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harveyn10244512
Participating Frequently
November 27, 2017
Answered

Darker Color blend mode behaviour

  • November 27, 2017
  • 7 replies
  • 4055 views

This came about while I was trying to understand the difference between

the Darken and Darker Color blending modes.

The documented explanation for the Darker Color blending seems straightforward:

Darker Color

Compares the total of all channel values for the blend and base color and displays the lower value color.

But this does not always seem to be the case in RGB mode.

If I fill a base layer with sRGB (225,161,49) and a blend layer with sRGB (174,174,174)

then I expect Darker Color blending will compare the totals of the RGB values (435 vs. 522),

determine that the base layer is "darker" (435<522) and thus display the base color,

which is indeed the result.

I would also expect similar results for any permutation of RGB values which total 522.

For example Darker Color does interpret the base color as darker than sRGB (194,154,174)

and sRGB (154,194,174).

However, when I fill a blend layer with sRGB (174,154,194), Darker Color blending

displays this color instead the base color which has the lower total RGB value.

Conversely Darker Color blending displays the base color instead of sRGB (200,200,0)

which has a lower total RGB value (400 vs. 435).

Clearly my understanding of this blend mode is inconsistent with my observations.

Could someone offer a more accurate explanation?

Or is this a bug?

(I am using CC 2018 Release 19.0.)

This topic has been closed for replies.
Correct answer The Scott Valentine

The actual comparison is done using luminance, which is a weighting of each channel based on human color perception:

Y(Luminance) = SUM(R*0.2126 + G*0.7152 + B*0.0722)

(these may not be exact for Photoshop - they're just common weights)

So for a base of (225, 161, 49), you get a summed value of 435, but a weighted value of 166.52

A blend of (174, 154, 194) gives a summed value of 522, but a weighted value of 161.14

In this example, the blend wins the weighting, but loses the summation.

7 replies

Bob_Hallam
Bob_Hallam
Legend
November 28, 2017

All of the color blend modes use CIELab as their basis for the changes.  There will be no easy to see relationships to RGB or CMYK values.  

ICC programmer and developer, Photographer, artist and color management expert, Print standards and process expert.
sharp_hands16B8
Community Expert
sharp_hands16B8Community Expert
Community Expert
November 28, 2017

You can check out Scott's book here. It has the calculations in it, and he's very much an authority on it. That's why so many of the industry best volunteered to be in his book. I doubt the engineering team is going to tell you anything more.https://www.amazon.com/Hidden-Power-Blend-Modes-Photoshop/dp/0321823761

https://www.amazon.com/Hidden-Power-Blend-Modes-Photoshop/dp/0321823761

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    November 28, 2017

    So I learned something new here.

    I normally don't use blend modes I don't understand, because then it just becomes a random effect I can't control or visualize. Darker Color is, I confess, one of them.

      The Scott Valentine
      Community Expert
      The Scott ValentineCommunity Expert
      Community Expert
      November 28, 2017

      Here's the spreadsheet: Darker Color Blend Mode - Google Sheets

      You can edit the white number cells with RGB values, and the red cells below them will update. The light blue cells are the weighting factors. As you change values, the empty cells to the right of the matrix will show which of the layers is going to show up on the canvas.

      ------------------------------------------------------------Never let your tools get in the way of your art!
      The Scott Valentine
      Community Expert
      The Scott ValentineCommunity ExpertCorrect answer
      Community Expert
      November 27, 2017

      The actual comparison is done using luminance, which is a weighting of each channel based on human color perception:

      Y(Luminance) = SUM(R*0.2126 + G*0.7152 + B*0.0722)

      (these may not be exact for Photoshop - they're just common weights)

      So for a base of (225, 161, 49), you get a summed value of 435, but a weighted value of 166.52

      A blend of (174, 154, 194) gives a summed value of 522, but a weighted value of 161.14

      In this example, the blend wins the weighting, but loses the summation.

      ------------------------------------------------------------Never let your tools get in the way of your art!
      Chuck Uebele
      Community Expert
      Chuck UebeleCommunity Expert
      Community Expert
      November 27, 2017

      Got a responds from my friend. He's going to look at it later then he gets home, but basically he said that while the darker color is suppose to be a comparison of all the channels, it also includes a normalization process that accounts for human perception. You can kind of see how Adobe applies this in Camera Raw when you do a B&W conversion. The default slider values aren't all at 0:

        H
        harveyn10244512Author
        Participating Frequently
        November 28, 2017

        Thank-you for your replies, Chuck.

        Your first reply reassures me that I am not alone in seeing this.

        Your second reply jives with Scott Valentine's later reply,

        so hopefully we're all heading towards a better understanding

        of this blend mode.

        The Scott Valentine
        Community Expert
        The Scott ValentineCommunity Expert
        Community Expert
        November 28, 2017

        I'm the guy Chuck is referring to 

        For a fun exercise, you can create a spreadsheet to do the calculations. I sent one to Chuck to play with that identifies whether the base or blend color shows up in the result - I'll try and create one in Google Docs so you can see how it works.

        ------------------------------------------------------------Never let your tools get in the way of your art!
        D Fosse
        Community Expert
        D FosseCommunity Expert
        Community Expert
        November 27, 2017

        This works per channel, you're not supposed to add the numbers up.

        If your base layer is red 255, and the blend layer red 174, then the result is red 174.

        And then in your example you have base green 161, and blend green 174, so the result is green 161.

        And so on...

        H
        harveyn10244512Author
        Participating Frequently
        November 27, 2017

        Thank-you for your reply, however your explanation is for DARKEN blending.

        DARKER COLOR blending is supposed to differ in the way "darker" is determined and

        is supposed to display one color or the other, not mix values to display a third color.

        Here are the explanations from the user guide page (dated February 15, 2017):

        Darken

        Looks at the color information in each channel and selects the base or blend color—whichever is darker—as the result color. Pixels lighter than the blend color are replaced, and pixels darker than the blend color do not change.

        Darker Color

        Compares the total of all channel values for the blend and base color and displays the lower value color. Darker Color does not produce a third color, which can result from the Darken blend, because it chooses the lowest channel values from both the base and the blend color to create the result color.

        Blending modes in Adobe Photoshop

        Chuck Uebele
        Community Expert
        Chuck UebeleCommunity Expert
        Community Expert
        November 27, 2017

        Interesting. I see what you're saying. Not sure how they calculate the results. Just asked a friend who wrote a book on it. Hopefully, he will chime in.

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