WebP animation support

[klausbrunner]

A sequel to #195 (containing the same changes as it's based on that branch), this adds animation support within a WebP device. Frames are collected in-memory and stitched into an animation on dev.off(). Example:

agg_webp_anim("animation.webp", delay=100)
for(i in 1:10) {
  plot(sin(1:100 + i)) 
  dev.flush()
}
dev.off()

[klausbrunner]

Just to validate this approach, the following little stress test (1000 frames at 4k resolution, lossless) takes about 5 minutes on my laptop and results in a webp file of about 45 MB. Unsurprisingly, CPU is the bottleneck as it maxes out a single core. Memory usage barely changes during the run.

test_that("agg_webp_anim handles large animations", {
  skip_on_cran()

  tmp <- tempfile(fileext = ".webp")
  nframes <- 1000
  agg_webp_anim(tmp, width = 3840, height = 2160, delay = 10, loop = 1)

  colors <- rainbow(nframes)
  cat("Processing", nframes, "frames at 4K resolution...\n")
  start_time <- Sys.time()

  for (i in seq_len(nframes)) {
    if (i %% 100 == 0) cat("Frame", i, "/", nframes, "\n")

    plot.new()
    rect(0, 0, 1, 1, col = paste0(colors[i], "08"), border = NA)
    text(0.5, 0.7, paste("FRAME", i), cex = 6, col = colors[i], font = 2)

    n_points <- 100
    for (j in 1:n_points) {
      x <- j / n_points
      y <- 0.5 + 0.15 * sin(8*pi*x + i/8) * cos(4*pi*x - i/12)
      points(x, y, pch = 19, cex = 3, col = colors[i])
    }

    rect(0.1, 0.1, 0.1 + 0.8 * (i / nframes), 0.15, col = colors[i], border = "black")
    text(0.5, 0.05, paste(i, "of", nframes), cex = 2)

    dev.flush()
  }
  dev.off()

  expect_true(file.exists(tmp))
  expect_gt(file.info(tmp)$size, 100000)

  if (debugging) {
    elapsed <- as.numeric(difftime(Sys.time(), start_time, units = "secs"))
    cat("Big WebP animation:", tmp, round(file.info(tmp)$size / 1024), "KB,", round(elapsed, 1), "seconds\n")
  } else {
    unlink(tmp)
  }
})

[thomasp85]

Thanks for all of this