8000 [Bug]: 3D tick label position jitter when rotating the plot view · Issue #29551 · matplotlib/matplotlib · GitHub
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[Bug]: 3D tick label position jitter when rotating the plot view #29551

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scottshambaugh opened this issue Jan 30, 2025 · 7 comments
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[Bug]: 3D tick label position jitter when rotating the plot view #29551

scottshambaugh opened this issue Jan 30, 2025 · 7 comments
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backend: agg topic: text

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@scottshambaugh
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scottshambaugh commented Jan 30, 2025 •
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It seems like there is some rounding going on with 3D tick label positions. When rotating the plot, the position of the labels move around a bit. This may be intentional to line up pixel values, or it may be an artifact, but the result is "jittering" that looks bad.

This becomes most noticeable in animations:
https://github.com/scottshambaugh/mpl_stereo/blob/7c20f7123173eed89370ffa4227be85de514b3a3/docs/trefoil_3d_animation.gif

May be related to #13044? Though there aren't 2D rotations happening here afaik.

TODO: check if the axis labels are also affected
@scottshambaugh scottshambaugh changed the title BUG: 3D tick label position jitter when rotating the plot view [Bug]: 3D tick label position jitter when rotating the plot view Jan 30, 2025
@WeatherGod
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Another possible reason to consider: antialiasing.

@timhoffm
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I would be surprised if #13044 is related because the offset for a certain text rotation angle (here 0°) is constant.

You could try a polar plot with an animated set_theta_offset() and see whether rotating the tick labels around the polar plot jitters as well.
< 8000 !-- no margin wins, so we check it last and use its value if true. -->

@QuLogic
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QuLogic commented Jan 31, 2025 •
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I'm pretty sure this isn't anything to do with 3D, and just that text is snapped to pixels:

import matplotlib.pyplot as plt
import matplotlib.animation as manim

fig = plt.figure(figsize=(2, 2))

text = fig.text(0, 0, 'ABC123')
change = 0.001


def update(*args, **kwargs):
    global change
    x, y = text.get_position()
    if not (0 <= x <= 1 and 0 <= y <= 1):
        change *= -1
    text.set_x(x + change)
    text.set_y(y + change)
    return text,


ani = manim.FuncAnimation(fig, update, interval=20, frames=int(2/change),
                          cache_frame_data=False)

ani.save('text.gif')

Image

If you open this GIF in GIMP, you can see that in a few frames nothing happens, and then it shifts by one pixel one way, and then one pixel the other.

@scottshambaugh
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scottshambaugh commented Feb 1, 2025 •
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Spent some time digging into it today, and I agree this is a result of the general text rendering pipeline. Some basic edits to remove rounding in the agg backend didn't change anything, so IMO this is related to the glyph -> bitmap conversion and the associated (anti)aliasing. I wonder if there's a better way to draw the text from the vector paths? I'm getting out of my depth though.

@QuLogic
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I experimented with subpixel positioning:
Image

It's nearly there, but 1 frame out of 5 or so is a pixel out of place, so you still see a bit of jitter if you stare at it closely.

@anntzer
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anntzer commented Feb 1, 2025

mplcairo looks immune to the problem (from a quick look), likely because it directly renders text to the final buffer rather than going through an intermediate one like agg/ft2font does. I don't know how hard it would be to attach agg's target buffer to ft2font to do the same?

@WeatherGod
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WeatherGod commented Feb 2, 2025 via email

@QuLogic QuLogic mentioned this issue Apr 28, 2025
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@anntzer anntzer mentioned this issue May 16, 2025
Drop the FT2Font intermediate buffer. #30059
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