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Documentation for blend modes
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Description

Blend modes can be used in several places in Blender:

  1. Shader Editor (mixRGB node): 18 modes
  2. Compositor (mix node): 18 modes
  3. Grease Pencil (layers): 6 modes
  4. Texture & vertex painting (brush): 24 modes
  5. Video Sequence Editor (strips): 27 modes

In the Video Editing section about Effect Strips (video_editing/sequencer/strips) and the Compositing section about the Mix node (compositing/types/color/mix) there is a bit of documentation. For the shading, grease pencil and texture painting there is only a link to the Glossary. This glossary in its turn contains a link to the external GIMP documentation; which uses more or less the same concepts.

It was my intention to update this documentation and therefore I did some background research. However, it soon became apparent that:

  • There is some controversy about the use of blend modes in a scene referred environment as Blender (cfr. comments by Troy Sobotka in https://devtalk.blender.org/t/solved-blend-modes-naming-inconsistency/8006/8)
  • Blend modes have intimate interconnections with other basic concepts as: color model, color space, color management, masking, … and is as such difficult to document.
  • The theoretical foundation is most of the time rather simple, e.g. the mathematical formula for multiplication is A * B. However, predicting the result of these blend modes with real images is often very hard (not in the first place because the RGB color model is not very intuitive for humans).
  • Most tutorials therefore stick to the advice: "try a few things" with no attempt to stimulate understanding of what is going on.

Proposal for a complete rewrite of the Blend mode sections. Feedback is appreciated.

  • Centralize the documentation for blend modes in the Video Editing section. The VSE is a display referred environment and has as-of-today the most blend modes. It 's also easier to demonstrate the real-life results of the blend modes in the VSE.
  • Add a specific chapter for the safe modes in a scene referred environment and link to this chapter in the other sections (mixRGB, Grease Pencil, …)
  • Each mode description contains a more theoretical and practical approach:
  • Theoretical: underlying mathematical formula and graph for a stylized input (e.g. grey scale gradients) + how to interpret this formula/graph.
  • Practical: demo with realistic images and/or techniques e.g. masking techniques.

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Event Timeline

Hugo Schouppe (HUSCH) changed the task status from Needs Triage to Needs Information from User.Wed, May 20, 11:56 AM
Hugo Schouppe (HUSCH) created this task.

I will add my opinions here, because I have some. Don't consider me an expert though :)

There is some controversy about the use of blend modes in a scene referred environment as Blender (cfr. comments by Troy Sobotka in https://devtalk.blender.org/t/solved-blend-modes-naming-inconsistency/8006/8)

I wouldn't say this is much a controversy. If some blend modes are funny like Linear Burn, according to Troy, I think second best thing is to document it as-is.

Blend modes have intimate interconnections with other basic concepts as: color model, color space, color management, masking, … and is as such difficult to document.

There is a correct method how they are applied. I don't think relationship is subject for much documentation. This should be handled properly by software.

In some cases we don't apply these blend modes in linear colorspace (eg in VSE). I have been looking in T75844: Blend modes are broken, and there seems to be wrong math in Lighten and Dodge, but these are subjects to be fixed, rather than documented.

Centralize the documentation for blend modes in the Video Editing section. The VSE is a display referred environment and has as-of-today the most blend modes. It 's also easier to demonstrate the real-life results of the blend modes in the VSE.

Personally I would create separate section for blend modes and point to it from all other sections, but I wouldn't mind anyway. Also keep in mind my previous point of VSE doing it wrong (likely) haven't looked too deep into this. It should be possible to use strips in linear colorspace and display in sRGB though.

If you will need any help with using VSE for this, feel free to poke me on chat.

Each mode description contains a more theoretical and practical approach:
Theoretical: underlying mathematical formula and graph for a stylized input (e.g. grey scale gradients) + how to interpret this formula/graph.
Practical: demo with realistic images and/or techniques e.g. masking techniques.

