Re: Quality factor and bits per pixel in JPEG


"fulltime" <PiaoChieh@xxxxxxxxx> wrote in message
news:1174669310.345962.236960@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi all,

I have an grayscale image 512 by 512 pixels = 262144bytes. I need to
encode this raw grayscale image into JPEG n find its PSNR at various
bpp. However, I do not quite get the definition of bits per pixel

Bits per pixel is just the file size (in bits) divided by the number of
pixels
(but I suspect you already knew that).

and its relationship with quality factor.

It depends on the complexity of the original image.
Quality factor just indicates the severity of the quantization.
For a given quality factor the file size will vary enormously,
depending on the original image complexity.

how do i calculate the quality factor in the quantisation matrix that
is needed to achieve a 0.25 bits per pixel and 0.5 bits per pixel
output?

You really just need to do it iteratively.
If you're doing this on many images you could plot a curve
of file size versus quality factor for a variety of images, and use that
to get you close to the final value based on the file size that
results from an initial quality factor. Otherwise, just try it with
various quality factors to get the file size you need.


.

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