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

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Mihnea Constantin 2020-11-2

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https://ww2.mathworks.cn/matlabcentral/answers/634054-what-are-ll-lh-hl-hh-in-dwt

评论： Dominic Mathew 2021-3-17

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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Answer 1

Saurav Chaudhary 2020-11-5

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https://ww2.mathworks.cn/matlabcentral/answers/634054-what-are-ll-lh-hl-hh-in-dwt#answer_534725

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 2020-11-7

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 :)

Dominic Mathew 2021-3-17