1. Calculate the screen size. For example 1920 x 1080. Make sure that the width is a multiple of 4. For example, for HD screen resolution of 1366x768, use 1368x768
2. dd if=/dev/fb1 bs=1368 count=3072 | gzip -c > screen.bgra.gz
Each pixel is 4 byte, B-G-R-A. So 1368x768 resolution takes 1368*768*4=1368*3072 bytes
3. transfer the file to a Linux computer with imagemagick installed (needs a version released after 2010), and then convert it:
gunzip screen.bgra.gz
convert -size 1368x768 -depth 8 screen.bgra screen.png
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