Descent heights still not right

[clauswilke]

An issue to be considered after 2.3.0: I have noticed that descent heights are still not correct. The fault lies in this line:

ggplot2/R/margins.R

Line 67 in 4db5122

descent <- descentDetails(temp)

It looks like a reasonable line, but it turns out that the function descentDetails() ignores the font settings of the grob it is given:

library(grid)
descentDetails(textGrob("pqgj", gp = gpar(fontsize = 5)))
#> [1] 0.0350748697916667inches
descentDetails(textGrob("pqgj", gp = gpar(fontsize = 10)))
#> [1] 0.0350748697916667inches
descentDetails(textGrob("pqgj", gp = gpar(fontsize = 20)))
#> [1] 0.0350748697916667inches

Instead it takes the font settings from the currently active viewport. The following works correctly:

pushViewport(viewport(gp = gpar(fontsize = 5)))
descentDetails(textGrob("pqgj"))
#> [1] 0.0146145290798611inches
popViewport()

pushViewport(viewport(gp = gpar(fontsize = 10)))
descentDetails(textGrob("pqgj"))
#> [1] 0.0292290581597222inches
popViewport()

pushViewport(viewport(gp = gpar(fontsize = 20)))
descentDetails(textGrob("pqgj"))
#> [1] 0.0584581163194444inches
popViewport()

The result in ggplot is that we're getting the exact same descent height every time, regardless of theme settings:

library(ggplot2)
df <- data.frame(x = 1)
ggplot(df, aes(x, x)) + geom_point() +
  ggtitle("gjpjQ") +
  theme(plot.title = element_text(size = 10))

ggplot(df, aes(x, x)) + geom_point() +
  ggtitle("gjpjQ") +
  theme(plot.title = element_text(size = 20))

ggplot(df, aes(x, x)) + geom_point() +
  ggtitle("gjpjQ") +
  theme(plot.title = element_text(size = 100))

The reason why things seem to work correctly is because the default fontsize is 12, so we're getting a descent height that is approximately right for most typical font sizes.

I have started collecting some grid code for text handling here, in the hopes that some of the code will be useful for future ggplot2 development. Among the code, there is a set of functions to correctly calculate descent heights: https://github.com/clauswilke/gridtext/blob/master/R/grob-descent.R

The approach I'm using works correctly for arbitrary fonts, fontsizes, angles, and vjust / hjust values.

library(grid)
library(gridtext)
library(tibble)

label_data <- tibble(
  label = c("Descenders: pgqjy"),
  x = unit(c(.2, .5, .8), "npc"),
  y = unit(c(.8, .5, .1), "npc"),
  box_hjust = c(0, 0.5, 1),
  box_vjust = c(0, 0.5, 1),
  fontsize = c(20, 17, 14),
  fontfamily = c("Comic Sans MS", "Helvetica", "Times New Roman"),
  angle = c(0, 35, -35)
)

grid.newpage()
g <- labels_grob(label_data, debug = TRUE)
grid.draw(g)

[hadley]

We should also engage with @pmur002 and see if we can make this better in grid itself.

[clauswilke]

Indeed, @pmur002, regardless of whether descentDetails() should or should not be modified, it would be tremendously helpful if a function could be added to grid that simply returns relevant font metrics for a given font. Something like this:
https://github.com/clauswilke/gridtext/blob/ac404acd3a3a28f1b78fddf288478b82a0e3423c/R/grob-descent.R#L17-L21

We would like the same descent height for any string rendered with a specified font family, size, and face, and getting that by making a text grob, then replacing the label, and then calculating properties thereof seems backwards. In fact, in my code I added a simple lookup table that caches descent values for different font parameters, and this is about 100 times faster than going the viewport/grob route every time:
https://github.com/clauswilke/gridtext/blob/ac404acd3a3a28f1b78fddf288478b82a0e3423c/R/grob-descent.R#L29-L55

[baptiste]

cf also #1882
(this part of the issue got overlooked)

[clauswilke]

@baptiste Thanks for the link. The gap between legend title and legend should be fine now either way, because I added code to calculate it based on font size:

ggplot2/R/guide-legend.r

Line 336 in 4db5122

vgap <- height_cm(theme$legend.spacing.y %||% (0.5 * unit(title_fontsize, "pt")))

library(ggplot2)
set.seed(123)
dummy <- data.frame(x=runif(10), y=runif(10), f = gl(2,5))
ggplot(dummy, aes(x,y, shape=f)) + 
  geom_point() +
  labs(shape="guide") +
  theme(legend.title = element_text(size = 28))

(The code based on grob height did cause weird alignment issues with multiple legends for me.)

