,1); % Read in the first frame of the video
sumImage23 = im2double(Ven23frame1); %Double the accuracy of the frame
for i=2:30
    Ven23Frames= read(Ven23,i);
    sumImage23 = sumImage23 + im2double(Ven23Frames);
end
avgVen23= imshow(sumImage23/30);
This all seems to work; I get an image which looks like the average. However, whenever I try to subtract this average image from any particular frame from the video I get this error:
   Undefined operator '-' for
  input arguments of type
  'matlab.graphics.primitive.Image'.
Then I tried to save the average image and frames as bmp files, load them back in and subtract them. Doing this just returns a black screen. I also tried doubling the accuracy of the reloaded images to no avail.
I'm obviously doing something wrong and I would be grateful if anyone could point it out to me. Thanks.

Best Answer

In view of the error, I would suspect that you're trying to subtract your mean image from a variable called exactly image. Unfortunately, at the point where you're doing the subtraction, the variable image does not exist, so instead matlab calls the function image which returns an object of type 'matlab.graphics.primitive.Image'. Subtraction is not defined for that (a graphics object), hence the error message. At the same time, you probably have an upside down picture of a boy's head popping up in a figure.
In conclusion:
  1. you have a bug in that you're using a variable you've never defined. Can't help you more, since you didn't show that part of the code.
  2. do not name your variable image
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