Dan Margulis Applied Color Theory

Scum Dots and Conversions

Date: Mon, 07 Jul 2003 13:44:18 -0700
   From: "Seth Campbell"
Subject: 1% cyan dot

Hello group,
    Sometime ago I believe that there was a discussion about photoshop leaving behind a "scum" dot. a 1% cyan dot in deleted areas. I was wondering if there is a fix for this? where I might find the discussion in the archives.

Thanks,
Seth Campbell
Smith/Walker Design
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Date: Mon, 7 Jul 2003 16:04:15 -0600
   From: Chris Murphy
Subject: Re: 1% cyan dot

On Monday, July 7, 2003, at 02:44  PM, Seth Campbell wrote:

 Sometime ago I believe that there was a discussion about photoshop
 leaving behind a "scum" dot. a 1% cyan dot in deleted areas. I was
 wondering if there is a fix for this? where I might find the discussion
 in the archives.

Photoshop 7; or Photoshop 6 with dither turned off. And also use ACE when making conversions instead of other CMMs.

Chris Murphy
Color Remedies (TM)
www.colorremedies.com/realworldcolor
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Date: Tue, 08 Jul 2003 09:07:56 -0400
   From: Dan Margulis
Subject: Re: 1% cyan dot

Seth writes,

Sometime ago I believe that there was a discussion about photoshop
leaving behind a "scum" dot. a 1% cyan dot in deleted areas.

There have been a disturbing number of such incidents, usually with scum appearing in all three CMY channels, not just the cyan.  It's been reported that certain third-party profiles have the problem when used with certain CMMs. It also frequently happens if the user chooses "Absolute Colorimetric" as a rendering intent on separation. It happened while saving JPEGs in Photoshop 7.0 (fixed in 7.0.1). And Chris says that it happens in Photoshop 6 if dither is turned off, which I wasn't aware of.

So, having already been burned, whenever I use a new separation method, I now always separate a test file that contains 255r255g255b, just to make sure it comes out to exactly 0c0m0y as it should. I'd recommend that everyone do the same.

Dan Margulis
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Date: Tue, 08 Jul 2003 07:18:39 -0600
   From: Andrew Rodney
Subject: Re: 1% cyan dot

on 7/8/03 7:07 AM, Dan Margulis wrote:

 There have been a disturbing number of such incidents, usually with scum
 appearing
 
Here we go again. Does that mean 100, 1000? Ten million? What on earth is the number that1s disturbing?
 
 It's been reported that certain third-party profiles have the problem when
 used with certain CMMs.
 
WHY would anyone use something other than the Adobe CMM INSIDE of Photoshop? If you use that profile/CMM combo outside of Photoshop you1ll get the same issue so you can1t blame Adobe. Stick with ACE and use good profiles.
 
 It also frequently happens if the user chooses "Absolute Colorimetric" as a
 rendering intent on separation.
 
Absolute Colorimetric is for proofing and for simulating a dot on paper to match another paper so that1s not at all unusual. If you didn1t get a paper white shift something would be broken for sure.
 
 So, having already been burned, whenever I use a new separation method, I now
 always separate a test file that contains 255r255g255b, just to make sure it
 comes out to exactly 0c0m0y as it should. I'd recommend that everyone do the
 same.
 
Anyone that tries any new profile/sep method and doesn1t test it out gets what they deserve.

Andrew Rodney
http://www.imagingrevue.com/  
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Date: Tue, 8 Jul 2003 09:51:17 -0600
   From: Chris Murphy
Subject: Re: 1% cyan dot

On Tuesday, July 8, 2003, at 07:07  AM, Dan Margulis wrote:

 It happened while saving JPEGs in Photoshop 7.0 (fixed in 7.0.1).

I believe it happened when using JPEG compression, not just the JPEG file format. So the JPEG, TIFF and EPS file formats were affected if you chose to use JPEG compression (which is always the case for the JPEG file format).

 And Chris says that it happens in Photoshop 6 if dither is turned off, which I wasn't aware of.

No it happens if dither is turned on, so you want to turn it off. (i.e. my response was the FIX for the problem; either PS 7, PS 6 with dither turned off, and the use of ACE in all cases).

I'm not sure why this happens, but I think it may be a Photoshop bug because I haven't heard this being a problem when using 3rd party profiles and CMMs outside of Photoshop (say with the tons of prepress equipment that's out there, or GretagMacbeth iQueue for automating conversions for separation and proofing).

Chris Murphy
Color Remedies (TM)
www.colorremedies.com/realworldcolor
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Date: Tue, 8 Jul 2003 09:57:34 -0600
   From: Chris Murphy
Subject: Re: 1% cyan dot

On Tuesday, July 8, 2003, at 07:18  AM, Andrew Rodney wrote:

 Here we go again. Does that mean 100, 1000? Ten million? What on earth is
 the number that’s disturbing?

Sorry but even 1 in the wrong place is potentially a $100,000 (or more) mistake that's going to come out of someone's pocket. I've had the problem myself, it's been on the ColorSync list, and I've heard of maybe a half dozen botched high profile (and high dollar) jobs. So it only takes a few such instances to catapult it to at least urban myth fear level. The likelihood of encountering it may be low, but if you had a 1% chance of losing a finger today, I wouldn't call that very good odds.

 WHY would anyone use something other than the Adobe CMM INSIDE of
 Photoshop?

