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_Shortys_
Participant
October 7, 2006
Question

RGB+SpotColor to RGB

  • October 7, 2006
  • 5 replies
  • 1524 views
Hi...

I'm working on the following problem for some weeks:
I need the algorithm or the mathematical formula for the method Photoshop uses to mix a spot color channel into the three RGB Channels.
For example:
I have an Image with 4 channels R,G,B and the Spot Color Channel with Solidity 0; So Photoshop CS and CS2 has the function to "mix" the spot color channel into the R,G,B Channels so that I only have three channels R,G,B !

I tried to figure out the formula by comparing the red, green and blue values before and after the "mixing", but the values make no sense for me. It's not the well known "Alpha Blending" formula.

Perhaps any of u can help me?

Thanks in advance
Cheers
Shortys
This topic has been closed for replies.

5 replies

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_Aaron_Sher_
Known Participant
June 29, 2008
No hurry - if I can't get the spot colors from Photoshop, the algorithm doesn't do me a lot of good. I'm still interested, but it's not time-critical.

Thanks,
Aaron
W
welfred88
Participant
May 18, 2010

Could anybody knows how to get the spot colors from photoshop?

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_Shortys_Author
Participant
June 25, 2008
Sry for the delay. At the moment I am very busy :(...
I did not work inside Adobe Photoshop SDK. I worked on developing an independent preview program which can preview TIFFs with spot colors.
I came to the algorithm by studying the RGB values of the original picture and the RGB values of the spot color channel before mxing them and then comparing them by the mixed RGB values.

The program is written in Java and I have to put the transformation algo in generic psedudo code. I will try to hurry ;)

Cheers
Sascha Homeier
SebLeon
SebLeon
Participant
September 18, 2017

Hi Sacha, Aaron,

If you still get the code that can mimmic Photoshop math behind mixing spot colors,I would be very happy to get it !

Thank you !

Sébastien

T
Test Screen Name
Legend
October 4, 2017

It's going to be something like this.

A spot colour has an alternative representation in Lab, RGB or CMYK. First convert to the alternative representation. If your tint is not 100%, scale appropriately unless you have a complete mapping to use instead.

Next, if you have Lab or CMYK convert to the RGB colour space. If you have RGB in a different space, convert to the working space.

If the ink has 100% solidity, replace the pixel values. If it has 0% solidity, throw away your values. Otherwise, use a transparency blend; my first instinct would be to use a blend mode of normal and solidity = opacity.

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_Aaron_Sher_
Known Participant
June 19, 2008
Yes, I'm definitely interested. Were you working inside a plugin? If so, how did you find out what the channel colors were? The information doesn't seem to be provided.

Thanks,
Aaron
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_Shortys_Author
Participant
June 19, 2008
Hi Aaron,

I found an interpolation which approximates the "real" RGB values in Photoshop. In the most cases it does not draw visual distinctions but in some cases with some colors and some Solidity value combinations it differs a bit from the real Photoshop mix.

If you are still interested in this I can see if I can put the algo into Pseudocode and post it to you ;)

Cheers
Sascha Homeier
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_Aaron_Sher_
Known Participant
June 18, 2008
Did you ever solve this? I need this information as well.

Thanks,
Aaron
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