Shallow Thoughts : tags : monitor

Akkana's Musings on Open Source, Science, and Nature.

Wed, 23 Sep 2009

LCD monitor burn-in, revisited

Back in March I wrote about a burn-in problem I was seeing with my Dell S-IPS LCD monitor. I got varying reports on the web on whether this was likely to be a temporary or a permanent problem; so here's an update six months later.

The problem I noticed back in March was that my xchat window, always positioned at the same spot on the screen, was getting burned into the monitor. Against a blank white or grey screen, I could clearly see where that window's titlebar normally was, and rows of horizontal lines where the text was.

[burned in LCD monitor] First, I changed my ways to avoid having windows in the same place all the time. I changed my window manager settings to remove most window placement settings, I removed directives to show any windows on all desktops, and a worked on developing a habit of moving windows around periodically to slightly different locations (I think I'll have my Firefox window on the upper right this afternoon). I don't really like that -- I guess I'm enough stuck in my ways that I like knowing that I can look to the upper left for web pages and the lower right for IRC -- but it's not that bit a deal.

And, happiness, my burned in xchat lines went away. My old bad behavior had not permanently burned in the pixels on my nice monitor.

But that's only part of the story -- because if you look at the photo from March, xchat is not all you see that's burned in. There's also the wavy stuff going across the lower 1/4 of the screen -- and that didn't correspond to any window I'd been running.

I thought maybe it was left over from some Windows wallpaper used by the monitor's previous owner. But none of the standard wallpapers on my Vaio's WinXP partition have this pattern. (What it reminds me most is the data I used to analyze from a cell-sorting machine in my first computer job. Somehow I suspect that's not the culprit.)

These patterns, unfortunately, are not going away. In fact, the ones along the top and bottom left edges are pretty clearly getting worse, and eventually, alas, I'll probably have to replace the monitor. But meanwhile, they vary a lot.

When I first turn on the monitor in the morning, most of the time I can't see the burn-in at all. After a long day of use, it's usually pretty obvious. In between, though, there's huge variation. Sometimes they appear after an hour of use; sometimes I can go most of the day before the burn-in starts becoming visible.

It doesn't seem to be particularly temperature sensitive, and it doesn't seem to vary much with which background image I'm using that day. Sometimes when I'm going to be away for a while, I display an all white screen -- I've read a few reports indicating that can help, and anecdotally I think it does. I should probably keep better statistics on temperature, background color and time to find out what's really affecting this. Maybe I could use it as a homework project in the new Linuxchix R/Stats course!

Update: Two days after I wrote this article, the patterns were unusually bad starting first thing in the morning, and stayed bad all day ... then at about 7 in the evening, as I typed away not doing anything special, over a period of about 15 minutes they disappeared almost entirely. Quite mysterious!

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[ 11:47 Sep 23, 2009    More tech | permalink to this entry ]

Sat, 28 Mar 2009

LCD Burn-In, er, "Image Persistence"

Mostly, I love my Dell 2005FPW S_IPS monitor. Nice colors, sharp image, incredibly w ide viewing angles. But one thing made me uneasy: while working on a smooth gradient in GIMP, I noticed some funky pixels in the bottom right of the screen, where I could see horizontal bars where the image clearly didn't have them. And if I moved the image, the bars stayed put. Oh no! My lovely monitor maybe wasn't so lovely!

Since then I've been pretending not to notice (I bought the monitor used, so warranty return isn't an option). But yesterday, I maximized Firefox on a page with a medium cyan background -- and that barred area was a lot worse.

In fact, it was so much worse that I could see detail in it: it was my xchat window. I could even read some of the menubar items.

It's an LCD! They don't get burn-in ... do they?

Well, yes, it turns out, they do. Only it's not called "burn-in", it's called "image persistence". And for some people, it happens very quickly, while other people never see it. Anecdotally S-IPS monitors seem to show image persistence a lot more easily than TN monitors, but nobody seems to know why.

The good news is that it's temporary -- it's not permanent burn-in like old CRTs sometimes showed. The solutions most people suggest:

On my monitor, about an hour and a half of all-white made it better, and after turning it off overnight, the next morning I could no longer see any trace of the persistent image.
Update two days later: Strangely enough, although the pattern seemed completely gone the next morning, that evening it returned, even though I hadn't had any window at all in that space all day. I gave it another hour or two of all-white over that area, then its usual evening of rest, and the next day it was gone again and stayed gone this time. At least, it's evening now and it hasn't returned yet.

So I guess I need to change my habits. I already use power saving mode so the screen sleeps when I'm not there (no screensaver); but on the other hand I'm at the machine day and night, and I like to keep windows in the same place.

