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Discussion Groups | Image Signal Processing | Quality of JPEG compression.
Technical Discussions related to Image Processing (image coding, compression, digital effects, mpeg, etc)
Quality of JPEG compression. - rukmanimohan04 - Mar 28 11:45:00 2005
Hi,
I want to know on what basis a Jpeg compressed image qualified?
ie., How does the images are said to have 100%, 90% and so on...
qualities???
Does it depend on the quantisation values??
waiting for reply,
rukmani,
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Re: Quality of JPEG compression. - Juan de Dios Santander Vela - Mar 31 13:31:00 2005
El 28/03/2005, a las 17:45, rukmanimohan04 escribió:
> Hi,
> I want to know on what basis a Jpeg compressed image qualified?
> ie., How does the images are said to have 100%, 90% and so on...
> qualities???
> Does it depend on the quantisation values??
> waiting for reply,
> rukmani,
Steps for JPEG compression, IIRC:
- Change of space: from RGB -> Lab or similar.
- DCT of L channel in 8x8 pixel blocks
- DCT of a & b channel (16x16 pixel blocks)
This gives you a different representation of your image, more suitable
for compression. We haven't lost any quality...
- Quantization of coefficients (this can be more or less agressive,
depending on quality settings)
- RLE encoding of coefficients
- Arithmetic Coding of the RLE-encoded stream
Even if your quantization is as fine as possible, you can get normally
a 2-4 compression ration due to the Arithmetic Coder.
It's a good idea to get a paper on JPEG ;-)
--
Juan de Dios Santander Vela
Diplomado en CC. Físicas, Ingeniero en Electrónica
Doctorando en Tecnologías Multimedia
Becario Predoctoral del Instituto de Astrofísica de Andalucía
Katharine Hepburn: A veces me pregunto si los hombres y mujeres se
complementan. Quizá deberían vivir puerta con puerta, y hacerse visitas
de vez en cuando.
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