Dot matrix boards

[dibble1]

Why do dot matrix boards make the scrolling writing appear slanted when even if you check the leds they appear perfectly vertical. Is this programmed in or an optical illusion?
Thanks

[HowardKennitby]

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http://www.doineedaneyetest.com/

[THECORBYLOON]

I think it's to do wi the way that each horizontal line is generated. If it is a slow rate, the led may have gone out and the one next to it may have lit up but the one immediately below the first one is on still because it is on the next line and it lags behind. If each line lags behind the one above, the letters appear to be slanting to the right. I suppose if each line is changed at the same time and not in sequence then it would not slant.

[methyl]

Off topic.
Late on night on the Motorway I saw a matrix board displaying a slanted error code f-zero-nine and wondered what had gone wrong with the system.
Then I hit a bank of FOG.