AP blend modes - technical detail, please

[dmstraker]

AP adds blend modes Average, Negation, Reflect, Glow, Contrast Negate and Erase.

 

Is there a detailed technical description of how these work, including the blend logic, please? Rationale for inclusion and ideas for use also would be appreciated.

 

Also, is there any reason why Divide blend mode is not included?

 

Thanks!

[James Ritson]

Hi Dave, I'll try and give a quick breakdown, though it has prompted me to think that the in-app help blend modes topic needs fleshing out a bit... Not sure I can suggest practical use cases for them all yet.

• Reflect darkens the image using values from the composite layer. It's great for selectively enhancing parts of an image like reflections or areas of light.
• Glow essentially performs the same as Reflect but flips the layer order, so it brightens the image using values from the composite layer. Great for widening the radius of artificial lighting and making it more intense.
• Average is exactly that: a mathematical average between the image and the composite layer. In most cases it's the same as setting the composite layer's opacity to 50%.
• Erase will subtract from the image using the composite layer as a "mask" - the strength of this is controlled through Opacity rather than the composite layer's actual contents.
• Negation is like Difference (which subtracts pixel values between the composite and image layer) but is additive, so pixel values are simply added together from the layers - resulting in brightening rather than darkening.
• Contrast Negate I'm not sure exactly, but it seems to invert the pixel values of the composite layer based on the image layer's content. Useful for certain designs. I've attached a really rough example to this post.

Not sure about Divide's no-show, I'll have to ask one of the developers at some point. Hope that helps!

[dmstraker]

Thanks, James. Very helpful. I'm using PS terminology, so assume 'composite layer' is top 'blend layer'.

 

I'm also trying to figure out the algorithm for each. For example I reckon that for Erase, Result opacity = Base opacity * (1 – Blend opacity). Are you able to ask developers about this? It would save some reverse engineering and guesswork.

 

I suspect the lack of Divide is because it's relatively recent (CS5 2005), although Subtract came in at the same time. Dissolve is also missing, though less of a loss.

[spinhead]

And 5 years later on, I'm still searching for the math behind Glow, Negation, and Reflect. Most tools and sites don't acknowledge the existence of these blend modes. Obviously, the Affinity engineers know what these three are doing, and even perhaps clarify Difference, for which I find differing and incompatible calculations.

I plan to play around and fully understand the blend modes as intuitively as I can, and it actually helps me to know the math.

[Lisbon]

Hi @spinhead

"Blend" is the layer that is on top.

If Im not mistaken these are the formulas youre looking for.

Negation
1-abs(1 - Base - Blend)

Reflect
If Blend=1 then final result = 1
else Base^2/(1-Blend)

Glow
Same as reflect, but with the layers swapped.

Hope this helps.

[spinhead]

Excellent. Just seeing the calcs for Negation, it already makes more sense.