Depth offset moves lines and points in (Web)GL

[s-nie]

This may be related to #1025. There are also mentions of reduced depth precision with GL in the source code, but I couldn't find anything mentioning this specific issue.

While implementing a custom viewport based on re_renderer, I noticed a strange behavior when lines and points have a depth offset value. There seems to be some sort of parallax effect when the objects are not centered in the view. The effect is more pronounced the larger the offset value is. In the video below, I used a value of 200 for the manipulator gizmos and 0 for the spiral objects.

output.mp4

To Reproduce
Steps to reproduce the behavior:

1. Create a viewport
2. Add points and lines to it with different depth offsets
3. Move the camera in the viewport
4. Run in (web)GL

Expected behavior
The depth offset does not cause any translation based on the camera position. In the video above that would mean that the gizmos stay centered on the spirals.

Rerun version
0.12.0

Additional context
Things look as expected with the vulkan backend.

[Wumpf]

Thanks for the report and the (pretty cool! :)) video documenting the behavior!
You're right this is related to

• Treat camera projected primitives in 3D space more like 2D objects #1025

The depth offset works with literally offsetting on the z axis which has all sort of problems. One of them being that it works very poorly in WebGL where we have to use much bigger steps which is the reason you're seeing this issue on WebGL but not with Vulkan. This happens because WebGL is the only platform where the normalized-device-coordinates depth range goes from -1 to 1 which messes with our depth precision (in particular because the conversion is patched in via wgpu which always assumes 0 to 1).

it's not written down, but we want to get away from the depth offset and instead have a way to influence the actual draw ordering, making this also related to

• Draw order & (order independent) transparency #702

[s-nie]

Thanks for the quick response and explanation! Makes sense, then I'll know to be careful with depth offsets for now. Feel free to close this issue then, as the overarching issue is tracked in #1025 anyway.

Kudos for making re_renderer modular by the way, that made it super convenient to use without pulling in a mountain of dependencies :)

[Wumpf]

Let's keep it around, it's a good issue that highlights specific shortcomings that we haven't written down anywhere else :)

[emilk]

We haven't tested it I think, but based on Andreas' comment this should work much better on WebGPU