Why imnoise function gives pictures with larger size than original image?

I have a dataset and after addding noise to images I write them using imwrite function to store them in my data set, but after looking at the generated pictures I see that their size are 2 or 3 times larger than original image, I thought they should be nearly same size(for example my original image is 322KB but my same image + gaussian noise is 780KB ... Can somone explain why?

Here is my code:

%%Speckle
var_speckle=0.05;
%%Salt & Pepper
d=0.05;
%%Gaussian
m=0;
var_gauss=0.01;
%%%%%%%%%%%%%%%
x=0;
imagefiles = dir('*.png');      
nfiles = length(imagefiles);    % Number of files found
for ii=1:nfiles
    currentfilename = imagefiles(ii).name;
    currentimage = imread(currentfilename);
    %%currentimage = rgb2gray(currentimage);
    %%adjusted = imadjust(currentimage);
    gaussian = imnoise(currentimage,'gaussian',m,var_gauss);
    poisson = imnoise(currentimage,'poisson');
    salt = imnoise(currentimage,'salt & pepper',d);
    speckle = imnoise(currentimage,'speckle',var_speckle);
    imwrite(gaussian,'C:\Users\dell\Desktop\Shabake_Asli\Dataset\ExtendedDataset\Mirror\SleepNoiseAddMirror\'+string(x)+'gaussian.png','WriteMode','append')
    imwrite(poisson,'C:\Users\dell\Desktop\Shabake_Asli\Dataset\ExtendedDataset\Mirror\SleepNoiseAddMirror\'+string(x)+'poisson.png','WriteMode','append')
    imwrite(salt,'C:\Users\dell\Desktop\Shabake_Asli\Dataset\ExtendedDataset\Mirror\SleepNoiseAddMirror\'+string(x)+'salt.png','WriteMode','append')
    imwrite(speckle,'C:\Users\dell\Desktop\Shabake_Asli\Dataset\ExtendedDataset\Mirror\SleepNoiseAddMirror\'+string(x)+'speckle.png','WriteMode','append')
    x=x+1;
end

[Geoff Hayes]

@Seyed Mousavikia - why are you using append in your calls to imwrite? Won't that just appeand data to the same file? And so if you have already created the file on a previous iteration of the loop, you would just be appending more data to it and so increasing its size. Try removing the append which would mean you would just overwrite the file (if it exists) on each iteration of the loop.

[Seyed Mousavikia]

Thank you for your edit Geoff, about append: I want to keep my original pictures and then add new images (images with noise) thats why I use append.. abou the size I mean that one original image + noise is much larger in size than original image without noise...

[Geoff Hayes]

@Seyed Mousavikia try comparing (usng the debugger) the currentImage and the gaussian image. Do both have the same dimension? Do both have the same data type? If so, then both images should be the same size when saved to file.

[Seyed Mousavikia]

Ok I will try Geoff.. yes they have both same size and same data type

Answer 1

[yanqi liu]

imwrite(mat2gray(gaussian),'C:\Users\dell\Desktop\Shabake_Asli\Dataset\ExtendedDataset\Mirror\SleepNoiseAddMirror\'+string(x)+'gaussian.png','WriteMode','append')

imwrite(mat2gray(poisson),'C:\Users\dell\Desktop\Shabake_Asli\Dataset\ExtendedDataset\Mirror\SleepNoiseAddMirror\'+string(x)+'poisson.png','WriteMode','append')

imwrite(mat2gray(salt),'C:\Users\dell\Desktop\Shabake_Asli\Dataset\ExtendedDataset\Mirror\SleepNoiseAddMirror\'+string(x)+'salt.png','WriteMode','append')

imwrite(mat2gray(speckle),'C:\Users\dell\Desktop\Shabake_Asli\Dataset\ExtendedDataset\Mirror\SleepNoiseAddMirror\'+string(x)+'speckle.png','WriteMode','append')

Answer 2

[Walter Roberson]

png file size depends on the choice of filter algorithms. imwrite does not offer any choice and is not necessarily offering the best filters.

https://en.m.wikipedia.org/wiki/Portable_Network_Graphics

Answer 3

[Seyed Mousavikia]

So you mean if I change the format of my images (e.g jpeg format) the imwrite result will be same size as original images?

[Walter Roberson]

No. .jpeg uses a different compression method that loses information. Comparing the file size it can create with the file size of PNG (which does not lose information) is not fair.

You might not be able to use imwrite() to get back the original file size.

I would suggest to you that immediately after you imread() the original file, that you imwrite() it out to a new name as a PNG file. And then later you would compare the size of the written image-with-noise PNG to the size of the PNG you saved. It might not be the same size as the original PNG file, but the two would have been saved with the same compression algorithm so it would be fair to compare them.