Use gganimate to display calculation of tweened data - r

Question

I would like to use gganimate to:
Graph two separate curved lines with geom_path
Call a function that performs a calculation with the data from those lines and returns a single coordinate (x, y)
Plot that coordinate as a geom_point
Move the lines around, with the geom_point updating as the lines move
This is simple if the movement is such that the single (x, y) coordinate moves linearly (just calculate it at each stage ahead of time and then animate it, it will move linearly from each stage to the next), but if it's not I'm not sure what to do. If I call a function within aes(), which seems like the natural solution, it seems to calculate it once at the beginning and then not update it as the lines move.
Here is an example.
library(tidyverse)
library(gganimate)
# A function to find the x and y coordinate of the minimum y value of either set
min_of_both <- function(x1, y1, x2, y2) {
cm <- bind_rows(tibble(x = x1, y = y1),
tibble(x = x2, y = y2))
return(list(x = cm[which(cm$y == min(cm$y)),]$x,
y = min(cm$y)))
}
# Create two parabola paths, curve A which moves downwards from t = 1 to t = 2
curveA <- tibble(xA = -50:50/10, yA = 5+(-50:50/10)^2, t = 1) %>%
bind_rows(tibble(xA = -50:50/10, yA = -10 + (-50:50/10)^2, t = 2))
# And curve B which is static in both time 1 and 2
curveB <- tibble(xB = -50:50/10, yB = 1 + (-30:70/10)^2)
data <- curveB %>%
bind_rows(curveB) %>%
bind_cols(curveA)
# Plot Curve A
p <- ggplot(data, aes(x = xA, y = yA)) +
geom_path(color = 'red') +
# And Curve B
geom_path(aes(x=xB,y=yB), color = 'blue')+
# Then plot a single point that uses both curves as input
# Note I also get problems if trying to run the function through data= instead of mapping=
# or if I define two separate functions, one for x and one for y, so as to avoid $
geom_point(aes(
x = min_of_both(xA,yA,xB,yB)$x,
y = min_of_both(xA,yA,xB,yB)$y),
size = 3,
color = 'black') +
theme_minimal()+
transition_states(t)+
ease_aes('sine-in-out')
animate(p)
This results in (not sure if the animation will play on StackOverflow but the parabola does indeed move):
The black dot is intended to mark the lowest y-coordinate on either parabola at each moment, but instead it marks the lowest y-coordinate on either parabola at any point in the animation (at the end).
Any tips appreciated.

Answer 1

After a lot of head-scratching I think I've understood your point and have found one solution. The best way forward might be to manually tween the paths and calculate the min values using your function whilst grouping by .frame before plotting:
# Same curve setup, but labelling points for grouping later
curveA <- tibble(xA = -50:50/10,
yA = 5+(-50:50/10)^2,
point = 1:101,
t = 1) %>%
bind_rows(tibble(xA = -50:50/10,
yA = -10 + (-50:50/10)^2,
point = 1:101,
t = 2))
curveB <- tibble(xB = -50:50/10,
yB = 1 + (-30:70/10)^2,
point = 1:101,
t = 1)
A_frames <- curveA %>%
tween_along(ease = 'sine-in-out', 100, along = t, id = point) %>%
filter(.phase == "transition") %>%
select(xA, yA, point, .frame) %>%
arrange(.frame, point) # arrange by point needed to keep in order
B_frames <- curveB %>%
bind_rows(curveB %>% mutate(t = 2)) %>%
tween_along(ease = 'sine-in-out', 100, along = t, id = point) %>%
filter(.phase == "transition") %>%
select(xB, yB, point, .frame) %>%
arrange(.frame, point)
data <- A_frames %>%
left_join(B_frames, by = c(".frame", "point")) %>%
group_by(.frame) %>%
mutate(xmin = min_of_both(xA,yA,xB,yB)$x,
ymin = min_of_both(xA,yA,xB,yB)$y)
# Plot Curve A
p <- ggplot(data, aes(x = xA, y = yA)) +
geom_path(color = 'red') +
# And Curve B
geom_path(aes(x=xB,y=yB), color = 'blue')+
# Then plot a single point that uses both curves as input
# Note I also get problems if trying to run the function through data= instead of mapping=
# or if I define two separate functions, one for x and one for y, so as to avoid $
geom_point(aes(xmin, ymin),
size = 3,
color = 'black') +
theme_minimal()+
transition_states(.frame)+
ease_aes('sine-in-out')
animate(p, fps = 24)