[Python] FuncAnimation: Fade out at the end of animation affecting fade in animations at the...

In an attempt to fade latex text in and out of the screen using FuncAnimation, I have written the code below. In particular, I have written two methods fade_in and fade_out that fade an equation in and out respectively. When tested individually these methods work as expected, yet when tested together (see the bottom of the code below) an issue arises. If, after creating the objects x and y displaying a LaTeX x and y respectively, I

1. Apply fade_in on x;
2. Apply fade_in on y;
3. Apply fade_out on y;

then, during the first third of the animation, the symbol y is displayed on the screen. In contrast, I'd prefer if y is invisible during the first third of the animation, only to start appearing during its fade_in animation. Funny enough, If I merely apply steps (1) and (2), then y is indeed invisible during the first third of the animation.

import matplotlib.pyplot as plt
from matplotlib.font_manager import FontProperties
from matplotlib.animation import FuncAnimation, FFMpegWriter

def chain_animations(animations):
# Extract all update functions and frame counts
update_funcs = [a._func for a in animations]
frame_counts = [a.frame_seq.__length_hint__() for a in animations]
intervals = [a._interval for a in animations]
figs = [a._fig for a in animations]

assert all(interval == intervals[0] for interval in intervals), "All animations must have the same interval"
assert all(fig is figs[0] for fig in figs), "All animations must use the same figure"

total_frames = sum(frame_counts)
cumulative = [0]
for count in frame_counts:
cumulative.append(cumulative[-1] + count)

def update(frame):
for i in range(len(frame_counts)):
if cumulative <= frame < cumulative[i + 1]:
local_frame = frame - cumulative
return update_funcs(local_frame)
return []

return FuncAnimation(figs[0], update, frames=total_frames, interval=intervals[0], blit=True)


class Symbol:
def __init__(self, ax, letter, size=20, color='black', position=(0, 0), alpha=1.0, zorder=1, row = -1, col = -1):
self.ax = ax
self.letter = letter
self.size = size
self.color = color
self.position = position
self.alpha = alpha
self.zorder = zorder
self.font = FontProperties(family='cmr10')
self.row = row
self.col = col

self.add_to_axes(ax)

def add_to_axes(self, ax):
self.ax = ax
# Manually reduce font size slightly
fontsize = self.size # scale down for visual match
x, y = self.position
self.patch = ax.text(
x, y,
self.letter,
fontsize=fontsize,
color=self.color,
alpha=self.alpha,
zorder=self.zorder,
ha='left',
va='baseline',
math_fontfamily='cm',
usetex=False
)

def update_position(self, new_pos):
self.position = new_pos

def fade_in(self, ax, time=1.0, fps=30):
total_frames = max(1, int(time * fps)) # avoid divide by zero
target_alpha = 1.0
self.alpha = 0.0
self.patch.set_alpha(0.0)

def animate(frame):
t = frame / (total_frames - 1) if total_frames > 1 else 1.0
current_alpha = t * target_alpha
self.alpha = current_alpha
self.patch.set_alpha(current_alpha)
return [self.patch]

return FuncAnimation(ax.figure, animate, frames=total_frames, interval=1000/fps, blit=True)

def fade_out(self, ax, time=1.0, fps=30):
total_frames = max(1, int(time * fps)) # avoid divide by zero
target_alpha = 1.0
self.alpha = 1.0

if self.patch is not None:
self.patch.set_alpha(1.0)

def animate(frame):
t = frame / (total_frames - 1) if total_frames > 1 else 1.0
current_alpha = (1 - t) * target_alpha
self.alpha = current_alpha
self.patch.set_alpha(current_alpha)
return [self.patch]

return FuncAnimation(ax.figure, animate, frames=total_frames, interval=1000/fps, blit=True)








import matplotlib.pyplot as plt
from Symbol import Symbol

fig, ax = plt.subplots()
ax.set_xlim(0, 200)
ax.set_ylim(0, 200)

# Create a Symbol instance
x = Symbol(ax, letter='$x$', size=30, color='blue', position=(50, 50))
y = Symbol(ax, letter='$y$', size=30, color='red', position=(55, 50))

# Doesn't work: x fade in, y fade in, y fade out
# Works: x fade in, y fade in, x fade out
anim1 = x.fade_in(ax, time=1.0, fps=30)
anim2 = y.fade_in(ax, time=1.0, fps=30)
anim3 = y.fade_out(ax, time=1.0, fps=30)
anim = chain_animations([anim1, anim2, anim3])
anim.save("fade.mp4", writer=FFMpegWriter(fps=30))

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