如何使用ggplot2合并不同颜色通道中的图

利用该layer_data()函数，我们可以获取在图层上计算的任何值，然后按需使用它。假设您的示例中已经包含了三个图。gp1，gp2和gp3。

让我们将十六进制转换之前的颜色值保存在新的data.frame中：

cols <- data.frame(r = layer_data(gp1)$value,
                   g = layer_data(gp2)$value,
                   b = layer_data(gp3)$value)

Since these are computed densities, it may be best to rescale them to be within [0,1] first before we transform them to colour hexademical notation:

cols <- apply(cols, 2, scales::rescale)
cols <- rgb(cols[,1], cols[,2], cols[,3])

Now, since the x and y data between each of the plots are the same, we can just grab the x-y coordinates from one of them and combine it with our new colours:

newdata <- cbind(layer_data(gp1)[,c("x","y")], cols)

Because our colours are already in the format that ggplot understands to interpret as colours, we'll use the scale_fill_identity() for plotting this:

ggplot(newdata, aes(x, y, fill = cols)) +
  geom_raster() +
  scale_fill_identity()

Which produced the following for me:

Alternatively, we can also plot each colour channel as a layer and use alpha to mix these:

cols <- data.frame(r = layer_data(gp1)$value,
                   g = layer_data(gp2)$value,
                   b = layer_data(gp3)$value)
newdata <- cbind(layer_data(gp1)[,c("x","y")], cols)

ggplot(newdata, aes(x, y)) +
  geom_raster(aes(alpha = r), fill = "red") +
  geom_raster(aes(alpha = g), fill = "green") +
  geom_raster(aes(alpha = b), fill = "blue")

Which gave me the following:

However, keep in mind that the order in which you add the layers is going to affect the look of the plot. In my case, blue came last, so there is a blue sheen over all the other values.

EDIT: The sheen can be largely removed by adding scale_alpha_continuous(range = c(0,1)). The resulting plot looks a lot like the next method, but doesn't mix red and green into bright yellow, which I would argue is more realistic. The extent of the data can no longer be estimated though! (end edit)

Another approach to the alpha strategy that avoids a dominant colour's sheen is to map the rgb values to hsv space, keeping the 'v' constant and set an alpha to the sum of the rows:

cols <- data.frame(r = layer_data(gp1)$value,
                   g = layer_data(gp2)$value,
                   b = layer_data(gp3)$value)
cols <- apply(cols, 2, scales::rescale)
hsv <- t(rgb2hsv(cols[,1], cols[,2], cols[,3]))
hsv <- hsv(h = hsv[,1], s = hsv[,2], v = 1, alpha = scales::rescale(rowMeans(cols)))

newdata <- cbind(layer_data(gp1)[,c("x","y")], hsv)

ggplot(newdata, aes(x, y, fill = hsv)) +
  geom_raster() +
  scale_fill_identity()

A question that you can ask yourself with that method however, is how accurately you think the yellow bit in between the red and green bit is in representing your data? Also, because we no longer have the background shape, we cannot see anymore what the extent of the data is.

Be aware that not every export method supports having alpha values for colours.

编辑：很遗憾，我不知道优美的图例解决方案，如果有人这样做，请告诉我！