Re: null conversion
Re: null conversion
- Subject: Re: null conversion
- From: Graeme Gill <email@hidden>
- Date: Sat, 26 Nov 2005 14:24:36 +1100
Michael Fox Photography News Account wrote:
A RIP vendor (which one doesn’t matter for this question) told me that
they will read the profile in the image as the input profile and convert
it to the output profile configured in the RIP.
I asked, “what if the two profiles are the same?” (i.e. Photoshop or
some other edition software was already used to do the profile
conversion before sending the file to the RIP.)
Is there a reason why a RIP wouldn’t be able to tell if the profile in
the image is the same as the output profile configured in the RIP?
Lots of reasons. The profiles might notionaly be the same colorspace,
but be based on different measurement charts, or they might be
based on the same measurement data, but be made with different
profiling software, or they might be the same profile, but rebranded
by different vendors and therefor be different in some non-essential way.
It's easy enough to tell if a profile is bit for bit identical with
another, but not if it's functionally identical.
Even if there is no way to tell for sure, then why wouldn’t the math
simply result in a null change?
Because unless the profiles are purely mathematical ones (such as a matrix
and gamma function), the interpolation tables are approximations of the
desired transformations, and mathematically cannot be exact inverses of
each other (most CMM's don't attempt to invert clut tables, they use the
A2B for a forward transform, and the B2A for the reverse transform.)
Also consider that if you are dealing with CMYK profiles, then processing through
the profiles will almost certainly be different from not, because the former
causes the black to be regenerated, while the latter leaves the black levels
unchanged. In some situations it might be desirable to do such a null-transform
to enforce total ink limit on a file.
Graeme Gill.
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| >null conversion (From: "Michael Fox Photography News Account" <email@hidden>) |