[Digikam-users] jpeg compression

[wjarosz at ucsd.edu]

You are computing the uncompressed size of your image incorrectly.
What you have computed is the number of bits needed to store the
image. You most likely want the number of bytes. There is an 8-fold
difference. For an uncompressed 24 bit image, each pixel takes up 3
bytes, therefore:

3264 x 2448 x 3 = 23,970,816 bytes

If digikam reports a compressed size of 3,974,526, then that's a 6x
compression factor, which sounds reasonable.

-w

On 8/21/07, Francesco Scaglioni <fgs at epulse.net> wrote:
> Hi,
>
> > This suggests, that maybe something like 70 was used inside of the camera?
> >
> > From the number of pixels Nx * Ny, the number of bytes needed
> > to store the image (without) compression should follow from
> > Nx * Ny * (8+8+8), i.e. one Byte per R, G, B.
>
> On a resolution of 3264 x 2448 I get jpeg file sizes of between
> 2.2 and 4 MB per image (subject matter depending).
>
> 3264 x 2448 x 24 = 191,766,528
>
> For a particular image of size 3,974,526 (size and
> resolution stated by digikam) the compression facto does not
> make sense to me (in that I don't fully understand ).  A
> factor of 48 cannot be possible : surely.
>
> Any suggestions ?
>
> Regards,
>
> Francesco
>
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