I'm still not entirely happy with the spacing choices for guides, but they are at least acceptable in most cases.

[baptiste]

@clauswilke indeed, this particular issue got fixed; I just wanted to point out that the descenders issue has been recurring (and closed issues generally disappear from sight).

How does "grid2" sound? IMO it'd be nice to sit down and think of a long-term rewrite of grid, learning from current feature requests, evolving needs and technologies (modern devices, for instance, notably bypassing grDevices), and also reimplementing some bottlenecks, harmonising/merging with gtable, etc.

[pmur002]

@clauswilke, an R-level function that returns metric info sounds reasonable, but my first thought is that this should be something in 'grDevices' rather than 'grid'. I will take a look.

[clauswilke]

@pmur002 Sure, grDevices instead of grid would be fine.

While we're at it, I have noticed that the value returned by grobAscent() can exceed the value returned by grobHeight(). Is this correct? I thought the grob height was the maximum ascent at the given font settings.

library(grid)
pushViewport(viewport(gp = gpar(fontsize = 20)))
convertHeight(grobHeight(textGrob("abc")), "pt")
#> [1] 14.3700927734375pt
convertHeight(grobAscent(textGrob("abc")), "pt")
#> [1] 14.3700927734375pt
convertHeight(grobHeight(textGrob("Q")), "pt")
#> [1] 14.3700927734375pt
convertHeight(grobAscent(textGrob("Q")), "pt")
#> [1] 14.624951171875pt
popViewport()

Created on 2018-06-07 by the reprex package (v0.2.0).

[thomasp85]

Maybe only tangentially related, but something to keep in mind. Experimentation with profiling gganimate code have shown that not rendering labels more than doubles the rendering speed of ggplot objects and the lion share of that falls into the call to descentDetails in titleGrob. If it is not possible to improve calculation of font metrics dramatically we should consider some sort of memoisation...

[clauswilke]

@thomasp85 I was hinting at this issue in my comment above about the lookup table that caches descent values. It gives a 100x speed increase on average, though you wouldn't see the whole effect in ggplot2 because my function also pushes and pops viewports to get the descent details right and ggplot2 currently doesn't do that.

library(gridtext)
library(microbenchmark)
gp <- grid::gpar(fontfamily = "Helvetica", fontsize = 12, fontface = "plain")
microbenchmark(
  gridtext:::lookup_font_details(gp),
  gridtext:::calc_font_details(gp)
)

#> Unit: microseconds
#>                                expr      min       lq       mean   median
#>  gridtext:::lookup_font_details(gp)   11.805   19.605   61.93275   27.079
#>    gridtext:::calc_font_details(gp) 2072.938 2428.107 2869.23680 2612.757
#>        uq       max neval
#>    37.491  3165.092   100
#>  3012.104 14423.639   100

Created on 2018-06-14 by the reprex package (v0.2.0).

I've been wondering more broadly whether reimplementing the whole label rendering system will make things faster or slower on the whole. The current code is overly complex, I think, but it also has issues, and fixing the issues might on the whole slow things down rather than speed them up.

[clauswilke]

To elaborate on the code complexity issue: To get descent heights right, we need to place each individual piece of text into a separate grob and create a viewport that includes a gap for the descent height. For rotated text, we then rotate the entire viewport. This works beautifully, as shown above, and removes the need for the awkward trigonometry in the current code, which isn't correct for all vjust settings and which is also not correct for multi-label grobs:

ggplot2/R/margins.R

Lines 69 to 73 in 4db5122

	# Use trigonometry to calculate grobheight and width for rotated grobs. This is only
	# exactly correct when vjust = 1. We need to take the absolute value so we don't make
	# the grob smaller when it's flipped over.
	text_height <- unit(1, "grobheight", text_grob) + abs(cos(angle / 180 * pi)) * descent
	text_width <- unit(1, "grobwidth", text_grob) + abs(sin(angle / 180 * pi)) * descent

