4.4 Automatic Text Contrast

In the previous section Figure 4.4, the overlap fill defined by 'label.fill' was set to blue. Text colors were also manually changed using label.color to improve visual contrast. This step may be automated by using make_color_contrast().

Given a set of input text colors in argument x, and background colors in y, the function returns a vector of colors to contrast with the background. The purpose is to retain some color saturation, as opposed to returning either white or black, while also prioritizing visual clarity.

An optional argument bg can be used to define the canvas background color, which is useful when the colors in y have partial transparency, and would also partly show the canvas color.

Two additional arguments to make_color_contrast() may be relevant: L_lo=40 and L_hi=95, which control the output color darkness and lightness, respectively. Adjustments may be passed through venndir() using '...' ellipses to modify the color saturation, for example: venndir(setlist, L_hi=85).

The argument do_plot=TRUE is used to visualize the input and output colors for review.

x <- c("red3", "blue", "gold", "pink", "black");
xc1 <- make_color_contrast(x, y="royalblue3", do_plot=TRUE);

xc2 <- make_color_contrast(x, y="#FFF9EE", do_plot=TRUE);

Figure 4.5: Two examples with blue background (top) and off-white background (bottom). In each plot, the top row labels indicate the input colors, the bottom row labels are the output colors adjusted to improve contrast.

Putting these techniques together, the example in Figure 4.6 uses modify_venndir_overlap() together with make_color_contrast() to ensure text labels have visual contrast.

v <- venndir(make_venn_test(do_signed=TRUE), do_plot=FALSE)

new_bg <- "darkgreen";
current_colors <- c("black", "firebrick", "dodgerblue3", "purple4");

v2 <- modify_venndir_overlap(v,
   overlap_set="set_A&set_C",
   params=list(
      label.fill=new_bg,
      label.border="black",
      label.color=make_color_contrast(current_colors, new_bg),
      label.count="outside",
      innerborder=new_bg,
      innerborder.lwd=3)
   )
plot(v2)

Figure 4.6: Venn diagram with modifications to the label for 'set_A&set_B'.

The modifications made in this figure:

• new_bg was defined as 'darkgreen'.
• new_bg was applied to the label.fill and the innerborder.
• innerborder.lwd was increased to 3.
• label.border was changed to black, to contrast with dark green.
• label.count was set to 'outside' to place count labels outside.
• current_colors were defined to match the existing label text colors.
• label.color was defined using make_color_contrast(), with arguments current_colors and new_bg.