Black point compensation question, Ps

[hugowolf]

Given, unless it has been fixed recently, BPC is always used for perceptual rendering in Photoshop whether you check that box or not, but it can be turned on or off for a relative colorimetric rendering, what happens if you select a perceptual rendering for a color space to color space conversion using simple matrix profiles?

It can't do a perceptual rendering, the AtoB and BtoA tables aren't there. So has to do a colorimetric rendering. But does checking or unchecking the BPC check box have an affect?

Brian A

[digitaldog]

I see no reason not to always have BPC on.

[hugowolf]

I see no reason not to always have BPC on.

I can't think of a case where I wouldn't, but that doesn't answer the question. It could even ignore the BPC checkbox when switching to relative colorimetric.

Is there any way to run a simple test to see what is going on?

Brian A

[digitaldog]

6. 2. Rendering intent
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.

http://www.color.org/AdobeBPC.pdf

[AlterEgo]

http://www.color.org/AdobeBPC.pdf

there is also something from Marti Maria of LCMS = http://www.littlecms.com/BlackPointCompensationTests.pdf

[hugowolf]

Interesting; thank you both for the links.

Brian A

[Josh-H]

I see no reason not to always have BPC on.

Andrew, correct me if I am wrong here.. (not impossible!) But.. the problem with BPC as I have always found (and lets assume a soft proof example with a RC rendering intent) is that there is no way to know exactly how much black 'lift' is being applied under the PS hood when you tick the BPC box?

I have found that if I print a test chart step wedge of black squares going from '0' to say '30' at 1 point incremental jumps with the profile I want to use with BPC turned off I can then see the exact point at which tonal gradation begins in the print. This is a very interesting way to also compare profiles against each other for shadow tonal gradation and can really highlight the difference between a good and bad profile. A good profile might see tonal gradation starting at say '5' for example on a matt paper, a bad profile may not start until '25'. Its an important number to know as once you know where tonal gradation begins for a given paper and profile you can then apply a custom bump to your prints for BPC.

I can then apply as an action a slight bump to the black level for my actual print to exactly the point at which tonal gradation begins in the step wedge printed chart (in other words a custom BPC) and get the ideal BPC for my print. Its a bit more work than just using PS's BPC, but its highly accurate.

I picked up this tip from an article here in Lula some time ago and apologies to the original author, but I cannot recall who wrote it. Point is, it works extremely well and enables one to set the exact point at which tonal gradation begins in a print. And it works just as well in the whites as well as the blacks.