Attachments sometimes flicker for a frame

[appgurueu]

Minetest version

5.5-dev

Summary

Animated models and attachments sometimes flicker for a frame. This is a "heisenbug" and hard to observe; the attachment issue appears to occur more frequently though. See this video. Watch key moments at 0.25x speed to see the bug.

Attachment bug (diamond axe temporarily rotated by 180° around the player model):

Steps to reproduce

MTG + wield3d or visible_wielditem mod, switch to 3rd person view, reduce viewrange, enable fast, run around.

[SmallJoker]

sfan5 already mentioned somewhere that the model disappears for one frame when crossing a re-positioning border each 200m. Although I cannot find the correct issue #...

[sfan5]

duplicate of #10027

[appgurueu]

@sfan5 only partially a duplicate; the attachment issue is lacking in #10027. I changed title and description accordingly, please reopen. The attachment issue is in fact more annoying to me.

[ghost]

Yes, a very CONFIRMED issue. For examples:

• In my Petz mod a Parrot is atached to the shoulder of the player model, and when it walks the rotated parrot ghost arises.
• My "The Samz" player model shows this issue with wielded items.
  This issue is old, some guy said in an old post here that not putting the body bone rotation to 0.0 can resolve this issue. In fact the MTG player model is hacked to avoid this issue. He said that could be a export b3d issue.

[appgurueu]

There also is this recent thread on the forum that seems to deal with it.

[sfan5]

Needs retesting.

[appgurueu]

I think I can explain what was probably happening here: The negative scale bug leads to wrong joint transforms (the 180° Y rotation). Attachments are off by one frame (#14818), so Sam looks correct, and the attachment looks wrong exactly as one would expect.

It should be gone for certain once #15687 is merged (and not that it should be necessary, but I will touch that code again later to avoid calculating the scale from the transformation matrix at all by instead only calculating the matrix from the animated TRS properties).