How do LCD's refresh, can they tear?

[Frosteh]

So I've got my new LCD and I'm loving it long time

I just wish to increase my understanding of LCD's and how they work, I just like to how things work.

I'm kind of under the impression that LCD's refresh each pixel at the same time rather than scanning from side to side and top to bottom like a traditional CRT does. I'm not quite sure why I assumed this I just did with the available information, maybe it's completely wrong but I'd like to be sure.

Does this mean that LCD's can't tear in the traditional sense? CRT's could tear because the image that is currently active on the cable (whatever is in the video cards buffer) would be rendered by the monitor despite where it was in the vertical scan, so if it was rendering frame 1 and got to the 30th line of pixels down the monitor and frame 1 was switched out to display frame 2 the monitor would carry on drawing row 31 from frame 2 and you'd end up with 1 image on the screen comprised of 2 or possibly more different frames.

If LCD's refresh 1 pixel, or row at a time, which order is it done?

Since LCD's also have a finite refresh time does that mean that they can have the frame change inbertween screen refreshes, if so do they use some kind of buffer to store the newest frame and then display it or what?

Thanks in advance for any info on the topic.

 

[Chernobyl1]

LCD screens work as an array of memory cells within the LCD itself.
As you probably know, memory cannot all be accessed at once so a full screen refresh will be handled one memory location at a time.
If the graphics card changes its primary screen buffer (which is very fast) in the middle of doing a refresh on the LCD (which isnt as fast), image tearing can occur.

By enabling VSync, the primary display buffer on the graphics card will wait for the LCD to finish its screen refresh before it gets updated, eliminating any changes occuring partway through an LCD refresh.

edit
to further explain VSync...
VSync requires the use of buffers in the graphics card memory to prepare the next screen image so there is no processing of the image on screen. ie the image will be drawn in a different buffer and then that buffer will become the primary buffer which is mapped to the output for the LCD.
This will give you a single screen refresh delay in displaying the image.
Most of the time you wont notice, but twitch shooter players might.


You can specify in the driver how many buffers you would like to use, the default is usually 3.

 

[Frosteh]

Thanks, I am aware of how Vsnc works and what it does, something I read up on a lot with CRT's

So the screen refresh isnt instant, do LCD's refresh in a typical side to side, top to bottom fasion?

I've not used vsync with my new LCD and I can't see any tearing what so ever, does tearing become harder to see with LCD's or do you think the Vsync implimentation I have running is broken, maybe the game is forcing it on over the drivers or something (I will double check this with my frame rate)

 

[Chernobyl1]

An LCD will refresh usually in the most straight forward manner for the LCD panels design.
This could vary between panels depending how the screen locations are mapped to memory addresses. Ususally the interface to the LCD will give consecutive memory allocation to consecutive cells on the screen.
This doesnt have to be the case though.

I dont notice screen tearing as much with my LCD but it is there.

Turn VSync off, load Winamp with the Milkdrop visual plugin running full screen and you will see heavy tearing at times.