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Author Topic: Hahnemühle "black point compensation" off  (Read 8691 times)
opgr
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« Reply #20 on: April 17, 2006, 05:33:50 AM »
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At the current state of affairs there is a difference between relcol and perceptual, because most profiling sw incorporate a form of BPC in the perceptual tables, which basically means that the blackest black is noted as having an L = 0 value. In such cases, Photoshop will skip its own routines for BPC, even if it's switched on.

BPC is not incorporated in the ICC specs, since it is not an obvious requirement other than for perceptual renderings. And for perceptual intent, the ICC can not make a recommendation, because it is then still open what curve is used for the compensation. A simple linear scaling? Or some form of shadow compression as is the case with Adobe's bpc. In other words, it becomes a judgement call.

And also there is one very important case where BPC is very much NOT a requirement and is to be switched off: namely If you want to make a press proof where the proofing printer blackpoint is darker than the press print blackpoint. In order to faithfully reproduce the lighter black point, you need to switch BPC off.

If you truly want to see the effect of BPC including its shadow compression, you always need to use the Photoshop softproof feature and switch the Black Point Simulation on. This will lighten the blacks if applicable, and allows you to better judge the effect of blacks relative to saturation. If you leave this off, some dark saturated colors may look oversaturated in preview. This is particularly apparent in dark, shadowed skintones...!

Having said all this, I must admit that the first profile I tried from the HM website (FineArt Canvas at the bottom) is indeed influenced by Photoshop BPC, even with Perceptual intent. However, not necessarily in a bad way, and likely because of a less than optimal perceptual table in the profile.


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Why is there a difference at all between the two?

Lastly why, since there's an obvious requirement, hasn't this been incorporated into the ICC specs?

Questions, questions ... :-)
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Oscar Rysdyk
theimagingfactory
Stephen Best
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« Reply #21 on: April 17, 2006, 07:13:46 AM »
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At the current state of affairs there is a difference between relcol and perceptual, because most profiling sw incorporate a form of BPC in the perceptual tables, which basically means that the blackest black is noted as having an L = 0 value. In such cases, Photoshop will skip its own routines for BPC, even if it's switched on.

BPC is not incorporated in the ICC specs, since it is not an obvious requirement other than for perceptual renderings. And for perceptual intent, the ICC can not make a recommendation, because it is then still open what curve is used for the compensation. A simple linear scaling? Or some form of shadow compression as is the case with Adobe's bpc. In other words, it becomes a judgement call.

And also there is one very important case where BPC is very much NOT a requirement and is to be switched off: namely If you want to make a press proof where the proofing printer blackpoint is darker than the press print blackpoint. In order to faithfully reproduce the lighter black point, you need to switch BPC off.

If you truly want to see the effect of BPC including its shadow compression, you always need to use the Photoshop softproof feature and switch the Black Point Simulation on. This will lighten the blacks if applicable, and allows you to better judge the effect of blacks relative to saturation. If you leave this off, some dark saturated colors may look oversaturated in preview. This is particularly apparent in dark, shadowed skintones...!

Having said all this, I must admit that the first profile I tried from the HM website (FineArt Canvas at the bottom) is indeed influenced by Photoshop BPC, even with Perceptual intent. However, not necessarily in a bad way, and likely because of a less than optimal perceptual table in the profile.
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The fact that perceptual tables incorporate some form of compensation of their own is borne out by my experience. The way I interpret Adobe's implementation of BPC however is that it's not just shadow compression, but an overall luminance compression of the image (see the referenced Adobe description). I still don't see a change in the soft-proof when toggling BPC when Simulate Black Ink (which I assume is what you meant) is on, even though the profile's black point isn't L=0 ... though I admit I haven't tested whether it makes a difference on output with the current paper/profile combination I'm using.

So to clarify, if the perceptual table is doing BPC it will set the black point L=0 which will nullify Photoshop's BPC (since there's no adjustment). And if it isn't, then you do want Photoshop to do it. What concerns me is that it seems some profiles are doing some compensation but still affected by Photoshop's. I'm not sure that hard and fast rules such as "always use BPC" applies in the last case and it seems best to test which gives the optimum results.

Thanks for your input.
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opgr
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« Reply #22 on: April 17, 2006, 07:53:15 AM »
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Agreed. Testing is paramount in such a case. Though I also agree with the idea that if you run into such a profile, then its usually the profile that is suspect. So if you build your own custom profiles and BPC does indeed affect the perceptual rendering, then it may be useful to review the profile build settings. For example, the max density discussed in another thread might be a culprit, and selecting a different papertype in the printer driver prior to profiling may alleviate the problem.

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What concerns me is that it seems some profiles are doing some compensation but still affected by Photoshop's. I'm not sure that hard and fast rules such as "always use BPC" applies in the last case and it seems best to test which gives the optimum results.
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Oscar Rysdyk
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Hermie
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« Reply #23 on: December 26, 2006, 08:30:20 AM »
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This is what Adobe says about BPC and rendering intents:

"Typically, BPC is performed for conversions using the Relative Colorimetric intent. BPC is not available for conversions using Absolute Colorimetric intent.

Color conversion using Perceptual intent already maps source white to destination white and source black to destination black. Because this mapping preserves the relationships of the shades, it is unlikely that a whole shadow section will be mapped to the same black value. Therefore, BPC should not be necessary. BPC is available, however, for this rendering intent, to be used with malformed profiles. For a given picture, the user can decide whether using BPC improves the color conversion and can select it or deselect it accordingly.

BPC is available for color conversion using the Saturation intent. As with Perceptual intent, the user may or may not find that selecting BPC improves the conversion of a given image."

Source: 'Adobe Systems' Implementation of Black Point Compensation' http://partners.adobe.com/public/developer...df/AdobeBPC.pdf See paragraph 6.2

Herman
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jjlphoto
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« Reply #24 on: December 26, 2006, 10:03:26 AM »
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Why not look at the image in soft proof and see whether on or off looks best? Theory only gets you so far, the rest is personal taste and making tests.
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Thanks, John Luke

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