On 11-06-02 9:25 AM, Thibouille wrote:
Mmm wouldn't a proper PS conversion to 8bit cure the problem and
saving later to 8bit Jpeg rather than 16bit image directly exported to
8bit Jpeg ?
That might work best in this case, but it depends entirely on the
quality of that 16-8 bit conversion. It would just have to be tried to see.
I was thinking about this during my doggy walk (she was not amused) and
I realized that the dithering will only work well if different random
noise is added to each of the R, G, and B channels separately. Adding
grain in LR will *not* do that, so while it might help, the noise may
well disrupt the image too much too. Depends on how much you like grain,
I guess. :-)
In Photoshop at least, you can easily open the Channels panel, select
each of the RGB channels in turn and add a small percentage of noise.
Then you could do the JPEG conversion and see what happens.
This problem exists in the audio domain too. In poorly mastered CDs you
can hear "popcorn noise" if you crank the volume during really quiet
passages. A couple of companies have made some good coin marketing 24-16
bit dithering devices (and plugins for digital audio workstations) that
introduce very specific noise to eliminate the audio-level banding that
would otherwise occur.
-bmw
2011/6/2 Bruce Walker<[email protected]>:
On 11-06-02 3:11 AM, Tim Bray wrote:
Check out
http://www.tbray.org/ongoing/When/201x/2011/06/01/-big/RUNE0790.jpg.html
Look at the green bokeh-fied background and observe the obvious lines
that look like elevation lines on a map, let's call them "bars", as
the green brightness drops off. They ain't there in the .dng, and
after the first cut, I specified 100% JPG quality and they're still
obvious.
It gets weird... I took a screen grab of the Lightroom window, in
which none of those bars are visible, and saved to a .png file, using
built-in OS X facilities, the PNG created by Preview. Obvious bars! I
put it online at http://www.tbray.org/tmp/screen-grab.png - I thought
.png was uncompressed!
PNG is not lossy like JPEG, but it is compressed. The compression is not as
efficient as JPEG because it can't play tricks that lossy conversions can.
I'm sure that a silky-smooth jpg of this picture could be created.
But I don't know how.
-Tim
I'm barely seeing your bars; they are not *that* obvious.
Assuming your toolchain is 100% 16-bit (or better) end-to-end, then your
image is suffering from banding by being dithered down to 8-bits at the
output. JPEG is 8-bits RGB, so you are most likely introducing banding
right there.
Also as for your PNG image, there's 16-bit PNG and 8-bit PNG, and you used
8-bit, which will show clear banding same as the JPEG.
screen-grab.png: PNG image, 764 x 727, 8-bit/color RGB, non-interlaced
Even 16-bit images can suffer from very slight banding in really gradual
tonal transitions. The solution to that is to dither the image a bit -- add
some intentional noise. You could use a mask in Photoshop to restrict the
noise addition to just the darker bokeh areas and avoid mucking-up the
bloom.
There's a grain feature in LR (bottom of the Develop section) that you could
play with to add some noise.
And that's a _great_ shot, btw. My only issue with it is I can't see the
stem which makes it look a little "floating in space", but I guess that's
because of the angle of view. You've struck an excellent balance with the
DoF / sharp edges / bokeh.
-bmw
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