BPC
BPC
- Subject: BPC
- From: "eugene appert" <email@hidden>
- Date: Sun, 21 Oct 2007 16:16:01 -0400
Greetings,
I have noticed an anomaly, which occurs when printing a colour image with a large area of subtle gradation from deep shadow (L*1 to L* 20). Soft proofing this image using BPC produces a horrible posterizing or banding which is often even worse when actually printed. In every case it seems to be a problem with black point compensation.
The remedy is always to target the printer black point manually and lift the tones to where the printer/paper can properly distinguish them or, printing using perceptual rendering which frequently amounts to much the same thing. In other words eliminating BPC from the equation.
When I examine each case closely I find two sets of problems. The first deals with what BPC thinks its doing and the second deals with what it is actually doing.
The example I am looking at now is Ilford smooth gloss and Epson 7800. When I convert an L* chart using BPC and configure my colour settings to absolute, I can read the new L* values as L*0=L*7, L*1=L*8, L*2=L*9 etc. Colorthink confirms that the profile black point is L*7, so all would appear to be in order. Part of the problem comes from the fact that BPC sends all values from L* 8 to L*12 to L*14, this is the plateaux of values that appears as banding in soft proofing. It doesn't seem logical to me that BPC would compress grey values that close to the fragile shadow detail it was conceived to preserve, rather than compressing them much further up the curve where its less crucial.
However when I print a L* chart on the printer/paper combination using BPC, it is clear that there is no separation all the way to L*5 or L*6. This is what I meant by problems with what BPC is actually doing. Density readings confirm what the eye sees on the print, that is, no visible density change until the L*5 patch after which visible densities are less than what the profile believes its printing, all the way to L*14 or L*15.
Black point compensation seems to be blindly remapping values above the black point in increments it doesn't really know it can achieve. As though it was only using the black point density and not any information concerning the reproduction curve.
I am curious as to why BPC fails and would like to know more about how it actually functions and what information it is able to use from the target profile. Also I have never noticed this problem in B&W and wonder if BPC functions differently with colour images.
Thanks for any help.
Eugene Appert
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