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Author Topic: Color matching - custom project  (Read 3648 times)
AFairley
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« on: June 25, 2009, 04:37:23 PM »
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I am working on a  project in which graphics are created in Photoshop using a palette of 8 colors, which are supposed to match specific Pantone swatches.  The graphics are composed of solid color areas only.  I would like to color match the output on an Epson 3800 to the Pantone swatches.  I am printing on Epson Photo Paper Pro Glossy (or whatever it's called) using the Epson canned profile for the paper.

This is not really a usual profiling situation, since (a) there are 8 discrete colors involved (no blending, shading, ect, just solid color areas) and ( I can tweak each of them individually in Photoshop to match my target swatches (in theory).

Is there a formula or program that will let me scan a print and a color swatch and then tweak the RGB values for the color in photoshop using the RGB values from the scans so that the print color matches the swatch after the tweak?

Hope this is clear.

Thanks, Alan.

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Desmond
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« Reply #1 on: June 26, 2009, 09:46:26 AM »
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If you have the pantone, why don't you scan in the graphic, select the colour and fill with known pantone, and print with Absolute Colourmetric? Or I missed something?
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Regards,

Desmond
AFairley
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« Reply #2 on: June 26, 2009, 10:51:35 AM »
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Quote from: Desmond
If you have the pantone, why don't you scan in the graphic, select the colour and fill with known pantone, and print with Absolute Colourmetric? Or I missed something?

The graphic is being created in photoshop, and I can fill with known pantone.  I will try absolute colormetric and see how that goes.  I had thought that some of the colors would have to be a little off with the canned profile because it is a compromise for rendering all possible color shades within the gamut, for example, yellows are slightly greenish.  But my tests have been with Relative Colormetric, so...
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Ethan_Hansen
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« Reply #3 on: June 26, 2009, 06:39:58 PM »
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Quote from: AFairley
Is there a formula or program that will let me scan a print and a color swatch and then tweak the RGB values for the color in photoshop using the RGB values from the scans so that the print color matches the swatch after the tweak?

What's your budget? If it is in the low thousands, you can use an Eye-one Photo + ProfileMaker is just the ticket. The profile editing component of the software has a capable spot color optimization feature. We use it for product work where a range of particular output colors needs to be an exact match to the swatch in question. Alternatively, you can print a large grid of possible colors and measure to see which one provides the closest match.

Assuming you are on a more limited budget, you should be able to make do with a combination of scanner and your eyes. Build a image containing a grid of color patches bracketing each pantone color. If you are using a RIP, building the grid in CMYK works, otherwise I would be tempted to work in LAB. Make a range of patches bracketing the Pantone color. In CMYK, try an initial grid using +/- 5% in each value. This create a file with 3^4 = 81 patches for each Pantone color; e.g. [C+5%, M, Y, K], [C, M, Y, K], [C-5%, M, Y, K], etc.

If you use the standard Epson driver, build the test file in LAB instead. You do not want to confound matters by shoving CMYK colors through the Epson driver or by relying on Photoshop's conversion from Pantone to your working RGB color space. Here I would do an initial pass using +/- 5 in LAB value, giving 27 total patches: [L+5, a, b], [L, a, b], [L-5, a, b], etc.).

Convert the test image to the canned Epson profile using Relative Colorimetric (not Abs. color because you do not want to add a color cast to lighter swatches). Print and scan along with the Pantone swatch. Open the file in Photoshop and sample the scanned Pantone swatch. Create a layer filled with the scanned swatch color and set the blending mode to difference. You are looking for the scanned color patch with the lowest total RGB value (nitpicker's note: what you really want is the patch with the lowest RMS error; i.e. minimize sqrt((R[patch] - R[pantone])^2 + (G[patch] - G[pantone])^2 + (B[patch] - B[pantone])^2).

If you have an older version of Photoshop on hand just build a Filter Factory filter to find the patch with the lowest difference. If you are at CS2 (I think; might have been CS) or higher, you can use the Filter Foundry (freeware) plug-in. A quick filter to highlight all patches less than X different from the Pantone swatch is:[div class=\'codetop\']CODE[div class=\'codemain\' style=\'height:200px;white-space:pre;overflow:auto\']r=sqr((dif(r,10)*dif(r,10)) + (dif(g,20)*dif(g,20)) + (dif(b,30)*dif(b,30))) < 5 ? 255 : 0
g=0
b=0
Notes: This example assumes the RGB value of the Pantone swatch is (10, 20, 30) -- that's where those numbers come from. Substitute with the actual values your scanner returns. Any pixel less than 5 RGB units different from the Pantone color will be pure red, otherwise it will be black. Change the 5 to any other value to find colors closer or farther from the target swatch. Important: You want to perform a Gaussian blur with a radius of several pixels first to average out scanner noise.

