appcoef2: -

https://www.mathworks.com/help/wavelet/ref/appcoef2.html

OR : if you use dwt2 - https://www.mathworks.com/help/wavelet/ref/dwt2.html

The "Ca" are the approximate coefficients

% MATLAB CODE

% Max_level = maximum number of levels you want to decompose the image in wavelet. for example 2 or 3 or .....

% Transformation_Method = The transformation method which you want to use for example 'haar' wavelet, debuchies wavelet('db1',.... etc)

% Level_of_transform = on how much level of transform you want to decompose the image (1, 2, 3.... etc)

I = imread('image.jpg');

[C1,S1] = wavedec2(I,Max_level,Transformation_Method );

data = appcoef2(C1,S1,Transformation_Method , Level_of_transform);

hope it will help you