Now, it would be easy to just add margins to the text label at the same time, into the same viewport, as seen here:

library(grid)
library(gridtext)
library(tibble)

label_data <- tibble(
  label = c("Descenders: pgqjy"),
  x = unit(c(.2, .5, .8), "npc"),
  y = unit(c(.8, .5, .1), "npc"),
  box_hjust = c(0, 0.5, 1),
  box_vjust = c(0, 0.5, 1),
  padding = list(ggplot2::margin(5, 10, 5, 10)),
  fontsize = c(20, 17, 14),
  fontfamily = c("Comic Sans MS", "Helvetica", "Times New Roman"),
  angle = c(0, 35, -35)
)

grid.newpage()
g <- labels_grob(label_data, debug = TRUE)
grid.draw(g)

However, this means that the margins are rotated in the frame of the label, which is not the current ggplot2 convention. To retain the current ggplot2 convention, we need to wrap the labels into another grob with its own viewport.

[clauswilke]

One more example, how things like x and y axes could be generated. All of this works in principle. The challenge will be to integrate into ggplot2 without breaking currently existing themes.

library(grid)
library(gridtext)
library(tibble)

label_data <- tibble(
  label = c("This", "is", "a", "test", "x-axis"),
  x = unit(c(.1, .3, .5, .7, .9), "npc"),
  y = unit(0.8, "npc"),
  box_hjust = 0.5,
  box_vjust = 1,
  hjust = 0.5,
  vjust = 1,
  angle = 0,
  fontsize = 10, fontfamily = "Comic Sans MS",
  padding = list(mar(5, 5, 3, 5)),
  margin = list(mar(5, 5, 5, 5))
)
grid.newpage()
grid.draw(labels_grob(label_data, align_frames = TRUE, debug = TRUE))

label_data$angle <- 45
label_data$hjust <- 1
label_data$box_hjust <- 1
label_data$box_vjust <- 0.5
label_data$y <- grid::unit(0.55, "npc")
grid.draw(labels_grob(label_data, align_frames = TRUE, debug = TRUE))

label_data$angle <- 90
label_data$hjust <- 0
label_data$y <- unit(0.3, "npc")
grid.draw(labels_grob(label_data, align_frames = TRUE, debug = TRUE))

Created on 2018-06-14 by the reprex package (v0.2.0).

[clauswilke]

One more comment about speed. For any somewhat complex rendering task, I think it makes sense to do all internal calculation in a fixed unit and convert to grid units only once, rather than doing calculations in grid units. The grid units are elegant but not that efficient, due to all the overhead they carry.

A very simple example follows, and things will only get worse the more unit calculations and conversions need to be done.

library(grid)
microbenchmark::microbenchmark(
  5 + 7,
  unit(5 + 7, "pt"),
  unit(5, "pt") + unit(7, "pt")
)
#> Unit: nanoseconds
#>                           expr   min      lq     mean  median      uq
#>                          5 + 7    83   124.5   184.49   177.5   221.5
#>              unit(5 + 7, "pt") 25132 26028.5 34192.16 26675.5 32975.5
#>  unit(5, "pt") + unit(7, "pt") 55266 56836.0 64465.85 58249.0 60979.0
#>     max neval
#>     687   100
#>  577149   100
#>  256298   100

Created on 2018-06-14 by the reprex package (v0.2.0).

[thomasp85]

I think with a proper caching mechanism it doesn’t matter much if your implementation is a bit slower as it will only impact the first plot (most font settings are reused across plots and can thus be fetched from the cache)

[thomasp85]

Agree on the units - I’ve long wanted something more performant. There might be instances where it is not possible to get the dimension before render time, but it should be possible to abstract that away and do as much work unit-less

[clauswilke]

@thomasp85 Yes, it caches across plots. What I was trying to say was that the realized speed gain in ggplot2 will be less than the benchmark indicates, because the current, buggy code in ggplot2 is faster than the correct but uncached code would be.