How do you invert the screen? You'd think there would be a gazillion programs to do that on X, but there aren't. You can compile a C program called sgamma then run sgamma -b -1 to invert, and sgamma -b 1 to restore. It restores to full brightness, though, so if you've changed your brightness using a program like xbrightness you'll have to adjust it again afterward. Alternately, Guillermo showed me a nice little C program called invgamma, by Ben Winslow, that just inverts whatever gamma curve you already have (run it again to undo the effects). Ben doesn't seem to have a page for it and it doesn't have any license info in it so I can't put it on my site either, but if you google for it you'll probably find a copy.

I'm a trifle bummed that the whizzy S-IPS monitor turned out to be so delicate. But I suppose it's good to change habits now and then anyway and not get too stuck on particular window positions. Maybe it'll help keep my brain from burn-in too.

References:

Update

I wrote an update, 6 months later, with more details: LCD monitor burn-in, revisited.

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[ 11:21 Mar 28, 2009    More tech | permalink to this entry ]

Wed, 26 Nov 2008

Software brightness control in X11

I love my new monitor. The colors are great, it's sharp, the angles are good. Only one problem: it's really really bright.

It has the usual baffling "push buttons at random trying to figure out how to navigate the menu system" brightness control -- which dims the monitor's perceived brightness by about .003% if I take it all the way to the bottom of the scale. (This is apparently a bug that some of these Dells have and others don't.)

It has contrast, too -- but the monitor won't change contrast when running through the DVI cable (this is even documented in the monitor's manual). I have no idea why. It makes me wonder whether there's normally a way of changing brightness over a DVI cable; but lots of googling hasn't brought enlightenment on that score.

I tried the VGA cable. The display was very noticeably less sharp, though pressing the monitor's "auto adjust" improved it a lot. Contrast adjustment did work (and helped) using the VGA cable, but it also turned everything green. I was able to improve the color cast a bit with
xgamma -ggamma .75 -bgamma .9
but this was all looking like quite a hassle. I wanted an easier way to change brightness. xgamma wasn't it: it works well for fine-tuning but its brightness curve is way off if you try to depart by much from full brightness.

Enter xbrightness and xbrightness-gui (Mikael Magnusson to the rescue again! He knew about these excellent programs, and perhaps equally important, he had a copy of xbrightness-gui, which seems to have vanished from the web.)

xbrightness is an excellent little command-line program that sets the X gamma curve to appropriate values. Just run xbrightness BRIGHTNESS passing it a value between 0 and 65535. xbrightness-gui is an interactive program that lets you drag curves around for each of the three color curves, or the combined image, with a user interface very similar to GIMP's Curves tool. You can even save and load curves.

xbrightness-gui's coolness notwithstanding, the simple xbrightness was really all I needed. It does a fine job of adjusting the monitor brightness while keeping colors neutral. The version I was using was Mikael's version, to which he'd added the ability to adjust colors too (much like xgamma does, but using more useful curves). It turns out I don't need the color correction, but it's nice to know it's there.

But what I did need was a way to query the current brightness, and, more important, a way to bump the brightness a little bit up and down. So I added those features. Getting the current brightness isn't actually something you can do, since the whole gamma curve for the three channels is what you perceive as brightness. I didn't try to estimate perceived brightness based on the whole curve; I just took the value of the highest value for each color, and their average or maximum.

Then I tied my new increment/decrement into key bindings in Openbox. I bound W-F5 (the Windows key plus F5) to xbrightness -2560, and W-F6 to xbrightness +2560, so I can go up or down in brightness by pressing keys without having to type any five-digit numbers.

I've made available the old xbrightness-gui, since it's no longer available anywhere else; a patch that integrates my changes and Mika's into xbrightness-0.3; and the patched xbrightness tarball. They're all at http://shallowsky.com/software/xbrightness/.

One other fun thing about using X gamma settings to adjust brightness. The first night I used it, I noticed at some point that my cursor looked very different -- it had become blindingly white. It turns out that the cursor is implemented at a lower level and doesn't go through the X gamma system. So turning the brightness down via gamma curves doesn't affect the cursor, which remains always at full brightness. It's quite a nice side effect -- the cursor is much more visible than it normally is.

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[ 11:37 Nov 26, 2008    More linux | permalink to this entry ]

Sat, 15 Nov 2008

Using (or not) an Apple Cinema Display on a non-Apple

Dave and I recently acquired a lovely trinket from a Mac-using friend: an old 20-inch Apple Cinema Display.

I know what you're thinking (if you're not a Mac user): surely Akkana's not lustful of Apple's vastly overpriced monitors when brand-new monitors that size are selling for under $200!

Indeed, I thought that until fairly recently. But there actually is a reason the Apple Cinema displays cost so much more than seemingly equivalent monitors -- and it's not the color and shape of the bezel.

