【问题标题】：Same/Fix alpha even for overlapping areas in ggplot2即使对于 ggplot2 中的重叠区域，相同/修复 alpha
【发布时间】：2022-08-18 19:24:05
【问题描述】：

我想先画一堆区域，然后用相同的单个 alpha 值显示生成的整体区域。所以代替这个：

library(tidyverse)

dat <- tribble(
  ~xmin, ~xmax, ~ymin, ~ymax,
     10,    30,    10,    30,
     20,    40,    20,    40,
     15,    35,    15,    25,
     10,    15,    35,    40
)

ggplot() +
  geom_rect(data = dat,
            aes(
              xmin = xmin,
              xmax = xmax,
              ymin = ymin,
              ymax = ymax
            ),
            alpha = 0.5)

我希望将其作为我的结果：

^{由reprex package (v2.0.1) 于 2022 年 7 月 26 日创建}

我觉得我的问题的答案可能与this thread 中的答案相似，但我并不完全理解，因此不确定。另请注意，我使用geom_rect() 作为reprex，但最终我希望它适用于ggforce::geom_circle()。

编辑 1

不幸的是，指向scale_alpha(range = ..., limits = ...) 的Quinten\'s first answer 没有回答我的问题，因为它显然只能导致不透明的区域。

编辑 2

Quinten\'s updated answer 是我可以接受的上述代表的解决方法。但是，正如我在上面提到的，我希望它适用于ggforce::geom_circle()。不幸的是，我想我现在必须更具体并创建另一个代表。（对不起）

library(ggforce)
#> Lade nötiges Paket: ggplot2

dat <- data.frame(
  x = c(1, 1.3, 1.6),
  y = c(1, 1, 1),
  circle = c(\"yes\", \"yes\", \"no\")
)

ggplot() +
  coord_equal() +
  theme_classic() +
  geom_circle(
    data = subset(dat, circle == \"yes\"),
    aes(x0 = x, y0 = y, r = 0.5, alpha = circle),
    fill = \"grey\",
    color = NA,
    show.legend = TRUE
  ) +
  geom_point(
    data = dat,
    aes(x, y, color = circle)
  ) +
  scale_color_manual(
    values = c(\"yes\" = \"blue\", \"no\" = \"red\")
  ) +
  scale_alpha_manual(
    values = c(\"yes\" = 0.25, \"no\" = 0)
  )

^{由reprex package (v2.0.1) 创建于 2022-08-17}

标签： r ggplot2 alpha ggforce

【解决方案1】：

ggblend

不久前我还看到了ggblend，但不确定它是否能解决您的问题，幸运的是它可以解决！你可以做两件事：

您可以将 Rstudio 中的 Graphics 更改为“Cairo”，如下所示：

代码：

#remotes::install_github("mjskay/ggblend")
library(ggblend)
library(ggforce)

# reprex 1 ----------------------------------------------------------------
library(tidyverse)

dat <- tribble(
  ~xmin, ~xmax, ~ymin, ~ymax,
  10,    30,    10,    30,
  20,    40,    20,    40,
  15,    35,    15,    25,
  10,    15,    35,    40
)

p1 <- ggplot() +
  geom_rect(data = dat,
            aes(
              xmin = xmin,
              xmax = xmax,
              ymin = ymin,
              ymax = ymax
            ),
            alpha = 0.3) %>% blend("source")

p1

# reprex 2 ----------------------------------------------------------------
dat <- data.frame(
  x = c(1, 1.3, 1.6),
  y = c(1, 1, 1),
  circle = c("yes", "yes", "no")
)

p2 <- ggplot() +
  coord_equal() +
  theme_classic() +
  geom_circle(
    data = subset(dat, circle == "yes"),
    aes(x0 = x, y0 = y, r = 0.5, alpha = circle),
    fill = "grey",
    color = NA,
    show.legend = TRUE
  ) %>% blend("source") +
  geom_point(
    data = dat,
    aes(x, y, color = circle)
  ) +
  scale_color_manual(
    values = c("yes" = "blue", "no" = "red")
  ) +
  scale_alpha_manual(
    values = c("yes" = 0.25, "no" = 0)
  )

p2
#ggsave(plot = p2, "p2.pdf", device = cairo_pdf)

您可以将对象保存为png 和type = "cairo"，如下所示：

library(ggblend)
library(ggforce)

# reprex 1 ----------------------------------------------------------------
library(tidyverse)

dat <- tribble(
  ~xmin, ~xmax, ~ymin, ~ymax,
  10,    30,    10,    30,
  20,    40,    20,    40,
  15,    35,    15,    25,
  10,    15,    35,    40
)

p1 <- ggplot() +
  geom_rect(data = dat,
            aes(
              xmin = xmin,
              xmax = xmax,
              ymin = ymin,
              ymax = ymax
            ),
            alpha = 0.3) %>% blend("source")
#> Warning: Your graphics device, "quartz_off_screen", reports that blend = "source" is not supported.
#>  - If the blending output IS NOT as expected (e.g. geoms are not being
#>    drawn), then you must switch to a graphics device that supports
#>    blending, like png(type = "cairo"), svg(), or cairo_pdf().
#>  - If the blending output IS as expected despite this warning, this is
#>    likely a bug *in the graphics device*. Unfortunately, several
#>    graphics do not correctly report their capabilities. You may wish to
#>    a report a bug to the authors of the graphics device. In the mean
#>    time, you can disable this warning via options(ggblend.check_blend =
#>    FALSE).
#>  - For more information, see the Supported Devices section of
#>    help('blend').

