Line Art - intersection lines do not receive weight transfers #94033

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System Information
Operating system: Win10
Graphics card: NVIDIA GeForce GTX 1660 Ti with Max-Q Design

Blender Version
Broken: blender-3.1.0-alpha master 03015a9b22 windows amd64 release
Broken: blender-2.93
Worked: None?

Line Art can use vertex weights transferred from meshes to modulate line effects. This works on contours and other type of lines, but not intersections.

Exact steps for others to reproduce the error
1- setup a Line Art scene with two spheres interescting
2- paint random weights in a vertex group
3- use the Vertex Weights Transfer to access the weights through Line Art
4- add a Thickness modifier to the line
5- set the Vertex Group on the Thickness modifier

intersection_weight_debug.blend

**System Information** Operating system: Win10 Graphics card: NVIDIA GeForce GTX 1660 Ti with Max-Q Design **Blender Version** Broken: blender-3.1.0-alpha master 03015a9b222e windows amd64 release Broken: blender-2.93 Worked: None? Line Art can use vertex weights transferred from meshes to modulate line effects. This works on contours and other type of lines, but not intersections. **Exact steps for others to reproduce the error** 1- setup a Line Art scene with two spheres interescting 2- paint random weights in a vertex group 3- use the Vertex Weights Transfer to access the weights through Line Art 4- add a Thickness modifier to the line 5- set the Vertex Group on the Thickness modifier ![lineart_intersection_weights.png](https://archive.blender.org/developer/F12737802/lineart_intersection_weights.png) [intersection_weight_debug.blend](https://archive.blender.org/developer/F12737804/intersection_weight_debug.blend)

Added subscriber: @AntoineP

Added subscribers: @ChengduLittleA, @lichtwerk

Changed status from 'Needs Triage' to: 'Needs Developer To Reproduce'

Can confirm the behavior.
However this might not be a bug (it is also not clear to me which of the underlying vertexgroups it should use in the case of intersections? after all there are at least two to consider then)

So it looks a bit like a feature request, will let @ChengduLittleA decide

Can confirm the behavior. However this might not be a bug (it is also not clear to me which of the underlying vertexgroups it should use in the case of intersections? after all there are at least two to consider then) So it looks a bit like a feature request, will let @ChengduLittleA decide

Hi there. Thanks for reporting.

Indeed this has not been implemented yet, but this function could be added relatively easily. I'll add this into my todo.

There's some ambiguity in this though, like if there's a point on one triangle, you could interpolate the weight pretty easily, but there's an intersection, it's coming from two points from two triangles, how would you like the weight to behave? I can do like maybe just an average, or min/max/multiply... This needs specific use case to determine. Maybe averaging can be enough? Would like to hear your thoughts @AntoineP :D

Hi there. Thanks for reporting. Indeed this has not been implemented yet, but this function could be added relatively easily. I'll add this into my todo. There's some ambiguity in this though, like if there's a point on one triangle, you could interpolate the weight pretty easily, but there's an intersection, it's coming from two points from two triangles, how would you like the weight to behave? I can do like maybe just an average, or min/max/multiply... This needs specific use case to determine. Maybe averaging can be enough? Would like to hear your thoughts @AntoineP :D

Changed status from 'Needs Developer To Reproduce' to: 'Confirmed'

Hello! Thanks for taking some time to think about this.

I'm using the vertex weights to apply a noise to the line in a way that is temporally stable. The noise was generated from a world-space texture, so for this specific case the blending method is not important: both objects will have the same weights if they are aligned in world space, such as in intersecting areas.
For general cases, averaging seems to be the obvious first choice, but it's not always suitable. For example: If I'm trying to hide some lines using a painted B&W mask linked to the opacity line modifier, the averaged mask may end up gray and half-transparent. In that case, I would prefer a solution with no blending at all, such as always favouring one of the objects. But that would require the objects to be sorted, I'm not sure if that is possible?

Would it be too confusing to expose the blending mode to the user?
In any case, the current behaviour of dropping all the data is the hardest to deal with artistically, I'd be happy with any blend mode!

Hello! Thanks for taking some time to think about this. I'm using the vertex weights to apply a noise to the line in a way that is temporally stable. The noise was generated from a world-space texture, so for this specific case the blending method is not important: both objects will have the same weights if they are aligned in world space, such as in intersecting areas. For general cases, averaging seems to be the obvious first choice, but it's not always suitable. For example: If I'm trying to hide some lines using a painted B&W mask linked to the opacity line modifier, the averaged mask may end up gray and half-transparent. In that case, I would prefer a solution with no blending at all, such as always favouring one of the objects. But that would require the objects to be sorted, I'm not sure if that is possible? Would it be too confusing to expose the blending mode to the user? In any case, the current behaviour of dropping all the data is the hardest to deal with artistically, I'd be happy with any blend mode!

Hi!

For the use case you described... it's quite easy to have a "min/max/multiply/average/add" mode because these are order-irrelevant. I actually couldn't really think there would potentially be more use cases out of these modes. Maybe it's good enough this way. The weight can be interpolated from the triangles they are on.

I'll add to my notes. Thanks!

Hi! For the use case you described... it's quite easy to have a "min/max/multiply/average/add" mode because these are order-irrelevant. I actually couldn't really think there _would_ potentially be more use cases out of these modes. Maybe it's good enough this way. The weight can be interpolated from the triangles they are on. I'll add to my notes. Thanks!

I would be very happy with the blend modes you mentioned and I agree it would be good enough. However, if you are thinking about use cases, my initial problem would be best solved without blending. The noise that generates the thickness differences is tuned to create a specific look, and any of the blend modes mentionned would have a pretty visible effect on it. In my opinion the perfect solution for this use case is to keep the weight from only one of the objects, a bit similar to how freestyle has a priority system for overlapping segments.

@AntoineP Hi! In the latest development of line art, it's technically possible to assign weight from one of the two objects, it may require more research as line art itself doesn't store weight info during the calculation at the moment,. But we are getting closer :)

Gotcha! Thank you for taking the time to reply and for all the amazing work!

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Line Art - intersection lines do not receive weight transfers #94033