FFmpeg: Rainbow trail chromakey effect

An effect loosely inspired by old Scanimate¹ analogue video effects. The process involves stacking progressively delayed, and colourised, instances of the input video on top of each other. These overlays are blended based on a chosen colourkey, or chromakey. The colour values and number of repetitions can be easily changed, though with higher numbers [in test cases, 40+], buffer underflows may be experienced.


# Generate ['Scanimate' inspired] rainbow trail video effect with FFmpeg
# (N.B. Resource intensive - consider multiple passes for longer trails) 
# version: 2017.
# source: http://oioiiooixiii.blogspot.com

function rainbowFilter() #1:delay 2:keytype 3:color 4:sim val 5:blend 6:loop num
   local delay="PTS+${1:-0.1}/TB" # Set delay between video instances
   local keyType="${2:-colorkey}" # Select between 'colorkey' and 'chromakey'
   local key="0x${3:-000000}"     # 'key colour
   local chromaSim="${4:-0.1}"    # 'key similarity level
   local chromaBlend="${5:-0.4}"  # 'key blending level
   local colourReset="colorchannelmixer=2:2:2:2:0:0:0:0:0:0:0:0:0:0:0:0
   # Reset colour after each colour change (stops colours heading to black)
   # 'smartblur' to soften edges caused by setting colours to white

   # Array of rainbow colours. Ideally, this could be generated algorithmically
   local colours=(
      "2:0:0:0:0:0:0:0:2:0:0:0:0:0:0:0" "0.5:0:0:0:0:0:0:0:2:0:0:0:0:0:0:0"
      "0:0:0:0:0:0:0:0:2:0:0:0:0:0:0:0" "0:0:0:0:2:0:0:0:0:0:0:0:0:0:0:0"
      "2:0:0:0:2:0:0:0:0:0:0:0:0:0:0:0" "2:0:0:0:0.5:0:0:0:0:0:0:0:0:0:0:0"

   # Generate body of filtergraph (default: 7 loops. Also, colour choice mod 7)
   for (( i=0;i<${6:-7};i++ ))
      local filter=" $filter
   printf "split [a][b];${filter}[a][b]overlay"

ffmpeg -i "$1" -vf "$(rainbowFilter)" -c:v huffyuv "${1}_rainbow.avi"
download: ffmpeg_rainbow-trail.sh

This is a top-down approach to building the effect. Another [possibly better] solution is to build the layers from the bottom up (pre-calculate the PTS delay for each layer i.e. "layer number x PTS delay"). This might improve the fidelity of the top layer in certain videos. Another idea is split input into three instances rather than two, and 'key overlay the third at the very end of the filter.

A concatenation of all videos generated during testing and development.

¹ Scanimate video synthesizer: http://scanimate.com/
original video: https://www.youtube.com/watch?v=god7hAPv8f0