png(filename = "plot1.png", type = "cairo")
# Output in your own folder:

p1
dev.off()

#ggsave(plot = p1, "p1.pdf", device = cairo_pdf)

# reprex 2 ----------------------------------------------------------------
dat <- data.frame(
  x = c(1, 1.3, 1.6),
  y = c(1, 1, 1),
  circle = c("yes", "yes", "no")
)

p2 <- ggplot() +
  coord_equal() +
  theme_classic() +
  geom_circle(
    data = subset(dat, circle == "yes"),
    aes(x0 = x, y0 = y, r = 0.5, alpha = circle),
    fill = "grey",
    color = NA,
    show.legend = TRUE
  ) %>% blend("source") +
  geom_point(
    data = dat,
    aes(x, y, color = circle)
  ) +
  scale_color_manual(
    values = c("yes" = "blue", "no" = "red")
  ) +
  scale_alpha_manual(
    values = c("yes" = 0.25, "no" = 0)
  )
#> Warning: Your graphics device, "quartz_off_screen", reports that blend = "source" is not supported.
#>  - If the blending output IS NOT as expected (e.g. geoms are not being
#>    drawn), then you must switch to a graphics device that supports
#>    blending, like png(type = "cairo"), svg(), or cairo_pdf().
#>  - If the blending output IS as expected despite this warning, this is
#>    likely a bug *in the graphics device*. Unfortunately, several
#>    graphics do not correctly report their capabilities. You may wish to
#>    a report a bug to the authors of the graphics device. In the mean
#>    time, you can disable this warning via options(ggblend.check_blend =
#>    FALSE).
#>  - For more information, see the Supported Devices section of
#>    help('blend').

png(filename = "plot2.png", type = "cairo")

# Output in your folder
p2

dev.off()
#ggsave(plot = p2, "p2.pdf", device = cairo_pdf)

^{创建于 2022 年 8 月 17 日，reprex v2.0.2}

更新

您可以做的是使用sf 包中的st_union 对区域进行UNION，这样您就可以得到一个区域，而不是像这样的重叠区域：

library(tidyverse)
library(sf)

dat <- tribble(
  ~xmin, ~xmax, ~ymin, ~ymax,
  10,    30,    10,    30,
  20,    40,    20,    40,
  15,    35,    15,    25,
  10,    15,    35,    40
)

area1 <- dat %>%
  slice(1) %>%
  as_vector() %>%
  st_bbox() %>%
  st_as_sfc()

area2 <- dat %>%
  slice(2) %>%
  as_vector() %>%
  st_bbox() %>%
  st_as_sfc()

area3 <- dat %>%
  slice(3) %>%
  as_vector() %>%
  st_bbox() %>%
  st_as_sfc()

area4 <- dat %>%
  slice(4) %>%
  as_vector() %>%
  st_bbox() %>%
  st_as_sfc()

all_areas <- st_union(area1, area2) %>%
  st_union(area3) %>%
  st_union(area4)

ggplot(all_areas) +
  geom_sf(alpha = 0.5, fill = "grey", colour = "grey") +
  theme(legend.position = "none")

ggplot(all_areas) +
  geom_sf(alpha = 0.8, fill = "grey", colour = "grey") +
  theme(legend.position = "none")

^{由reprex package (v2.0.1) 于 2022-08-17 创建}

第一个答案

也许你想要这个，你可以使用 scale_alpha 和 range 和 limits 来保持相同的区域 alpha 像这样：

library(tidyverse)

dat <- tribble(
  ~xmin, ~xmax, ~ymin, ~ymax,
  10,    30,    10,    30,
  20,    40,    20,    40,
  15,    35,    15,    25,
  10,    15,    35,    40
)

ggplot() +
  geom_rect(data = dat,
            aes(
              xmin = xmin,
              xmax = xmax,
              ymin = ymin,
              ymax = ymax,
              alpha = 0.5
            )) +
  scale_alpha(range = c(0, 1), limits = c(0, 0.5)) +
  theme(legend.position = "none")

^{由reprex package (v2.0.1) 于 2022 年 7 月 26 日创建}

【讨论】：

如果我错了，请纠正我，但这种方法只能使整个区域不透明（alpha = 1），但不能使整个区域具有相同 alpha 值的半透明，例如0.5?
@PaulSchmidt，是的，你是对的，所以保持 alpha 0.5 有点棘手。所以这固定有一个区域。
@PaulSchmidt，我更新了我的答案。您可以使用sf 包中的st_union 将这些区域合并为一个区域。正如您在答案中看到的那样，alpha 现在是相同的，您甚至可以更改它。
对于我上面的代表，这确实是我会接受的答案。但是，正如您现在在我的第二次编辑中看到的那样，我的 reprex 不够精确是我的错，因为您的解决方案无法转换（我认为？）当我最终想要使用 ggforce::geom_circle() 时。

