#!/bin/bash
ffmpeg -i "$1" -r 2 -f image2 frames%05d.png &> /dev/null
for f in frames*.png; do
    convert $f -matte -fuzz 30% -transparent red -channel A -negate +channel -background white -flatten ${f}-result.png
    convert ${f}-result.png ${f}-result.pbm
    convert ${f}-result.pbm ${f}-result.tif
    #gocr ${f}-result.pbm >> lst.txt
    tesseract ${f}-result.tif ${f}-result &> /dev/null
done
grep -Pioh '^[a-z0-9]+$' *.txt|sort|uniq -c| sort -nr

---
$ ~/l337/captcha.sh /tmp/FlashCUVj3l
      4 GWZ
      2 GNL
      1 M2
      1 JZ
      1 GW
---

yes, GWZ is correct :P