agg_supertransparent: Draw to a PNG file, modifying transparency on the fly

Description

The graphic engine in R only supports 8bit colours. This is for the most part fine, as 8bit gives all the fidelity needed for most graphing needs. However, this may become a limitation if you need to plot thousands of very translucent shapes on top of each other. 8bit only afford a minimum of 1/255 alpha, which may end up accumulating to fully opaque at some point. This device allows you to create a 16bit device that modifies the alpha level of all incomming colours by a fixed multiplier, thus allowing for much more translucent colours. The device will only modify transparent colour, so if you pass in an opaque colour it will be left unchanged.

Usage

agg_supertransparent(
  filename = "Rplot%03d.png",
  width = 480,
  height = 480,
  units = "px",
  pointsize = 12,
  background = "white",
  res = 72,
  scaling = 1,
  snap_rect = TRUE,
  alpha_mod = 1,
  bg
)

Arguments

filename: The name of the file. Follows the same semantics as the file naming in grDevices::png(), meaning that you can provide a sprintf() compliant string format to name multiple plots (such as the default value)
width, height: The dimensions of the device
units: The unit width and height is measured in, in either pixels ('px'), inches ('in'), millimeters ('mm'), or centimeter ('cm').
pointsize: The default pointsize of the device in pt. This will in general not have any effect on grid graphics (including ggplot2) as text size is always set explicitly there.
background: The background colour of the device
res: The resolution of the device. This setting will govern how device dimensions given in inches, centimeters, or millimeters will be converted to pixels. Further, it will be used to scale text sizes and linewidths
scaling: A scaling factor to apply to the rendered line width and text size. Useful for getting the right dimensions at the resolution that you need. If e.g. you need to render a plot at 4000x3000 pixels for it to fit into a layout, but you find that the result appears to small, you can increase the scaling argument to make everything appear bigger at the same resolution.
snap_rect: Should axis-aligned rectangles drawn with only fill snap to the pixel grid. This will prevent anti-aliasing artifacts when two rectangles are touching at their border.
alpha_mod: A numeric between 0 and 1 that will be multiplied to the alpha channel of all transparent colours
bg: Same as background for compatibility with old graphic device APIs