【解决方案2】：

使用sf 和purrr：

创建多边形列表（矩形）
创建一个多多边形作为这些矩形的联合
用geom_sf 显示结果

library(tidyverse)
library(sf)

dat <- tribble(
  ~xmin, ~xmax, ~ymin, ~ymax,
  10,    30,    10,    30,
  20,    40,    20,    40,
  15,    35,    15,    25,
  10,    15,    35,    40
)

sf_rectangle <- function(xmin,xmax,ymin,ymax) {
  st_polygon(list(rbind(c(xmin, ymin), c(xmin, ymax), c(xmax, ymax), c(xmax, ymin), c(xmin, ymin))))
}

rect_union <- dat %>% purrr::pmap(sf_rectangle) %>%
                      purrr::reduce(st_union)


ggplot(rect_union) + geom_sf(alpha=.2)

【讨论】：

【解决方案3】：

好吧，我偶然发现了{ggblend} 和它的blend() 函数。基本上，我只需要在geom_ 之后添加%>% blend("source")，它将覆盖多个半透明区域，它完全符合我的要求。

但是，它确实具有以下限制，可以在下面的 reprex 的 Warning 中看到，即您实际上无法在 RStudio 中预览绘图，甚至可能无法将其导出到某些文件类型，因为并非所有设备可以处理这个。但是，通过ggsave(..., device = cairo_pdf) 导出到 pdf 效果很好，我在下面显示了它的屏幕截图。

不过，我将保留这个问题，直到我找到同时导出 .png 和 .svg 的方法，或者有人提出适用于 reprex 1 和 reprex 2 的替代解决方案。

# remotes::install_github("mjskay/ggblend")
library(ggblend)
library(ggforce)
library(tidyverse)


# reprex 1 ----------------------------------------------------------------
library(tidyverse)

dat <- tribble(
  ~xmin, ~xmax, ~ymin, ~ymax,
  10,    30,    10,    30,
  20,    40,    20,    40,
  15,    35,    15,    25,
  10,    15,    35,    40
)

p1 <- ggplot() +
  geom_rect(data = dat,
            aes(
              xmin = xmin,
              xmax = xmax,
              ymin = ymin,
              ymax = ymax
            ),
            alpha = 0.3) %>% blend("source")
#> Warning: Your graphics device, "png", reports that blend = "source" is not supported.
#>  - If the blending output IS NOT as expected (e.g. geoms are not being
#>    drawn), then you must switch to a graphics device that supports
#>    blending, like png(type = "cairo"), svg(), or cairo_pdf().
#>  - If the blending output IS as expected despite this warning, this is
#>    likely a bug *in the graphics device*. Unfortunately, several
#>    graphics do not correctly report their capabilities. You may wish to
#>    a report a bug to the authors of the graphics device. In the mean
#>    time, you can disable this warning via options(ggblend.check_blend =
#>    FALSE).
#>  - For more information, see the Supported Devices section of
#>    help('blend').

ggsave(plot = p1, "p1.pdf", device = cairo_pdf)
#> Saving 7 x 5 in image


# reprex 2 ----------------------------------------------------------------
dat <- data.frame(
  x = c(1, 1.3, 1.6),
  y = c(1, 1, 1),
  circle = c("yes", "yes", "no")
)

p2 <- ggplot() +
  coord_equal() +
  theme_classic() +
  geom_circle(
    data = subset(dat, circle == "yes"),
    aes(x0 = x, y0 = y, r = 0.5, alpha = circle),
    fill = "grey",
    color = NA,
    show.legend = TRUE
  ) %>% blend("source") +
  geom_point(
    data = dat,
    aes(x, y, color = circle)
  ) +
  scale_color_manual(
    values = c("yes" = "blue", "no" = "red")
  ) +
  scale_alpha_manual(
    values = c("yes" = 0.25, "no" = 0)
  )
#> Warning: Your graphics device, "png", reports that blend = "source" is not supported.
#>  - If the blending output IS NOT as expected (e.g. geoms are not being
#>    drawn), then you must switch to a graphics device that supports
#>    blending, like png(type = "cairo"), svg(), or cairo_pdf().
#>  - If the blending output IS as expected despite this warning, this is
#>    likely a bug *in the graphics device*. Unfortunately, several
#>    graphics do not correctly report their capabilities. You may wish to
#>    a report a bug to the authors of the graphics device. In the mean
#>    time, you can disable this warning via options(ggblend.check_blend =
#>    FALSE).
#>  - For more information, see the Supported Devices section of
#>    help('blend').

ggsave(plot = p2, "p2.pdf", device = cairo_pdf)
#> Saving 7 x 5 in image

^{创建于 2022 年 8 月 17 日，reprex v2.0.2}

【讨论】：

太好了，您找到了解决方案。我还看到了ggblend 包，不确定它是否适合您的问题。我在答案中添加了一些代码以将输出另存为 png。您可以将png 与type = "cairo" 一起使用。