You should also do an eyeball comparison of your selected patch to the swatch under the lighting you plan on using (D50, etc.). You do not want to get burned by a potential metamerism problem between Epson ink and the scanner lightsource. Assuming the patch looks reasonable, you can do another optimization session with finer grid points surrounding the color combination you found to be closest.

If you need help building an image containing the necessary grid points, send me an email or PM. The software we use for building profiling targets can do this easily.
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neil snape
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« Reply #4 on: June 30, 2009, 11:04:14 PM »
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Quote from: AFairley
I am working on a  project in which graphics are created in Photoshop using a palette of 8 colors, which are supposed to match specific Pantone swatches.  The graphics are composed of solid color areas only.  I would like to color match the output on an Epson 3800 to the Pantone swatches.  I am printing on Epson Photo Paper Pro Glossy (or whatever it's called) using the Epson canned profile for the paper.

This is not really a usual profiling situation, since (a) there are 8 discrete colors involved (no blending, shading, ect, just solid color areas) and ( I can tweak each of them individually in Photoshop to match my target swatches (in theory).

Is there a formula or program that will let me scan a print and a color swatch and then tweak the RGB values for the color in photoshop using the RGB values from the scans so that the print color matches the swatch after the tweak?

Hope this is clear.

Thanks, Alan.


For 8 patches A ColorMunki from X-Rite will allow sending out a palette of the measured colours. You could then tweak those colours to match your output in Photoshop and reduce the delta E visually or by measurement.
Ethan's way described below would be a good way if you don't have access to, or know anyone with a Color Munki or better.

I assume that Colorvision's Spyder 3 package with patch reader would also do something similar by creating a swatch palette, but you'd have to check on their site.
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xilvar
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« Reply #5 on: July 04, 2009, 01:18:40 AM »
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With a correct profile you could print those colors accurately if the printer can produce them at all. As you mentioned you can just use absolute colorimetric and see what you end up with.

Anyway, if you want to use your scans to refine those colors you can just do it by hand too:
1) look at the Lab values of a reference sample. Say those values are L1, A1, and B1
2) look at the Lab values of your printed best attempt using the profile. Say those values are L2, A2, and B2
3) Then Delta L = L1-L2, Delta A = A1-A2, Delta B = B1-B2
4) Add the calculated deltas to the color values you printed
5) Change the colors to the newly calculated color values
6) Print again and see if you're happy.
7) If you're not happy, scan again and see if you either
 a ) didn't change a given channel enough
 b ) went too far
Cool if its ( a ) then just do the above again for that particular channel
9) if its ( b ) then take your original delta for that channel and halve it, then apply that instead halving the differences repeatedly until you're happy.

Incidentally, you'll definitely want to have the glossy or matte pantone swatches appropriate to your print surface. The scan simply wont work right for comparison otherwise.

(alternatively you can get a profile made specifically for those colors, I could easily do it with my Isis for example)

xilvar
« Last Edit: July 04, 2009, 01:23:45 AM by xilvar » Logged
ormike
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« Reply #6 on: July 09, 2009, 05:01:29 AM »
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There is a piece of software built specifically for matching spot colors on digital printers.
It is called CATZper and can be found at www.nazdarconsulting.com
There is a 15 or 30 day full function trial version available at N/C.
(BTW, I work for another division of this company)
Good Luck
« Last Edit: July 09, 2009, 05:02:09 AM by ormike » Logged
SarahNewman
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« Reply #7 on: August 20, 2009, 07:42:11 AM »
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Alan,

Spectraflow has developed a software solution to achieve your
goals. Please give Clark a call if you would like to discuss further.


Clark Omholt
Spectraflow
Color Workflow Solutions
clark@spectraflow.com
Office: 415-382-8681
Mobile: 415-225-6644
www.spectraflow.com
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Happy Printing!

Sarah Newman
Spectraflow, Inc.
Arkady
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« Reply #8 on: September 03, 2009, 02:10:37 PM »
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Quote from: AFairley
This is not really a usual profiling situation, since (a) there are 8 discrete colors involved (no blending, shading, ect, just solid color areas) and ( I can tweak each of them individually in Photoshop to match my target swatches (in theory).

Alan. Generally a spectral approach will be the ticket but would be an overkill sometimes. The mentioned I1 colormatch would be another reasonable path, but it does address color match only under one light source - D50.

Though the simplest way could be

1. Take Lab value of a patch (it is printed on the Panton swatch book)
2. Open you PS document.
3. Convert the doc to the Lab space
4. In PS palet of you favorite filling tool dial Lab numbers from the swatch book (found in item 1)
5. Create, let say a square and fill with the selected color.
6. Convert back to the original space you use for editing (probably sRGB or Adobe RGB).
7. Roll mouse over the square and read the RGB numbers you should used to create appropriate color.

Keep in mind that it will only more or less work for D50 light source.  And more over do gamut check because Panton colors maybe not printable on your printer.
« Last Edit: September 03, 2009, 02:11:40 PM by Arkady » Logged

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