GCR - Friend or Foe?

[mglouis]

To rule GCR in or out as the problem, what are your profile and total area coverage settings? You did not mention if the problem scales up as you increase GCR. Link Profiler/Optmizer has more than one setting that can make or break a workflow. It seems suspect that GCR would be the problem if you are having similar problems between a web press and an HP Indigo.

One way to isolate the problem is to make a link using a device profile that is known to work well in your shop. Link it with absolute colorimetric using the same profile as source and destination. Use GCR settings from the ICC profile. Get the TAC from the profile; don't over-ride it.

Then make a nearly identical link (or queue), where the only difference is Alwan Dynamic Maximum Black "7" instead of "from profile".

Now run the images side by side and if they look different then the only variable is the Alwan flavor of GCR.

Matt Louis

 

[JudP]

To rule GCR in or out as the problem, what are your profile and total area coverage settings? You did not mention if the problem scales up as you increase GCR. Link Profiler/Optmizer has more than one setting that can make or break a workflow. It seems suspect that GCR would be the problem if you are having similar problems between a web press and an HP Indigo.

One way to isolate the problem is to make a link using a device profile that is known to work well in your shop. Link it with absolute colorimetric using the same profile as source and destination. Use GCR settings from the ICC profile. Get the TAC from the profile; don't over-ride it.

Then make a nearly identical link (or queue), where the only difference is Alwan Dynamic Maximum Black "7" instead of "from profile".

Now run the images side by side and if they look different then the only variable is the Alwan flavor of GCR.

Matt Louis

Yes indeed and I did do this, well a variation of what you state above anyway...

I did some more research thanks to everyone who posted and found that running the file through Alwan does in fact apply GCR to the file and what looked like was hurting us was the Dynamic GCR settings. Now, I'm not saying that the dynamic settings aren't good, it definitely has it's use. On all our images apart from flesh tones, the Dynamic Black does great and really proves that GCR is helpful however I just notice subtle greyish tones in faces and hands which we don't like.

I did a test of the same image at different levels of GCR and found that just using Alwan's basic GCR settings with Dynamic Black turned off gave us the best flesh tones.

 

[Lukas Engqvist]

Thank you for posting your observations, great to know what to be aware of in an Alwan workflow.

 

[JudP]

Hi All,

Well, just thought I would update you on my so-called battle with our GCR settings. I've been testing different GCR settings in Alwan to get rid of what is best described as a muddy flesh tone and believe it or not I solved it by INCREASING the amount of GCR being applied to the file.

We had earlier had our GCR to start at 35% (K Start setting for those familiar with Alwan) and our thoughts were to keep bringing this up so it wasn't effecting the flesh tones however nothing changed. We then tried tested going down with the K start value and magically our muddy flesh tones disappeared. We have settled on a K start of 0.

Looks like what was happening is by setting the K start too high it was creating these harsh transitions from the point it started applying GCR to the file.

So...problem solved! And with this higher GCR setting, I have noticed that the press is printing more stable and color is holding more consistent.

Best, Jud

 

[Lukas Engqvist]

Thanks for the feedback. Great to know. So the Start value has to do with the "depth" of the GCR and not the "width"? Starting at 0 means that a 1% K will not be converted to CMY but remain K? I don't have Alwan so it's an honest question

 

[JudP]

Thanks for the feedback. Great to know. So the Start value has to do with the "depth" of the GCR and not the "width"? Starting at 0 means that a 1% K will not be converted to CMY but remain K? I don't have Alwan so it's an honest question

Hi Lukas,

I believe that you are correct. I'm not an Alwan expert myself yet but from what I've been told setting the K start point to zero applies GCR in all areas where CMY make up grey starting at 1%...

Jud

 

[rich apollo]

Thanks for the feedback. Great to know. So the Start value has to do with the "depth" of the GCR and not the "width"? Starting at 0 means that a 1% K will not be converted to CMY but remain K? I don't have Alwan so it's an honest question

You're setting the black the start point, just as you would do when building an ICC profile.

