Re: ICC PROFILES (dealing with untagged RGB)
Re: ICC PROFILES (dealing with untagged RGB)
- Subject: Re: ICC PROFILES (dealing with untagged RGB)
- From: "Stephen Marsh" <email@hidden>
- Date: Wed, 30 Jan 2002 11:07:06 +1100
Mark writes:
>
> I have just started to delve into the mysterious world of profiling
and wondered if there are any basic rules to converting from RGB
to CMYK? <<
RGB to CMYK has the same basic concerns, whether using profiles or an old
sep table - but output profiles usually have different rendering intents and
in Adobe software BPC can also have a big impact on things. If you are
familiar with converting from RGB to CMYK then you know about the critical
things to look for.
Presuming Photoshop 6 -
Choose the correct CMYK for the printing conditions in use in the final
print. This could also affect the choice of RGB you use (refer to the
current thread of ColorMatch vs. Adobe RGB) - but that's another post.
Do a softproof and gamut warning preview from the RGB to the CMYK output
profile used in the final print. Decide whether these areas are critical or
not - and either adjust them in RGB first or just leave them as is.
Usually a Relative Colorimetric conversion with BPC (black point
compensation) in Photoshop is performed for most image types. If your image
has lots of gamut issues or fine gradations - it is probably best to use
gamut compression or Perceptual/Image rendering from RGB to CMYK with BPC
(makes little difference for perceptual in most cases, unless I have been
lucky - just make sure that you _do_ use BPC when doing a RelCol transform
to CMYK).
Some people leave the conversion alone, others enhance the sep from the base
conditions (as RGB and CMYK are different beasts there can be a lot of
interpretation on the conversion or post sep edits depending on many
things - for example some images convert between modes fine, while others
such as a deep blue in A98 might turn to weak mud in some CMYK moves and
then post sep edits are often required).
Whether or not use the 8 bit dither option or not is another story - I
personally leave it off most of the time but it can be helpful if
posterization is a big issue.
>
> What would you do if the RGB image supplied to you to convert does
not have a profile attached? <<
Make a 'self informed' guess about the originating files conditions, if I
could not find out any other way. The idea is to assign a profile which
produces the best results for the unknown description of those RGB numbers.
It would seem a safe bet to try Apple RGB or ColorMatch RGB, sRGB and Adobe
RGB as the source for the assign profile command (common workspaces in use).
You could also try other profiles if these do not produce good results - but
probably start with these.
If the image looks too washed out, the 1.8 gamma of Apple/ColorMatch might
be too weak. In this case I pretty much treat ColorMatch and Apple as the
same thing for legacy files - but if you actually know what should be used
then choose the correct one. Next try sRGB...if the 2.2 gamma of sRGB makes
things better than the 1.8 gamma of ColorMatch RGB. If the luminosity is now
correct - but it looks a bit desaturated, then perhaps assign A98 instead.
In all these cases, you are simply assigning a profile - not converting any
numbers.
This only works for files from working spaces - if the file is a digicam or
something else, the device RGB is going to be so far off from a work space
profile that simply assigning a WS is not going to help that much - but you
have to choose something. This is where you need the device profile
(scanner, camera or software space creating the file). For digicams where no
tag is supplied and no workspace helps, then a generic DCS digicam profile
is used, in lieu of a workspace for the transform into workspace - as this
often helps, but not always (assign device profile and convert to workspace
once you find the right device profile, or one you are willing to use in
lieu of the right profile.)
These are visual decisions in - I also judge things via CMYK and LAB info
palette number readouts, but this takes experience - but it does help verify
and clarify the visual checks. At the end of the day, if I can't find out
for sure what was originally used (by asking the supplier if practical) then
an intelligent decision based on how things visually look on the monitor and
how they convert to CMYK is the way I judge things.
For digicam images, this tool might also help in making input decisions by
viewing the hidden meta-data (free Photoshop 6 plug):
http://www.pixelgenius.com/products.html
Hope this helps,
Stephen Marsh.