Feature Request: support for drawing glow::POINTS and glow::LINES

[donkeyteethUX]

Thanks for the great project. I was wondering if you have plans to add support for POINTS and LINES. This would be really helpful for building data visualization tools. For reference, kiss3d has examples of this.

https://github.com/sebcrozet/kiss3d/blob/master/examples/lines.rs
https://github.com/sebcrozet/kiss3d/blob/master/examples/points.rs

[asny]

Thanks for the kind words 🙏

Points and lines are actually already supported, just draw a sphere (or whatever shape you want) for each point and a cylinder (or whatever) for each line using a Mesh and any Material. If you have many points and/or lines with the same material, use the InstancedMesh instead of Mesh for performance reasons. Also, see the wireframe example and the recently added point cloud example. Does this meet your needs? I'll be happy to discuss adding something to make it easier and more clear, for example a specific Line struct that impl Geometry or another example?

Also, it seems like kiss3d actually uses the legacy way of drawing points and lines, which might be what you're asking for. However, it is much better supported and much more flexible to draw a 3D object that resembles a point/line. It's always difficult to say something about performance since it very much depends on hardware, but it shouldn't be much slower to do it like this (might be faster on some hardware). What is important is to use InstancedMesh if you have many points/lines.

[donkeyteethUX]

Makes sense. If I understand correctly, the key difference between the "legacy way" of drawing a line and simply representing it as a cylinder, is that a true line is scale-independent whereas a cylinder is not. No matter how much I zoom in or out, it's still a line with the same fixed width. This is because the width is defined in pixels, not an actual 3d geometry.

Say you want to visualize a graph of connections between 3d points (which can be very close together or far apart, in an absolute sense) with an OrbitControl camera. If you use spheres for points and cylinders for lines, you have to worry about trying to adjust the width of shapes dynamically based on the zoom level. Otherwise, they will disappear when far away, and become too "thick" when close. The desired property ends up being a fixed pixel-width, which is what the "legacy way" already provides.

Does this use case make sense? I know lines and points might be "legacy" on some graphics backends, but we can perhaps let the backend-abstraction layer worry about that, and glow still happily supports them.

[asny]

Yeah, wanting fixed width lines makes a lot of sense. However, to me at least, that sounds like you want 2D lines, not 3D lines. My reasoning is that if you draw a line with fixed width, then you loose the third dimension, ie. you have no idea if a line is close or far away. So if I understand you correctly (please correct me if I'm wrong), you can use the Line struct for drawing 2D lines (se the shapes2D example). To find the end points of the line on the screen, you can use the uv_coordinates_at_position method on the Camera to get the uv coordinates on the screen for a given 3D position. To go to a pixel position, just multiply the uv coordinates with the width/height of the viewport and add the viewport offset if any. I probably should have made a pixel_at_position method, I will add that 💪 Do you think this approach will work for your use-case?

And I agree, you shouldn't worry about what is legacy and what is not (I probably shouldn't have brought it up). So if you want 2D lines - which is exactly what is supported by legacy OpenGL - just use the Line struct.

[asny]

Added pixel_at_position in 9712b198e6ed0b84c8db472d548dbe114fd72d5c 💪

[donkeyteethUX]

This sees very reasonable; I'll try it out. Thanks for the response!