Find a black pixel's position on multiple images

Hi,

I have an film of an object (black pixels) that move (background in white pixels). From this film i've exported all images, I have a folder with a lot of black and white images.

I'm using a loop with this to "load images in matlab" and it seems to work :

string= [num2str(i) '.jpg'];
I=imread(string);

Then what I would like to do is a loop and for each image, find the y position of the first black pixel of a specific column (i'm trying to observe an axial displacement, from top to bottom). Store the y value for each image and at the end plot the y position vs the time.

But I don't konw how in a loop I could say for each image "on x = 40, find from the top the first black pixel and store this value"?

Any help would be appreciated

Thanks

[khaled abdessalem]

Hi Mael

please I need the conmplet matlab function that you use to plot y position vs the time for 120 images

All the best

Answer 1

[Ameer Hamza]

Outside the for loop initialize a variable Y as

Y = zeros(1, m); % here m is total number of images. I am assuming that you already know this in advance. Otherwise use Y = []

Then under the following line add

string= [num2str(i) '.jpg'];
I=imread(string);
columnNumber = ; %define the column number you want to check
Y(i) = find( I(:, columnNumber) == 0, 1); % find first 0 pixel in specified column
% again here i am assuming that i goes from 1:m in for loop

[mael thevenot]

Hi,

Thanks for the answer! it seems to work, I have just some questions :

• My images are .jpg, how can I display it so that I can find the column I want to check?
• The "find" goes from top to the bottom of the image right? Not the opposite?
• In the "find", the 0 is for black pixels right? White would be 255?
• It seems that I have 354 columns but when I try big numbers in "columnNumber" I have an error, exemple for columnNumber = 350 :

In an assignment  A(:) = B, the number of elements in A and B must be the same.

Error in (line 19)
    Y(i) = find( I(:, columnNumber) == 0, 1);

Sorry for my naive questions, thanks again.

[Ameer Hamza]

1) For displaying image use image() or imshow(). Thes will show the image but it will be difficult to get actual column number like this. You will have to look at matrix I directly.

2) If you want to go from bottom to top use it like this

find( I(:, columnNumber) == 0, 1, 'last');

3) Yes, 0 => zero brightness and 255 => maximum brightness.

[Ameer Hamza]

For point (4), it is happening because that column contains no black pixel, you can use this to avoid this problem

try 
  Y(i) = find( I(:, columnNumber) == 0, 1);
catch
  Y(i) = 0;   % if no black pixel is found then set it to zero
end

[mael thevenot]

Ok it work just fine and I now understand how it works :) Thanks you very much for your help you explined it very nicely.

[Ameer Hamza]

You are welcome.