The difference is that Apple cinema displays are a technology called S-IPS, while normal consumer LCD monitors -- those ones you see at Fry's going for around $200 for a 22-inch 1680x1050 -- are a technology called TN. (There's a third technology in between the two called S-PVA, but it's rare.)

The main differences are color range and viewing angle. The TN monitors can't display full color: they're only 6 bits per channel. They simulate colors outside that range by cycling very rapidly between two similar colors (this is called "dithering" but it's not the usual use of the term). Modern TN monitors are astoundingly fast, so they can do this dithering faster than the eye can follow, but many people say they can still see the color difference. S-IPS monitors show a true 8 bits per color channel.

The viewing angle difference is much easier to see. The published numbers are similar, something like 160 degrees for TN monitors versus 180 degrees for S-IPS, but that doesn't begin to tell the story. Align yourself in front of a TN monitor, so the colors look right. Now stand up, if you're sitting down, or squat down if you're standing. See how the image suddenly goes all inverse-video, like a photographic negative only worse? Try that with an S-IPS monitor, and no matter where you stand, all that happens is that the image gets a little less bright.

(For those wanting more background, read TN Film, MVA, PVA and IPS – Which one's for you?, the articles on TFT Central, and the wikipedia article on LCD technology.)

Now, the comparison isn't entirely one-sided. TN monitors have their advantages too. They're outrageously inexpensive. They're blindingly fast -- gamers like them because they don't leave "ghosts" behind fast-moving images. And they're very power efficient (S-IPS monitors, are only a little better than a CRT). But clearly, if you spend a lot of time editing photos and an S-IPS monitor falls into your possession, it's worth at least trying out.

But how? The old Apple Cinema display has a nonstandard connector, called ADC, which provides video, power and USB1 all at once. It turns out the only adaptor from a PC video card with DVI output (forget about using an older card that supports only VGA) to an ADC monitor is the $99 adaptor from the Apple store. It comes with a power brick and USB plug.

Okay, that's a lot for an adaptor, but it's the only game in town, so off I went to the Apple store, and a very short time later I had the monitor plugged in to my machine and showing an image. (On Ubuntu Hardy, simply removing xorg.conf was all I needed, and X automatically detected the correct resolution. But eventually I put back one section from my old xorg.conf, the keyboard section that specifies "XkbOptions" to be "ctrl:nocaps".)

And oh, the image was beautiful. So sharp, clear, bright and colorful. And I got it working so easily!

Of course, things weren't as good as they seemed (they never are, with computers, are they?) Over the next few days I collected a list of things that weren't working quite right:

The brightness problem was the easiest. A little web searching led me to acdcontrol, a commandline program to control brightness on Apple monitors. It turns out that it works via the USB plug of the ADC connector, which I initially hadn't connected (having not much use for another USB 1.1 hub). Naturally, Ubuntu's udev/hal setup created the device in a nonstandard place and with permissions that only worked for root, so I had to figure out that I needed to edit /etc/udev/rules.d/20-names.rules and change the hiddev line to read:

KERNEL=="hiddev[0-9]*", NAME="usb/%k", GROUP="video", MODE="0660"
That did the trick, and after that acdcontrol worked beautifully.

On the second problem, I never did figure out why suspending with the Apple monitor always locked up the machine, either during suspend or resume. I guess I could live without suspend on a desktop, though I sure like having it.

The third problem was the killer. Big deal, who needs text consoles, right? Well, I use them for debugging, but what was more important, also broken were the grub screen (I could no longer choose kernels or boot options) and the BIOS screen (not something I need very often, but when you need it you really need it).

In fact, the text console itself wasn't a problem. It turns out the problem is that the Apple display won't take a 640x480 signal. I tried building a kernel with framebuffer enabled, and indeed, that gave me back my boot messages and text consoles (at 1280x1024), but still no grub or BIOS screens. It might be possible to hack a grub that could display at 1280x1024. But never being able to change BIOS parameters would be a drag.

The problems were mounting up. Some had solutions; some required further hacking; some didn't have solutions at all. Was this monitor worth the hassle? But the display was so beautiful ...

That was when Dave discovered TFT Central's search page -- and we learned that the Dell 2005FPW uses the exact same Philips tube as the Apple, and there are lots of them for sale used,. That sealed it -- Dave took the Apple monitor (he has a Mac, though he'll need a solution for his Linux box too) and I bought a Dell. Its image is just as beautiful as the Apple (and the bezel is nicer) and it works with DVI or VGA, works at resolutions down to 640x480 and even has a powered speaker bar attached.

Maybe it's possible to make an old Apple Cinema display work on a Mac. But it's way too much work. On a PC, the Dell is a much better bet.

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[ 20:57 Nov 15, 2008    More tech | permalink to this entry ]