The simulation advances in whole 50 Hz steps and the renderer draws
whenever it can, so the drawn position only changed fifty times a second
and was held for however many frames fell inside a step. That is visible
as stepping, and it got WORSE the faster the machine: at 240 fps each
position is held for nearly five frames, which is why a fast PC looked
like a rabbit on crack while TARGETFPS=50 looked perfect. Matching the two
rates hid it, but locking the frame rate to the physics rate throws away
the entire point of having a fixed step.
So drawing now blends. Entity keeps renderPreviousOrigin - its origin at
the start of the step it is in, snapshotted by Mover::BeginStep - and
renderStepFraction, how far through that step the frame falls, which is
the leftover Entity::PerformAndWatch deliberately does not simulate.
GetRenderToWorld blends the two with Origin::Lerp, which already did
position and shortest-arc quaternion with normalisation.
It is RENDER ONLY. localOrigin and localToWorld are untouched, so physics,
collision, scoring, the nav map's queries and the network update records
all still see exact stepped values.
Three call sites. RootRenderable::Execute, which was the single place a
vehicle's transform reached the matrix stack - the renderable already ran
per frame and simply re-read a value that changed at the physics rate. The
eye needed its gate widened as well: it rebuilt the view only when
localToWorld CHANGED, so the world would have glided while the camera went
on stepping and the judder would have moved rather than gone. And a
teleport must stay a cut - that falls out free, because VTV::BeginStep
applies a scheduled respawn and THEN calls Mover::BeginStep, so the
snapshot lands post-teleport and the blend has nothing to travel.
The picture trails the simulation by up to one step, 20 ms at 50 Hz. That
is the standard price of interpolating rather than extrapolating, and much
the lesser evil: guessing forward overshoots and shimmers every time the
guess is corrected.
RP412INTERP=0 turns it off so the stepping can be seen again without a
rebuild.
Determinism proved rather than asserted: a scripted lap (RP412INPUTSCRIPT,
throttle and steering and pitch) at 240 fps with interpolation on and off,
90 PHYSTRACE samples over 22 seconds of driving, zero differ. Two earlier
attempts at that comparison were invalid and both were my method - the
first did not pin RP412SPAWNZONE so the runs began on different pads, and
the second had no input script, so a joystick sitting on the desk drove
the two runs differently. The template warns about the first of those in
as many words.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
CydandClaude Opus 5
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