always_redraw works in 0.18.0 but not in 0.18.1 (single ValueTracker)

[fierval]

Description of bug / unexpected behavior

The following code which does not work in 0.18.1 and is fine on 0.17.3:

for i, idx_r in enumerate(r_idxs):

    r = points[idx_r]

    if i == 0:
        track_pt = PointValueTracker(r)

        # lines denoting the prq angle
        pr = always_redraw(lambda: Line(p, track_pt.get_value(), color=YELLOW, buff=0))
        qr = always_redraw(lambda: Line(q, track_pt.get_value(), color=YELLOW, buff=0))
        grp = VGroup(pr, qr)

        self.play(LaggedStartMap(Create, grp, runtime=2, lag_ratio=0.2))

    # move the point of two "angle" lines along the line
    # connecting points[idx_r] and points[idx_r + 1]
    if i < len(r_idxs) - 1:
        r_next = points[r_idxs[i + 1]]
        self.play(track_pt.animate.set_value(r_next))

System specifications

System Details

• OS Windows11:
• RAM: 128 Gb
• Python version (python/py/python3 --version): 3.10.12
• Installed modules (provide output from pip [list):](url)

PASTE HERE

correct.mp4 and wrong.mp4 show the porblem at 15 sec

correct.mp4

wrong.mp4

[behackl]

Please provide a self-contained (minimal) example that illustrates the problem. I can't run the snippet you have posted.

[fierval]

The bug is actually present in 0.18.1 only. 0.18.0 works fine.

From the repo: https://github.com/fierval/beta-skeleton

Run:

python concept.py

[behackl]

While I can now theoretically run the snippet, I'd like to avoid digging through a custom Scene class and a medium-sized animation. Can you help us narrow the problem down, can it be isolated to appear with the default Scene class with a short construct method that has at most 1-2 play calls?

[fierval]

I have simplified the file in the repo (https://github.com/fierval/beta-skeleton) as you requested and also here is the gist, with the bug identified.

It looks like the LaggedStartMap applied to the VGroup causes problems for the value tracker down the line. Replacing this LaggedStartMap with simple Create works fine. Not a problem for 0.18.0 or below.

[behackl]

Thanks, this has made it easier -- although, just for the record, a "minimal example" with 105 subanimations isn't really minimal at all; what I've been looking for is more along the lines of

from manim import *

class BugReport(Scene):
    def construct(self):
        lines = VGroup()
        end_tracker = ValueTracker(0)
        for start in [LEFT*2 + DOWN, RIGHT*2 + DOWN]:
            moving_line = always_redraw(lambda start=start: Line(start, end_tracker.get_value() * RIGHT))
            lines.add(moving_line)

        self.play(LaggedStartMap(Create, lines, lag_ratio=0.5))
        self.play(end_tracker.animate.set_value(2))

In any case, this bug was introduced with #3542; we'll investigate. In the meantime, you can simply add grp.resume_updating() after playing the LaggedStartMap as a workaround.