Compression - two question on compression

Author

two question on compression

syntax

2004-05-12, 9:29 pm

i have two simple question

1. i have usued gaussian mask(5x5, 7 x7) in my image . i want to know
what gaussian mask is doing actually? i am getting bad output image .i
have all decimal (ASCII data i.e .pgm gray image) data.

can u explain what actually it is doing on those data ?

2. is wavelet a lossless or lossy compression ? which is one is
correct ? bcoz i have read the subband decomposition LL,LH,HL,HH
.....this is not lossy, i beleive(or is it?)

i am getting references in some text that wavelets are loosy and in
some other text it is referred as lossless . i am

. so which
one is correct ?

Thomas Richter

2004-05-12, 9:29 pm

Hi,

> 1. i have usued gaussian mask(5x5, 7 x7) in my image . i want to know
> what gaussian mask is doing actually? i am getting bad output image .i
> have all decimal (ASCII data i.e .pgm gray image) data.

> can u explain what actually it is doing on those data ?

Basically, for each filter, you are performing

output(x,y) = \sum(i,j = -n to n) filter(i,j) * input(x-i,y-j)

with filter() the corresponding gaussian filter kernel.

> 2. is wavelet a lossless or lossy compression ? which is one is
> correct ? bcoz i have read the subband decomposition LL,LH,HL,HH
> ....this is not lossy, i beleive(or is it?)

Wavelets as a mathematical operation are lossless. However, some
wavelets can only be lifted with floating point values (or have
a kernel without a suitable integer approximation) and thus
are ill-suited for a lossless computer implementation. The 9/7
wavelet is a typical lossy one. The 5/3 filter and the 13/7 filter
are well-suited for lossless. As mathematical operation, all three
are lossless (on a theoretical infinite precision machine).

So long,
Thomas

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