Re: BCP or no BCP to epson without rip
Re: BCP or no BCP to epson without rip
- Subject: Re: BCP or no BCP to epson without rip
- From: Steve Upton <email@hidden>
- Date: Tue, 9 Jun 2009 22:49:39 -0700
At 7:45 PM -0700 6/9/09, Marco Ugolini wrote:
>In a message dated 6/9/09 4:53 PM, Graeme Gill wrote:
>
>> Marco Ugolini wrote:
>>
>>> As for the Perceptual rendering intent, it is supposed always to map source
>>> black to destination black. In that sense, BPC is redundant with Perceptual.
>>> But I have seen instances in which a poorly-built output profile will produce
>>> different results depending on whether or not BPC is activated -- though I
>>> have reason to believe that such behavior is anomalous, and that a
>>> properly-made output profile will always scale source black to destination
>>> black in the Perceptual rendering intent.
>>
>> It's a fine theory, but of course with one major flaw - the destination
>> profile in general doesn't know what the source profile black point is, so it
>> can't know what to map to what.
not wanting to speak for Graeme but I can hopefully shed some light on the discussion.
A typical output profile will "expect" the range of color coming from the source profile to go all the way down to 0,0,0. The intent will scale values from 0,0,0 up into the range of the output device.
When converting from a "normal" working space like sRGB / aRGB / etc using the perceptual intent, this expectation is for the most part, correct.... at least as far as the color spaces go. The image itself could lie anywhere of course.
In this case, BPC will probably not have a noticeable effect on the resulting conversion.
But consider the case of a file which is in scanner, camera or printer space. Its darkest color will be limited by the bottom of the device gamut but that certainly could be higher than 0,0,0. It could be as high as 10-15 L* and not be exceptional. If you were wanting to convert this file into a different printer's color space you will likely *want* and *notice* a significant difference when BPC is applied whether the perceptual or relative colorimetric intents are used.
So I still stand by the general advice which is, use BPC unless you don't want it explicitly. Translated to client-speak I say it's the "give me great blacks" checkbox. In the rare case when people don't want great blacks (typically proofing), then you shut it off.
Also
At 9:25 AM -0500 6/9/09, G Mike Adams wrote:
>Well, fact is you're going to continue to be confronted with contradictory opinions because that's pretty much what they are, opinions.
I don't mean to be particularly contrary but I personally feel that BPC is more of a known science that that.
>
>However, even though it isn't always entirely true, the way it's supposed to work is that BPC doesn't even work in perceptual, because the quickest, easiest, shorthand way to think about what it is and what it does is that it acts as perceptual rendering intent for the black channel only. Therefore in perceptual, since all the channels have been rendered using whatever perceptual formula has been applied, the black has as well.
This is a point I wanted to clarify.
BPC is about mapping *tones* NOT *channels*. So the effect of BPC is the same regardless of whether the resulting (or source) spaces actually have a black channel or not.
Also, the effect DOES work in the perceptual intent and as far as I know is completely intent agnostic. Its purpose is to map the black points (black as color, not as channel) between two color spaces. It has been proven over time to do a good job of this as well.
At first I thought this may have simply been a matter of terminology but upon rereading this post I felt it necessary to post a clarification.
Hope it helps
Regards,
Steve
________________________________________________________________________
o Steve Upton CHROMiX www.chromix.com
o email@hidden 206.985.6837
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