"Saturation" Blend Mode does not work on Yellow

mx-lu · August 23, 2025, 5:03pm

Hello! I signed up here just because this bug is so funny.
I am messing around with color swatches, and trying to desaturate my colors using a new layer in the blend mode “saturation”. And it does not work if the hue is a perfect yellow. The value does not matter, and if the hue shifts only one to the left or right, it also works, but perfect yellow specifically is unaffected by the blend mode.

To reproduce:

Step 1: Have a bunch of colors

Step 2: Put a grey rectangle on top

Step 3 (optional): Change the blend mode. Here “Multiply” to show Yellow is normally affected in other blend modes.

Step 4: Set blend mode to “Saturation.” Watch all colors except yellow be affected. You can even see I mis-swatched in the bottom row.

behreandtjeremy · August 24, 2025, 11:38pm

Also looks like Hue blend mode is impacted when the gray layer is beneath a colorful layer.

Your issue might be similar to this issue from 4 years ago

github.com/aseprite/aseprite

Bug in Saturation layer blend mode

opened 09:56PM - 09 Mar 21 UTC

alpine-alpaca

bug render

I noticed that the [`set_sat`](https://github.com/aseprite/aseprite/blob/master/…src/doc/blend_funcs.cpp#L400) function has a bug when the two lower components are the same value. This can be shown in the following test file: [blend_saturation_bug.zip](https://github.com/aseprite/aseprite/files/6111637/blend_saturation_bug.zip) It looks as follows: ![blend_saturation_bug](https://user-images.githubusercontent.com/80179070/110542948-fc366e80-8129-11eb-9995-91c499a1667f.png) ![blend_saturation_bug_4x](https://user-images.githubusercontent.com/80179070/110543350-8088f180-812a-11eb-9754-b9cf775734ae.png) The top layer is all the same color and uses blend mode "Saturation". The bottom contains three strips of slightly different blues. The middle has R value = G value. Note how the merged value suddenly jumps colors when they should all have a very similar hue? ### Aseprite and System version * Aseprite version: 1.2.25-x64 * System: Windows

where multiple blend modes rely on helper methods defined in a reference doc here.

github.com/aseprite/aseprite

src/doc/blend_funcs.cpp

3129fda97


      
          {
            if (i == 0b001)
              return r;
            if (i == 0b010)
              return g;
          
            return b;
          }
          
          // TODO replace this with a better impl (and test this, not sure if it's correct)
          static void set_sat(double& r, double& g, double& b, double s)
          {
          #undef MIN
          #undef MAX
          #undef MID
          #define MIN(x, y) (((x) < (y)) ? (x) : (y))
          #define MAX(x, y) (((x) > (y)) ? (x) : (y))
          #define MID(x, y, z)                                                                               \
            ((x) > (y) ? ((y) > (z) ? (y) : ((x) > (z) ? (z) : (x))) :                                       \
                         ((y) > (z) ? ((z) > (x) ? (z) : (x)) : (y)))

The replacement method given in the comments by alpine-alpaca seems to fix the issue:

Gasparoken · August 25, 2025, 5:29pm

Thank you @mx-lu and @behreandtjeremy. I was able to reproduce it. We’ll fix it soon.