Correct (active matrix transitors keeping voltage constant to pixel).. But the LCD pixels can stick while it's momentuming to its final color value. They are prone to retention, and they tend to resist, get stuck (not sure of physical phenomenae; probably stiction in the molecules? not sure). That's why inversion exists. The voltage alternates to positive, negative (LCD inversion explanation), this kicks the LCD pixels out of its tendency to image-retention, pushing it closer to the final color. I believe, for the most part, that's why subsequent refresh passes "kicks" an LCD pixel ever closer and closer to its final color value. There may be other reasons, but this is the main explanation I can think of. There could also be decay in the active matrix transitors that needs to be re-refreshed (like a DRAM refresh pass), as TN LCDs will gradually fade to white when there's no refreshing -- no repeated voltage-resetting passes at the active matrix transitor gates...blargg wrote:Doesn't the LCD hold the voltage between passes?
A little known factoid: This is why I had to make modifications to http://www.testufo.com/flicker to prevent image retention (aka LCD burn in) of a forgotten browser window -- the flicker test causes image retention after a minute or so. Because flicker patterns often 'defect' LCD inversion ability to erase image retention. Every 10-20 seconds, it adds a repeat refresh (e.g. black-white-black-white-black-black-white-black) to reverse the phase. Even so, if you drag your browser window half an inch up/down once every 5 second at http://www.testufo.com/flicker on certain monitors there will be a faint evidence of the old boundaries.
I should create a TestUFO LCD test pattern that demonstrates image retention (inversion-defeating animation, such as text flickering for 30 seconds every other refresh cycle), followed suddenly by a solid image (to show LCD image retention) followed by an image-retention-erasing flicker sequence (black-black-white-white-black-black-white-white-black-black). Flickering items every refresh tend to defect current LCD inversion algorithms, while flickering items every other refresh tend to undo the image retention quite quickly.