[Digikam-users] jpeg from DSLR workflow: to 16bit or not

[Leonardo Canducci]

2009/9/25  <stefan at binaervarianz.de>:
>
>>> I know jpeg is 8bit but stretching histograms in 16bit color space
>>> should avoid posterization anyway. read the third tip at the end of
>>> this page:
>>> http://www.cambridgeincolour.com/tutorials/posterization.htm
>>
>> This is wrong. Switching 8 to 16 do not feel missing data. Histogram
>> will be full of holes.
>>
>> To wrap around this problem you need powerfull algorithm to recompose
>> histogram. There is nothing like this in opensource world.
>>
>
> Sorry to just jump into that conversation, but I couldn't help to notice
> that everyone seems to miss the question.
> It's not if the picture itself gains from changing the color depth, but if
> filters applied to it do.
>
> Just a little example: Lets say his original image is that of a
> checkerboard and it's encoded in just 2 bit - black and white.
> Now we change the depth to 8 bit. Of course the image doesnt change. It
> still has only two colors, black and white. All the new possible colors are
> not used.
> No gain whatsoever.
>
> Now we apply a very heavy blur filter to both images. The original 2bit
> image will transform into something resembling a black & white noise map.
> Very ugly.
> But the 8bit image will become a smooth gray picture. So the filter output
> is improved by converting the original image to a higher color resolution.
>
>
> So the answer is: it depends on the filter. If whatever you want to do with
> the picture afterwards needs to create new color values for pixels, you
> gain from the increased color depth.
> But be aware that you need to decrease it again to save the image into .jpg
> again. And you may loose anything gained beforhand depending on the
> algorithm used for that decrease.
>
> Now someone with more knowledge about digikam needs to say to which filters
> this applies.
That is more or less what I was thinking about. I'm mostly interested
in levels and curves filters because that's what I tweak in pictures
usually (ok, sharpness too sometimes).
Thanks!
-- 
Leonardo Canducci