Jud, sounds like the "muddy" tone may have actually been from cyan. Flesh tones are basically red/orange tinted grays (for caucasians). The contaminate color would be cyan or blue. One of the principles behind GCR is that two inks are all that ever contribute to a color. Magenta and yellow make red, cyan and yellow make green, cyan and magenta make blue. The third ink is always a contaminate which reduces saturation. By starting the GCR early you replace much of the contaminate colors with black, which has less impact on saturation.

 

[mglouis]

Setting the width has an affect of the black start. I made two links with a black start of 1% with the only difference being in the amount of GCR. The "Light GCR (5.0)*" Setting on P2P Column 4 shows the black coming in around 40%K. The ""Maxi GCR (1.0)*" setting makes the black start 2%. Since the P2P lacks a 1% gray I painted one in and the result was 0 0 0 1. We are Alwan Link Profiler users. To find "our" settings we made test strips and selected the strip we liked the best. On one extreme there was no gray protection. On the other extreme the screens made of mostly black ink were mottled in comparison to the 4 color rosettes. Since gray balance is stable up to HC-CMY, I start my black just below at 18%.

Matt Louis

 

[JudP]

You're setting the black the start point, just as you would do when building an ICC profile.

Jud, sounds like the "muddy" tone may have actually been from cyan. Flesh tones are basically red/orange tinted grays (for caucasians). The contaminate color would be cyan or blue. One of the principles behind GCR is that two inks are all that ever contribute to a color. Magenta and yellow make red, cyan and yellow make green, cyan and magenta make blue. The third ink is always a contaminate which reduces saturation. By starting the GCR early you replace much of the contaminate colors with black, which has less impact on saturation.

Hey Rich, Yes, you're totally correct which is why by setting a more "aggressive" GCR to the files our flesh tones have now lost that muddy look we were seeing. It definitely had our prepress guys stumped for a while but the principle of the contaminate color is exactly what we were seeing. Our flesh tones are now bright and clear and do really look good.

The press is also printing really stable through long runs with this heavy GCR setting.

Best, Jud

 

[JudP]

Setting the width has an affect of the black start. I made two links with a black start of 1% with the only difference being in the amount of GCR. The "Light GCR (5.0)*" Setting on P2P Column 4 shows the black coming in around 40%K. The ""Maxi GCR (1.0)*" setting makes the black start 2%. Since the P2P lacks a 1% gray I painted one in and the result was 0 0 0 1. We are Alwan Link Profiler users. To find "our" settings we made test strips and selected the strip we liked the best. On one extreme there was no gray protection. On the other extreme the screens made of mostly black ink were mottled in comparison to the 4 color rosettes. Since gray balance is stable up to HC-CMY, I start my black just below at 18%.

Matt Louis

Hi Matt,

We are currently using MAXI GCR 7 with a K start of 0 and it is significantly better than when we were using Dynamic Black with a K start of 35.

Our flesh tones are better and I just looked at what was on press now and it seems our very smooth gradients are still a little rough. We've definitely come a long way since my first post on this thread.

Have you noticed this in your work?

Jud

 

[mglouis]

Hi Jud,

I have only noticed banding problems I would attribute to reseparating images in lighter tones along gradients and in all cases the customer art was not perfect. Alwan has a control for grid points so you might try lowering the resolution of the profile with the goal of smoothness over accuracy.

The nature of moving tonality from CMY over to the black plate may be exposing problems in your black unit. Pushing the k start up away from zero into the mid tones seems to help highlight banding in our case for whatever reason.

Banding may also be caused by CTP curves. Steep curves in highlights or steep differences between two overprinting colors are problems areas to keep an eye on. As an experiment if you have the luxury of a press test you might try the same files that gave you a problem on uncalibrated plates to isolate their contribution to the problem.

Matt Louis