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dmstraker

AP blend modes - technical detail, please

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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!


Dave Straker

Cameras: Sony A7R2, RX100V

Computers: Win10: Chillblast Photo with i7-3770 + 16Gb RAM + Philips 40in 4K; Surface Pro 4 i5

Favourite word: Aha. For me and for others.

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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!

post-8578-0-08298000-1498648262_thumb.jpg


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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.


Dave Straker

Cameras: Sony A7R2, RX100V

Computers: Win10: Chillblast Photo with i7-3770 + 16Gb RAM + Philips 40in 4K; Surface Pro 4 i5

Favourite word: Aha. For me and for others.

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