HeLLoWorld wrote:Schemes where the desired value is deliberately overshooted (ie does not settle to correct value but higher), but is correct (but still rising) precisely at the moment of the flash, and with different target values for top and bottom of screen (even if already using accelerated scanout)
Unfortunately the exact crossing occurs at different times for different GtG combinations, and at different temperatures (hotter = faster). Don't forget the frozen/cold LCD knowledge too. So you've got a big engineering challenge to make the zero crossing occur simultaneously for the whole screen. You'll need 3D or 4D buffers (X times Y times multiple refresh history), and probably thermosistors, to keep track of how much ghosting is occuring where, and compensate for that, as well as using temperature information to create such precise voltage/timing information, to aim the voltage crossing at correct times for all pixels simultaneously. Ugh. Precise control of a wide voltage range.
Why not do something simpler such as enlarging the blanking interval instead?
It's easier to just wait-out the overdrive rippling. Create a 4ms pause between refreshes, for a 1ms LCD, for example.
Or why not do a double-pass refresh? (heavily overdriven refresh, immediately followed by a clean-up refresh)
Some strobed LCDs use this approach.
HeLLoWorld wrote:(ie, bottom pixels overshoot more, because they must rise faster, because they are commanded later than top pixels).
Only if you use Y-axis-compensated overdrive algorithms, like LightBoost does.
But not all strobe backlights does this, and it is also often beyond the scope of hobbyist hacking.
Easier is to simply use a high bandwidth LCD (capable of, e.g. 240Hz scanout -- like some HDTV LCDs are) -- and just enlarge the blanking interval as large as possible. Many new 240Hz LCDs now exist in the television industry, but refreshed from interpolation engines rather than direct-driven from a computer. But this is new DIY hacking territory that cirthix has finally accomplished (240Hz LCD), and hopefully manufacturers will follow suit. But 240Hz LCDs present immense opportunities for accelerated-scanout (e.g. refreshing a slower refresh cycle in a 1/240sec scanout). Direct-driving a 240Hz-capable LCD is geek territory. Ordering bare panels from places like Alibaba (china factory bazaar), designing your own driver circuit to drive the panel, etc. DIY territory finally broken by the likes cirthix.
HeLLoWorld wrote:So the whole screen changes at different rates
Only for Y-axis-compensated overdrive. Not for most LCDs, but only on some LCDs.
It is not used for the purposes of timing the strobe for overdrive overshoot, but to attempt to speed up the LCD pixels along the bottom edge of the screen. During Y-axis compensated overdrive, it appears to sometimes have quantitization bands that occur due to rounding errors: e.g. viewing
http://www.testufo.com/flicker in LightBoost (Height -> "Full Screen") reveals the bands of Y-axis compensated overdrive.
HeLLoWorld wrote:I guess the overshooting algos (like overdrive probably is) must be complicated enough to additionnaly take in acccout the value of the desired color of the individual pixel and maybe the previous value can be a valuable information too.
They already do, overdrive by definition, always uses previous color (at the minimum). That's what the memory chips in an LCD screen is often used for -- overdrive color processing. Overdrive always use previous values, and sometimes the previous 2-3 values, if the LCD is slow enough that multiple-level history helps. Sometimes they also buffer so they know the NEXT value too. That's why some LCDs add input lag when you enable overdrive because lookforward requires adding input lag. Fortunately, this does not happen with Asus Trace Free or BENQ AMA, they use only history buffers, not lookforward buffers.
The VG248QE is pretty good at ghost-free strobing. The overdrive ghosting is almost below human perceptible levels (<1% delta between visible value and correct value) along the horizontal center band of the screen.