Good idea in general. Some blend modes are better used as brushes though so it may be good to use different editor than VSE I would say. It can be demonstrated in VSE though by using mask as image 2, but, I doubdt, that a lot of people will make the connection "oh so I can use this as a brush"

Overall I was using to https://docs.gimp.org/en/gimp-concepts-layer-modes.html as my reference, which we link from our manual, and this looks to be indeed good enough balance between theory, use-case description and demonstration.

Topics like image/video processing are out of scope. Anything more then select this color mode here. We should focus on 3D.
But yes, composing/mix needs to be cleaned up since more than 5 years.
It would be too hard to maintain, because, as you said, you have to be expert and there is a lot of half knowledge around.

There is some controversy about the use of blend modes in a scene referred environment as Blender (cfr. comments by Troy Sobotka in https://devtalk.blender.org/t/solved-blend-modes-naming-inconsistency/8006/8)

This is correct, currently only add, subtract, multiply, and divide properly respect values outside the 0-1 range.

Centralize the documentation for blend modes in the Video Editing section. The VSE is a display referred environment and has as-of-today the most blend modes. It 's also easier to demonstrate the real-life results of the blend modes in the VSE.

I would leave information in the glossary and not one specific page.

Add a specific chapter for the safe modes in a scene referred environment and link to this chapter in the other sections (mixRGB, Grease Pencil, …)

I do not think this is necessary, just explain if the particular blend mode is suited for scene or display referenced imagery.

Theoretical: underlying mathematical formula and graph for a stylized input (e.g. grey scale gradients) + how to interpret this formula/graph.
Practical: demo with realistic images and/or techniques e.g. masking techniques.

I think these points are not worth the extra work of maintaining.

I also do not think it is important to include formulas, this is more technical than the manual needs to be.

Aaron Carlisle (Blendify) changed the task status from Needs Information from User to Confirmed.Thu, May 21, 2:34 AM
Aaron Carlisle (Blendify) changed the subtype of this task from "Report" to "Design".

Thanks for the feedback.

  • Blend modes are used quite a lot at several stages in production. I don’t think they are out of scope in Blender and therefore they should be properly documented, even only as-is.
  • The GIMP documentation is OK but they have lesser/different modes (e.g. dissolve) and use different terms (burn vs color burn, linear burn, ...). This is the same for other software (Krita, Photoshop, Nuke, …). The additional difficulty of display/scene referred environment remains.
  • A separate section (instead of VSE) looks indeed better. Integrating this in the glossary is in my opinion not feasible and also does not serve the purpose of a glossary.
  • I do think that a theoretical foundation (incl formulas) is necessary (there is nothing so practical as a good theory :-). The documentation in the manual is at the moment insufficient to get a clear understanding and the info outside (tutorials, etc) is most of the time along the lines of “click this”, “try that” or a few screenshots of the results.
  • That said, of course, it remains a complex matter. Maybe someone/ expert is interested in working together on this topic? I’m certainly willing to do the preparatory work.

Blend modes are used quite a lot

But that's exactly what makes it basic, thus required, CG knowledge. We wouldn't explain what a mesh is if that wouldn't exist already.

Expert

The thing is there has to be an expert to be there whenever Blender changes and that not the case.
Like it is with color management.

I tend to disagree. Even if (?) Blend modes are ‘basic’, they should be properly explained, be it by a non-expert. In my view, the Blender manual should be the first and most important information source for the Blender user. I don't know if there are any statistics but it's my gut feeling that when a user encounters a problem, he or she will not open the manual first (if at all). There is a reason why there are so many tutorials on YouTube and the internet.

This is partly because of the arguments in the previous discussion (no basic stuff, only experts, …). But maybe, this is the standpoint of the Manual Team. I don’t know.

I only recently started contributing to the documentation. But I have to say, this wasn’t made easy by the Manual Team and the used software. Before continuing on this journey, I would like to hear from the Team if there is room for contributions:

  • About basic things
  • From non-experts
  • In a more ‘open’ format

I would recommend slightly improving the documentation in the glossary and make sure it is linked to by other areas in the manual. The glossary can then link to: https://en.wikipedia.org/wiki/Blend_modes for users who want a more in-depth understanding.