Am 01.06.2005 um 09:59 schrieb Chris Murphy:
Using ProPhoto
RGB leaves you with just one choice of color
rendering: colorimetric. If you are using photographic rendering your
resulting image will be very small compared to any other RGB
colorspace.
Why would that be? I've had few problems with perceptual rendering
with ProPhoto RGB as source, even compared to sRGB.
When using perceptual rendering, the whole source gamut has to fit into
the destination colorspace. Rendering does not look for how much of the
source gamut your image is using. If it would do so you would receive
different results when moving from image to image. So when your source
gamut is huge, it gets compressed a lot when rendering to a small CMYK
output gamut.
This does not happen with colorimetric rendering (either absolute or
relative). In this case everything outside the smaller output
colorspace is just clipped to the edge of it. If your image is small
enough, you don't see any problem. If it is bigger, you end up loosing
details.
I put something online a couple of months ago. It is written in German,
but I believe you will still get the idea by looking at the pictures:
- all pictures are at the same scale, looking from top onto the
colorspace
- the first section shows the relation of the gamuts to each other
(Wide Gamut RGB is close to ProPhoto RGB)
- the second section shows how RGB workspace gamuts may fit into
ISOcoated (with colorimetric rendering). Dimmed colors are unprintable.
- the third sections shows what happens to your data moving from
scanner gamut to color workspace gamut. All but Wide Gamut are rendered
with photographic intend. Since Wide Gamut RGB is big enough it is
better to use colorimetric rendering
- the forth sections shows what happens to your data moving from color
workspace to ISOcoated using photographic rendering. For reference
there is also a direct rendering from the scanner gamut to ISOcoated
using a photographic rendering intend.
But
colorimetric rendering is often not sufficient, because there
are likely out of gamut colors in digital photography (i.e. in blue
tones).
There is a possibility that you'd have sufficiently out of gamut
colors to fluxor the gamut compression built into relative
colorimetric rendering. But in practice, real images, not synthetic
ones, take up a small portion of ProPhoto RGB, so this ends up not
being a problem.
Take a picture of neon advertisement at night or flowers with vivid
colors in bright sunlight with an EOS 20D. Render them to ISOcoated.
Using photographic rendering intend you get decent results. Using
colorimetric rendering intend you end up loosing a lot of details and
the emotions from your pictures are gone.
An other
drawback of ProPhoto RGB is that you will hardly find a
monitor that is able to reproduce the whole colorspace. So you end up
editing data you are not able to see on your monitor and not able to
print out later...
While this is true, in practice I don't find it to be much of a
problem, again because the amount of ProPhoto RGB filled by the image
is comparatively small. There is logic in selecting an editing space
that is going to require clipping of colors for some workflows, but
one has to realize that this is throwing away data even before it's
viewed or edited. I for one don't want to relegate my images to the
lowest common denominator device in my image editing workflow - the
display. Especially as technology improves with wider gamut displays,
and very soon we will be seeing HDR displays as well, which means some
issues with respect to the ICC and how that's going to be handled.
Ideally I'd like to see us using a perversely enormous source space
for editing and archiving our images, and to see color management
improve so that it can better gamut map our images into the limited
spaces we have to live with for viewing, as well as printing. I remain
skeptical of clobbering images from the get go, in particular shadow
detail much of which can be reproduced in print but does not exist in
smaller gamut editing spaces. Really what we have here, with this
religious debate about editing spaces, is a hack because color
management today is unsophisticated enough to do a better job without
us resorting to faking out the CMS so it will do a better job than it
can currently do. ICC v4 brings us a little bit farther, but there
needs to be more widespread adoption, and I'd like to see some
compelling results that take full advantage of v4.
I like to use an other setup. Why should I keep and manage data I am
not able to reproduce. I start at the output process available for a
given job. Do I output to a film printer - LStar-RGB. Do I output to a
CMYK press - LStar-RGB or ECIrgb (depending on customers requirements)
plus Photoshop softproof set to ISOcoated or whatever the printing
service needs. Do I output for internet or PowerPoint presentations -
sRGB. My archive: camera raw, settings, delivered data to customer.
This covers 98% of my needs.