10 Replies Latest reply on Apr 13, 2018 11:49 AM by D Fosse

    Brush Blend Modes Question

    HOWARD_UK

       

      Could somebody please help me understand something I've long wondered about...

       

      Why, when we paint on non transparent areas of a layer with a Brush Blend Mode other than Normal, the effect is as expected (Hue, Sat, etc change accordingly)  but... when brushed over unfilled, transparent areas, it is as if we are painting with Normal mode?

       

      I have never understood this: why, if we are painting only to change e.g. the Hue of what's there would we want blank areas to not remain blank?

       

      I hope this screenshot illustrates what I mean...

       

      Dropbox - Adobe Forum - Brush Blend modes.png

       

       

        • 1. Re: Brush Blend Modes Question
          davescm Adobe Community Professional

          A blend (brush or layer) can only happen if there is something to blend with. If there are no pixels to blend with then the result will just be the brush (or layer) content.

           

          Dave

          1 person found this helpful
          • 2. Re: Brush Blend Modes Question
            HOWARD_UK Level 1

            Thank you Dave...

             

            But I hoped my illustration made clear that I do (not expertly) understand what happens...that, as you say: If there are no pixels to blend with then the result will just be the brush (...) content.

             

            My question is "Why?"

             

            Does not logic suggest that if a brush is set not to cover but to blend, and there are no pixels to be blended, then there would/should be no result at all?

             

            Because isn't that the point: to Blend is not - by definition - to Cover?

             

            A brushstroke of blue on a Layer with Blend mode set to Hue does not cause empty areas of the Layer beneath to appear as blue.

             

            So why should a Brush with Blend Mode of Hue cover empty areas with Blue?

             

            I have never myself come across a situation where that is what I'd choose to happen. Perhaps you, or somebody, could suggest where or how it could be of any practical use?

            • 3. Re: Brush Blend Modes Question
              JohanEl54 Adobe Community Professional

              A brush paints pixels, so if you paint over an empty area, it fills that empty area with pixels too. The blend mode of the brush determines how existing pixels are 'mixed' with the pixels painted by the brush. If there are no existing pixels, then nothing gets mixed and so the result is what you painted.

               

              A brushstroke of blue on a layer with Blend mode set to Hue does not cause empty areas of the Layer beneath to appear as blue, because the area beneath is still empty. That's the difference between painting with the brush set to a certain blend mode, and painting on a layer set to the same blend mode.

              • 4. Re: Brush Blend Modes Question
                HOWARD_UK Level 1

                Johan, thank you. Yes, you and Davesscm describe well what happens. Sorry if I haven't expressed myself clearly: My curiosity is about why.

                 

                Put crudely (and please understand, I’m no technician!)… :  if pixels to interact with, then Blend; if no pixels, then Cover.

                 

                My question (my puzzlement) is why, when defining Brush Blend Mode behaviour, was it decided that, where there are no pixels to interact with, it should be Cover rather than Do Nothing ?

                 

                To give a simplistic example… (and leaving aside use of masks)…

                We have on a transparent layer a fishing net. We want to brushstroke a change of hue across a part or parts of that net.

                As things stand, a brush loaded with Blue, and with Blend Mode set to Hue, stroked across would paint all areas between the mesh of that net Blue.

                 

                Who would want that? To me, that behaviour seems (always has) incomprehensible. Counter-intuitive. Of no sense at all..

                 

                Why not have it that those blank, no-pixel-content, areas are left unaffected?

                 

                Perhaps I could ask it this way: what advantage or benefit is there to the current behaviour -  blank areas being covered? What disadvantage would there be to a behaviour that left blank areas blank?

                 

                If somebody could offer an example, I would sincerely be interested to know.

                • 5. Re: Brush Blend Modes Question
                  davescm Adobe Community Professional

                  Hi Howard

                  Blending modes require two layers or (in the case of brushes)  two sets of pixels to work. A brush will always lay down pixels - the blending mode just describes how those new pixels react with any existing pixels. Without two sets there is no blending so the pixels in the upper layer (or brush stroke) are applied without blending.

                   

                  In the case of layers the upper layer could be created first then the lower layer added second. If blending did not show pixels until the lower layer was available then there would be nothing to see.

                   

                   

                   

                  In your net with transparency example , just paint onto an empty layer (set to Hue blending mode) and clip that layer to the layer below (Alt-Click on the border in the layers panel)

                   

                  Dave

                  • 6. Re: Brush Blend Modes Question
                    JohanEl54 Adobe Community Professional

                    The why is probably best explained if you realise that blending is simply math. The new pixel is calculated based on a certain 'blend formula', and that formula gives you the original value when the other value is zero.  Mind you, some other blend modes may indeed result in nothing, I haven't tried them all. Just like 1+0=1, but 1x0=0.

                    • 7. Re: Brush Blend Modes Question
                      HOWARD_UK Level 1

                      I promise I'm not being stubborn for the sake of it...!  But the thing I'm still questioning is why (with Brush Blend modes) it seems to be considered a written-in-stone, law-of-physics inevitability that if there are no pixels to blend with then 'no blending' has to mean laying down pixels as normal, as if that is the only technical possibility?

                       

                      As you say, Johan, it is all mathematics. A formula decides: if this then a, if that then b.  My whole point is why does b have to be 'normal pixel' and not 'no pixel'?

                       

                      To me this seems to be just an accepted, generally adopted, convention. But one I really can't see has anything obvious in its favour.

                       

                      That we need (as in your example, Dave) to use masking to have a blend mode affect only the drawn areas seems just to beg the question of why that should be necessary - when the result could be so simply and directly achieved if 'no blending' was formulated to result in 'no pixel'...?

                       

                      I can only ask again what would be either a) the impossibility or b) the disadvantage of that?

                       

                      (Or, conversely, what are the benefits of how it is now?)

                       

                      [ Just to be clear: In all this, I'm only talking about Brush Blend modes - the logic behind Layer Blend Modes is clear ].

                      • 8. Re: Brush Blend Modes Question
                        davescm Adobe Community Professional

                        The fact is we would all be guessing as to why historically, the brush blend mode was made to work the way it does, as we don't work for Adobe.
                        My guess would be - to make the blending mode work the same between layers and brushes.

                         

                        Dave

                        • 9. Re: Brush Blend Modes Question
                          JohanEl54 Adobe Community Professional

                          HOWARD_UK  wrote

                           

                          I promise I'm not being stubborn for the sake of it...!  But the thing I'm still questioning is why (with Brush Blend modes) it seems to be considered a written-in-stone, law-of-physics inevitability that if there are no pixels to blend with then 'no blending' has to mean laying down pixels as normal, as if that is the only technical possibility?

                           

                          As you say, Johan, it is all mathematics. A formula decides: if this then a, if that then b.  My whole point is why does b have to be 'normal pixel' and not 'no pixel'?

                           

                          I don't think it is a formula like 'If a then this, if b then that', it's a purely mathematical formula like 'result = a + b' or 'result = a x b'. If b is zero then the first result will be a, and the second result will be zero. Even if you don't like it and would prefer 'a + zero = zero'.

                           

                          I'm going to end this because it will lead to nothing. It is as it is.

                          • 10. Re: Brush Blend Modes Question
                            D Fosse Adobe Community Professional & MVP

                            My favorite zen koan: What is the sound of one hand clapping?