Well, it is an option...if it's that dangerous then maybe Adobe shouldn't allow us to use CMM's other than their own?

 If you use that profile/CMM combo outside of Photoshop you’ll get the same
 issue so you can’t blame Adobe. Stick with ACE and use good profiles.

I'm not sure that's true. I'm pretty sure I remember someone - maybe it was John Gnaegy - I may have performed the test myself using AppleScripts to do the conversion using the Apple CMM. And as I recall the problem would occur using the Apple CMM in Photoshop but not through AppleScripts. So I can't say I'm 100% confident this is not a Photoshop bug.

 Anyone that tries any new profile/sep method and doesn’t test it out gets
 what they deserve.

Fair enough, but what a hassle to have to test for this kind of problem.

Chris Murphy
Color Remedies (TM)
www.colorremedies.com/realworldcolor
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Date: Tue, 08 Jul 2003 11:59:52 -0600
   From: Andrew Rodney
Subject: Re: 1% cyan dot

on 7/8/03 9:57 AM, Chris Murphy wrote:

 Sorry but even 1 in the wrong place is potentially a $100,000 (or more)
 mistake that's going to come out of someone's pocket.

Again, if that kind of money is on the line, doing a blind conversion for the first time and sending it to print without viewing the numbers/preview or pulling a proof is just stupid.

 Well, it is an option...if it's that dangerous then maybe Adobe
 shouldn't allow us to use CMM's other than their own?

I wouldn't go that far. There are rare occasions where you might be using Photoshop to do a conversion AND some other application that hasn't got access to ACE and you'd want to match them up. But that's the only case where I'd even consider not using ACE and I'd test the CMM because as we both know, some have bugs. Again, testing is key here.

 Fair enough, but what a hassle to have to test for this kind of problem.

Once?

Andrew Rodney
http://www.imagingrevue.com/
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Date: Wed, 09 Jul 2003 08:58:13 -0000
   From: Stephen Marsh
Subject: Re: 1% cyan dot

Andrew Rodney wrote in response to Chris Murphy:

 Again, if that kind of money is on the line, doing a blind conversion for
 the first time and sending it to print without viewing the numbers/preview
 or pulling a proof is just stupid.

I generally agree, ensuring that areas that should contain no ink is a basic task, but I can understand if it get's missed every now and then (even if it should not).

A potential problem with CTP digiproofing that I had not encountered with film based proofs - is that it is commonly done via inkjets, and there is substrate simulation is applied to the stock, so there is no pure paper white on the image-area as it all has 'scum dots' as part of the press simulation. With luck the RIP operator may catch this before the plate is burned - but it should still be verified in Photoshop via the info palette.

Stephen Marsh.
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Date: Thu, 17 Jul 2003 16:59:13 -0000
   From: Alan Harris
Subject: 1% black in default white using Colorsync

Hi folks,

I have tried searching the archive here for anything relating to the Colorsync engine giving a 1% black in the default white in the Photoshop (7.01) toolbox.

I found the thread about the 1% cyan scum spot, but I don't think they are related. (?)

Changing back to the Adobe ACE fixed the problem. I only discovered it when working on masks and noticed transparency appearing in the shadow areas when restoring parts of the mask with this dirty white.

Any info or links about this would be appreciated.

Thanks.
Alan Harris
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Date: Fri, 18 Jul 2003 10:52:12 EDT
   From: Dan Margulis
Subject: Re: 1% black in default white using Colorsync

Alan writes,

I have tried searching the archive here for anything relating to the Colorsync
engine giving a 1% black in the default white in the Photoshop (7.01) toolbox.
I found the thread about the 1% cyan scum spot, but I don't think they
are related. (?)

They are indeed related by their failure to grasp a fundamental point of colorspace conversions. Conversion algorithms are naturally variable. If you have some RGB or LAB value that converts to approximately 50c40m40y, three different programs may give three very slightly different results, even assuming they all generate black in the same way. Nothing wrong with this, and no way of proving which variation is right.

Some of the rocket scientists who design these things, however, forget the exception, which is, unless you are attempting for some reason to compensate for the color of the paper, if you have a value of 255r255g255g, or 100L0a0b, or 0c0m0y0k,it absolutely, positively, without exception, unconditionally, and without fail, must convert to pure white, period, amen.

So, basically, every time you adopt a new conversion method, you have to try converting white to see if the problem has reared its head again.

Dan Margulis
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Date: Fri, 18 Jul 2003 21:04:00 -0000
   From: Alan Harris
Subject: Re: 1% black in default white using Colorsync

Thanks for the reply Dan.

The problem was not in converting anything from one colour space to another. It was the default White in a new RGB Photoshop document, in the toolbox default background swatch. It was only 99% white. The Colour palette white was OK.

If I chose the palette white, all was well. But If I hit the default foreground/background button, I got the dirty white.

Regards, Alan Harris
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Date: Fri, 18 Jul 2003 16:23:12 -0600
   From: Chris Murphy
Subject: Re: 1% black in default white using Colorsync

If Color Settings is set to use a rendering intent of Absolute Colorimetric, new document white will have a color associated with it, but usually it's not 1% black. It's usually some combination of only CMY. Maybe that's what's going on?

Chris Murphy
Color Remedies (TM)
www.colorremedies.com/realworldcolor

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