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

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

Answer 1

[Saurav Chaudhary]

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

[Mihnea Constantin]

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]

I had tried to do hyperchaotic image encryption for the decomposed LL image but it is showing error could you explain why and how to resolve it.