"Undefined variable" error even though variable is defined

I'm using part of a code I borrowed from @imageanalyst where I'm trying to run through frames of a video to look at how the intensity changes over time. I'd like to perform a background subtraction with the code below

if frame1 == 1
Background1 = thisFrame1;

else
for t=1:frame1

% Change background slightly at each frame
Background1 = (1-alpha1)* thisFrame1 + alpha1 * Background1;

end
end

I get an error stating:

"Unrecognized function or variable 'Background1'.

Background1 = (1-alpha1)* thisFrame1 + alpha1 * Background1;"

even though I have the function defined. when I run the same line of code with a different file, it works fine, even though they're the same file type. any idea as to why this is happening? thanks!

[Stephen23]

""Undefined variable" error even though variable is defined"

But is it really defined? On the first loop iteration, what value does Background1 have?

Answer 1

[Steven Lord]

I'm guessing that you want to initialize Background1 when t is equal to 1 and to use the formula to update Background1 when t is greater than 1. In that case:

for t=1:frame1
    if t == 1
        Background1 = thisFrame1;
    else
        % Change background slightly at each frame
        Background1 = (1-alpha1)* thisFrame1 + alpha1 * Background1;
    end
end

Except that when t is 2, Background1 will be (1-alpha1)*thisFrame1 + alpha1*thisFrame1 = thisFrame1 (assuming there's no code later in the for loop after the if / else block that changes Background1. So perhaps if you describe in more detail what you want to do we can suggest some modifications to your code to achieve your goal.

Answer 2

[Walter Roberson]

if frame1 == 1
Background1 = thisFrame1;
else

This does not define Background1 if frame1 ~= 1

[LaraS]

I see, how do you recommend I fix this? Would it be more straightforward if I just wrote a line that directly subtracts the background?

[DGM]

It's not clear why the loop which increments through frames is inside the if-else and not the other way around.

[LaraS]

I actually tried correcting that while debugging (after posting this), but that didn't seem to help. Essentially what I'm trying to do is have the code update and adapt the background to each frame while it's running. It's really odd because it was running fine before when I was only running one video file. But I'm trying to update the script to run multiple files but it just throws this error about the undefined function now

[DGM]

Well without knowing what the code or process is, there really isn't much I can suggest.