Curve evaluation / to points for instancing #87429

New Issue

Hans Goudey · 2021-04-12T15:40:03+02:00

Hans Goudey commented

2021-04-12 15:40:03 +02:00

Use Case

For many use cases it is necessary to instance geometry along a curve, for things like rock paths, roads, etc. In those situations the following information is needed:

Position
Rotation
Scale (sometimes)
Alternatively a transform matrix could be used directly, but that isn't currently possible in geometry nodes, since the information is stored as attributes.

Background

Currently in geometry nodes you can generate instances on any point data. That includes point clouds, meshes, and curve control points.
With their evaluated tangent and normal information, curves have the unique ability to adjust these parameters very efficiently (quick changes can have big results on many instances).

However, curve control points do not have the required tangent, normal, (and therefore rotation) information for instancing.
To retrieve that information, evaluating the curve is necessary. This can create tangent, normal, and rotation information.

The question is where to store this information:

On curves, this is derived information, meaning changing the curve would "logically" change this data (not necessarily in practice).
On point clouds, this data is simply a flat list. In other words, there is no correspondence between it and the original points.

Generate a new point cloud

This method would not modify the curve but create point cloud data instead, copying over all of the curve attributes to the result.
More operations like "resampling" could be in the same node.

Pros

Simplicity: Separation of data logically (point clouds are "points" more directly than curve splines
No confusion about derived data (the curve is no longer around)

Cons

Complexity: There is no essential need to create a point cloud to do instancing. This is evident with how we handle meshes, where you can easily do instancing on points (even faces in the old system)
Performance: Copying attributes around during evaluation is not at all trivial
Arbitrary: Does not use the fact that attributes are stored on geometry to our advantage. In this case the point cloud is just an "array container"
Code: The required code is harder to maintain, more confusing, and requires more duplication
If there are more operations in this node, they are duplicated from operations that keep the curve a curve.

Evaluate splines

This method takes advantage of the 1 to 1 correspondence of evaluated points and control points on poly splines.
All Bezier and NURB splines are converted to poly splines, and three attributes are added: tangent, normal, and rotation.

Pros

Use curves to our advantage: We can adjust the resulting attributes using methods unique to curves with the maintained "topology" information.
Performance: In many cases only three attributes need to be added. The operation is much simpler. And the data can be used later for other curve-related things.
Standard: Sverchok and AN both have similar nodes
Simplicity: There is no unnecessary change of data type

Cons

Complexity: Exposes users to the different spline types when they might not have known about it before.
Subsequent curve operations maintain these attributes, even they they are derived

{[F10158372](https://archive.blender.org/developer/F10158372/image.png) size=full} ### Use Case For many use cases it is necessary to instance geometry along a curve, for things like rock paths, roads, etc. In those situations the following information is needed: - Position - Rotation - Scale (sometimes) - *Alternatively a transform matrix could be used directly, but that isn't currently possible in geometry nodes, since the information is stored as attributes.* ### Background Currently in geometry nodes you can generate instances on any point data. That includes point clouds, meshes, and curve control points. With their evaluated tangent and normal information, curves have the unique ability to adjust these parameters very efficiently (quick changes can have big results on many instances). However, curve control points do not have the required tangent, normal, (and therefore rotation) information for instancing. To retrieve that information, evaluating the curve is necessary. This can create tangent, normal, and rotation information. The question is where to store this information: - On curves, this is *derived* information, meaning changing the curve would "logically" change this data (not necessarily in practice). - On point clouds, this data is simply a flat list. In other words, there is no correspondence between it and the original points. ### Generate a new point cloud This method would not modify the curve but create point cloud data instead, copying over all of the curve attributes to the result. More operations like "resampling" could be in the same node. **Pros** - Simplicity: Separation of data logically (point clouds are "points" more directly than curve splines - No confusion about derived data (the curve is no longer around) **Cons** - Complexity: There is no essential need to create a point cloud to do instancing. This is evident with how we handle meshes, where you can easily do instancing on points (even faces in the old system) - Performance: Copying attributes around during evaluation is not at all trivial - Arbitrary: Does not use the fact that attributes are stored on geometry to our advantage. In this case the point cloud is just an "array container" - Code: The required code is harder to maintain, more confusing, and requires more duplication - If there are more operations in this node, they are duplicated from operations that keep the curve a curve. ### Evaluate splines This method takes advantage of the 1 to 1 correspondence of evaluated points and control points on poly splines. All Bezier and NURB splines are converted to poly splines, and three attributes are added: tangent, normal, and rotation. **Pros** - Use curves to our advantage: We can adjust the resulting attributes using methods unique to curves with the maintained "topology" information. - Performance: In many cases only three attributes need to be added. The operation is much simpler. And the data can be used later for other curve-related things. - Standard: Sverchok and AN both have similar nodes - Simplicity: There is no unnecessary change of data type **Cons** - Complexity: Exposes users to the different spline types when they might not have known about it before. - Subsequent curve operations maintain these attributes, even they they are derived

