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RGB to CMYK problem

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  #1  
Old 09-29-2006, 04:24 AM
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saby saby is offline
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RGB to CMYK problem

Hi All again,

I saw a thread in a few days, about photo like this picture but I can't find it. When I changed this picture to cmyk (profil EuscaleCoated v2.). I've lost the detail of light at the upper right, as U see. I tried some other profile but it was the same. Can someone help to find the thread or to correct this problem, please?
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Old 09-29-2006, 07:48 PM
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Cameraken Cameraken is offline
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Re: RGB to CMYK problem

Hi Saby.

I am not quite sure what you are asking for Saby.

Are you converting to CMYK for a reason or do you just want to repair the blue cast?

The red channel is damaged and that will make a damaged CMYK.

It would be better to repair the damage before the conversion.
I replaced the red channel with the green channel to repair the damage. This will then convert to CMYK without any noticeable loss.

There are several methods to repair this (Search for ‘Scuba Diving’)

Hope this helps.

Ken.
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Old 09-29-2006, 09:19 PM
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Photo678 Photo678 is offline
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Re: RGB to CMYK problem

you can't just blindly convert to cmyk, you need to show the gamma warning first adjust accordling to remove the out of gamma portions of the image (hue saturation works.....) convert, and then adjust again in levels to get as close as possible to the rgb image.

Because the color ranges are so different between the two, and rgb is basically what your monitor is, and cmyk would be the print version of the image, you will never be able to fully simulate cmyk onscreen.

so, as stated above unless you are printing this image at an offset printing facility, or its required to be cmky for a client, leave it in rgb.
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Old 10-13-2006, 06:42 AM
paroroonman paroroonman is offline
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Re: RGB to CMYK problem

you can work on RGB mode but always put the preview in "WORKING CMYK" so if you will convert it to CMYK mode no data would be loss.. and by the way working in RGB is only good for video and web.. but for printing always use CMYK mode .. CMYK is easier to adjust because it has four channels
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Old 10-13-2006, 11:45 AM
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Photo678 Photo678 is offline
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Re: RGB to CMYK problem

Quote:
Originally Posted by paroroonman
but for printing always use CMYK mode .
for offset printing, yes....but for desktop printers no...because your printer driver is set to convert from rgb to cmyk...if you send it as cmyk the will convert from cmyk to rgb back to cmyk and you can get some pretty muddy prints.
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Old 10-14-2006, 09:08 PM
edgework edgework is offline
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Re: RGB to CMYK problem

Quote:
Originally Posted by Cameraken
Hi Saby.

I am not quite sure what you are asking for Saby.

Are you converting to CMYK for a reason or do you just want to repair the blue cast?

The red channel is damaged and that will make a damaged CMYK.

It would be better to repair the damage before the conversion.
I replaced the red channel with the green channel to repair the damage. This will then convert to CMYK without any noticeable loss.

There are several methods to repair this (Search for ‘Scuba Diving’)

Hope this helps.

Ken.
It's not really accurate to say the red channel is damaged. The image is essentially blues and greens. By definition, there will be no red information. It is the lack of red that causes the heavy cyan cast, but if that is the desired color, forcing a duplication of green/magenta information in the red/cyan channel keeps detail, but the color shift creates an entirely new image. If the intent is to keep the colors, it's no solution.

The real problem is that blues and greens take a heavy hit when moving from RGB to CMYK. Cyan is a weak ink, which is why you need 10% more of it to balance with Magenta and Yellow to create neutral. The tonal range simply isn't there. Any image that relies on varieties of blues and greens for detail will necessarily suffer in the conversion.

The default conversion intent is Relative Colormetric. This will attempt to create CMYK duplicates of the original RGB colors, where possible, and where there are out of gamut colors, they just get crushed at the edges. Using Perceptual as the conversion intent will provide a precise replication of the relationships between each tone, though pulling the out of gamut tones into CMYK range will result in the entire image shifting color. Sometimes a channel from the RGB file (or perhaps even a greyscale conversion) can be copied into a layer atop the CMYK version, in luminosity mode, to help bring back some detail, but usually it's a toss-up between color and detail, and no solution will be totally satisfying.

This is where the blue detail went: On the left, the blues and greens in a generic RGB profile. On the right, the withered remnants in CMYK.
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Old 10-15-2006, 05:59 AM
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Cameraken Cameraken is offline
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Re: RGB to CMYK problem

Hi Edgework

Thanks for a great explanation of where the blue has gone.

I have never been scuba diving but I was under the impression that most underwater photographers try to eliminate the blue cast (but I guess this mainly applies to close up shots)

I do agree that this image looks better with the blue cast than without it though.

I tried replacing the cyan channel with the red channel. This gives an even blue to the top right corner. But the only answer to keeping those colours is to keep the image in RGB. Attached is the gamut warning. It shows (in grey) the colours that will shift in saturation.

Ken.
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Old 10-15-2006, 07:07 AM
edgework edgework is offline
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Re: RGB to CMYK problem

Quote:
Originally Posted by Cameraken
Hi Edgework


I tried replacing the cyan channel with the red channel. This gives an even blue to the top right corner. But the only answer to keeping those colours is to keep the image in RGB. Attached is the gamut warning. It shows (in grey) the colours that will shift in saturation.

Ken.
Hold that thought. It's a valuable trick, but for precisely the opposite condition.

Often, when converting to CMYK, an image that might already be somewhat deficient in Cyan will turn up in CMYK with a nearly worthless Cyan plate. Photoshop assumes a 20% dot gain for printed material when it moves colors into CMYK and lowers values accordingly in the quarter and mid-tones to compensate. Information that is already weak can get blasted into vapor in this process.

The red channel is analogous to the cyan channel, minus the reduction for dot-gain and retaining the shadow detail that gets ciphoned off into the black plate in CMYK. There will always be more quarter-tone and mid-tone detail in the Red channel than in the corresponding Cyan channel.

Enter the technique you tried. Copying the original red channel into the current cyan will bring back lost detail, and it will be actual color detail, not the forced compromise of substituting one plate for another. Usually it results in too much cyan, a much happier problem that can be treated with a quick curve.

When everything looks sunburned and all shadows are hot, this is a good way to increase cyan. Even if you don't have an original in RGB, duping the 4c version and converting to RGB will to some extent reverse the dot-gain reduction, resulting in better detail than you started with.

Of courses, this is only relevant if your end result is going to a printing press.
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