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What are LL,LH,HL,HH in DWT?

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Mihnea Constantin
Mihnea Constantin on 2 Nov 2020
Hello everyone, I have a small project where my first step is to decompose an image using DWT. Could anyone help me explain
1:what are LL,LH,HL,HH in DWT?
2: what code do i need to write if I want to do a second level and even third level decompostion to extract the value of LL

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Answers (1)

Saurav Chaudhary
Saurav Chaudhary on 5 Nov 2020
DWT is an algorithm used to reduce dimensionality of an image, feature extraction process. DWT algorithm decomposes the image into 4 sub-band (sub-image) ie,LL,LH,HL,HH.
  • LL is the approximate image of input image it is low frequency subband so it is used for further decomposition process.
  • LH subband extract the horizontal features of original image.
  • HL subband gives vertical features .
  • HH subband gives diagonal features .
You can decompose LL band image further, look at the following MATLAB answer to get an idea.https://www.mathworks.com/matlabcentral/answers/163844-how-to-write-matlab-code-for-third-level-dwt

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Mihnea Constantin
Mihnea Constantin on 7 Nov 2020
Thank you for the clariffication. I found two methods that I could do it, But im not sure which would be better.
Method 1:
I = imread('cameraman.tif');
[LL1,LH1,HL1,HH1]=dwt2(I,'db1');
[LL2,LH2,HL2,HH2]=dwt2(LL1,'db1');
[LL3,LH3,HL3,HH3]=dwt2(LL2,'db1');
LvlDecom_LL1 = LL1
LvlDecom_LL2 = LL2
LvlDecom_LL3 = LL3
Method 2:
I = imread('cameraman.tif');
[LL1,LH1,HL1,HH1] = dwt2(I,'sym4','mode','per')
[LL2,LH2,HL2,HH2] = dwt2(LL1,'sym4','mode','per')
[LL3,LH3,HL3,HH3] = dwt2(LL2,'sym4','mode','per')
LvlDecom_LL1 = LL1
LvlDecom_LL2 = LL2
LvlDecom_LL3 = LL3
I personally I'm not sure if both would be correct or if one is better than another....If you or anyone could clarrify this for me, I would highly appericate it :)

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