Hans Goudey commented

2021-04-12 15:40:03 +02:00

Changed status from 'Needs Triage' to: 'Confirmed'

Hans Goudey commented

2021-04-12 15:40:03 +02:00

Added subscriber: @HooglyBoogly

Miro Horváth commented

2021-04-12 22:06:10 +02:00

Added subscriber: @MiroHorvath

Hans Goudey self-assigned this 2021-05-06 01:40:26 +02:00

Hans Goudey commented

2021-05-07 07:02:21 +02:00

So, in the first pass I implemented a "Curve Resample" node. I would like to think about changing "Point Instance" to "Geometry Instance" and making it work on evaluated curves with the point, normal, tangent, and radius information. Otherwise you would have to construct a rotation from this information, which would be slower and move involved.

A "Sample Points" node also makes sense, but since the initial goal is instancing (I think), I'd like to do "Resample" first.

So, in the first pass I implemented a "Curve Resample" node. I would like to think about changing "Point Instance" to "Geometry Instance" and making it work on evaluated curves with the point, normal, tangent, and radius information. Otherwise you would have to construct a rotation from this information, which would be slower and move involved. A "Sample Points" node also makes sense, but since the initial goal is instancing (I think), I'd like to do "Resample" first.

blender-admin commented

2021-05-07 22:37:06 +02:00

This issue was referenced by 3185084efb

This issue was referenced by 3185084efbe493bf232fb0de62b341ae2a2e2936

Hans Goudey removed their assignment 2021-05-11 04:21:46 +02:00

Hans Goudey changed title from ~~Curve Sample Points Node~~ to Curve to Points Node

2021-05-11 04:24:36 +02:00

Hans Goudey changed title from ~~Curve to Points Node~~ to Curve evaluation for instancing

2021-06-05 00:13:07 +02:00

Hans Goudey self-assigned this 2021-06-05 00:13:07 +02:00

Hans Goudey changed title from ~~Curve evaluation for instancing~~ to Curve evaluation / to points for instancing

2021-06-05 00:13:35 +02:00

Paul Larson commented

2021-06-08 19:46:59 +02:00

Added subscriber: @GeorgiaPacific

Hans Goudey commented

2021-06-08 21:00:28 +02:00

For the records: I've been outvoted here and we're going with the "Curve to Points" method. I'll create a patch with that node and the different modes shortly.

Jean-Michel Argaud commented

2021-06-14 18:08:49 +02:00

Added subscriber: @jmargaud

blender-admin commented

2021-06-14 19:51:25 +02:00

This issue was referenced by fcbb20286a

This issue was referenced by fcbb20286a3163d1d6669502375aa3f096e9547a

Dalai Felinto commented

2021-06-15 11:58:58 +02:00

Changed status from 'Confirmed' to: 'Resolved'

Dalai Felinto closed this issue

2021-06-15 11:58:58 +02:00

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Curve evaluation / to points for instancing #87429

Use Case

Background

Generate a new point cloud

Evaluate splines