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Fill and Opacity

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  • Fill and Opacity

    Another thing that bugs me that I thought I understood is the difference between Fill and Opacity: Opacity changes the whole layer, and Fill only pixel transparency.
    But I've read many years ago that some Blend modes behave differently if you lower the opacity or the fill (sometime referred as the 'special 8'). Why is that? How it works exactly?
    Can you explain it to me? Thank you very much.

  • #2
    Re: Fill and Opacity

    Does this help?
    Original image by Vitaly Druchinin retouching by Natalia Taffarel I'm going to in Buenos Aires, Colombia, Rome, Asia Find out more here Retouch…


    • #3
      Re: Fill and Opacity

      I knew that one, but she didn't explain it. In the 1st part she shows the thing about I wrote before (pixel vs effect). And in the 2nd part she just shows the light, but never explains why it works like that. I'm interested in what is really happening.

      Tank you for the video!


      • #4
        Re: Fill and Opacity

        Check the following 2 tutorials and I guess you will get clear idea -
        Learn the difference between the Opacity and Fill transparency options in the Layers panel in Photoshop.

        Best of luck


        • #5
          Re: Fill and Opacity

          Thank you Shumicpi.

          The Retouching Academy video actually raises the issue of blend modes differences under fill/opacity, but it won't explain why. I'm not nitpicking here, I just feel, (just like with the frequency separation), that if I use something I don't understand (not fully, but at least the big picture), it's like wearing someone else's clothes... I'm doing it (for many-many years now), mimicking the methods, but I don't understand what is behind the technique.


          • #6
            Re: Fill and Opacity

            Perhaps a better description of how the modes work and the maths behind modes including the so called special 8 will be found here
            A Detailed description of how Photoshop blend modes work. The math involved, and their interrelationships with each other are discussed in depth.


            • #7
              Re: Fill and Opacity

              Actually it's really simple, but not widely known (or at least I didn't find this information when I first researched it).
              Normal opacity does its math after calculating the blend mode results, fill opacity does it before.
              I'm going to call the background layer B for short and the above layer A. The result will be called C.
              Let's take an image with R = 200 and use "Linear Dodge (Add)" to add an image with R = 100 to it (I'm using this mode because it's really just A + B = C).
              The opacity formula is Opacity * A + (1 - Opacity) * B = C.
              When you have 100% opacity and fill you get C = A + B = 300 = 255 because the values are clipped.
              Now when you lower the opacity to 50% this becomes 0.5 * C + (1 - 0.5) * B = 0.5 * 255 + (1 - 0.5) * B = 227.5 = 228. It's important to notice here that we're not using the original 300, but the clipped 255 from the addition.
              Now let's try 50% fill opacity instead: 0.5 * (A + B) + (1 - 0.5) * B = 250. Simply by not clipping the results before applying the opacity formula the result changes dramatically.

              PS: If you try the same in 32bpc mode, both fill and normal opacity will be the same obviously.


              • #8
                Re: Fill and Opacity

                Um... Isn't it actually the division by two of the top pixel before the calculation?

                Like A+0.5*B ?


                • #9
                  Re: Fill and Opacity

                  Yes, when you simplify it, it's just background + opacity * top layer for this blend mode.
                  However it gets more complicated when you use a different blend mode (e.g. "Linear Light" - B + 2 * A - 255), that's why I didn't simplify it :-).

                  Example: A = 215, B = 136, C (100% opacity and fill) = 311
                  Using normal opacity 30%: C = 0.3 * 255 + 0.7 * 136 = 171.7 = 172
                  Using fill opacity 30%: C = 0.3 * 311 + 0.7 * 136 = 188.5 = 189


                  • #10
                    Re: Fill and Opacity

                    Thank you very much! That's what I was looking for.


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