Wikipedia:Reference desk/Archives/Computing/2010 June 8

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June 8

Keeping Windows error messages in the background.

Is there a setting or shareware product that would prevent Windows errors window from popping in front of the current running application window? I'm trying to design a Kiosk that runs in fullscreen mode, but occasional OS warnings and messages pop in front of it. How can I keep my front most window always in front? --70.167.58.6 (talk) 00:11, 8 June 2010 (UTC)

I'm not sure you can totally overcome all modal windows. A better approach is to set up the windows machine to not display said messages, which can be done fiddling with the administrative settings. --Mr.98 (talk) 00:51, 8 June 2010 (UTC)

Writing a DVD

How do I write files(movies) to a DVD such that it could play on DVD players??I use Nero 10 —Preceding unsigned comment added by 117.204.2.32 (talk) 08:20, 8 June 2010 (UTC)

You really should consult the help file that comes with the Nero software. It'll tell you how. --Mr.98 (talk) 13:10, 8 June 2010 (UTC)

You can use a conversion software like DVD Flick. Nero can write .iso or make a DVD from .vob .bup and .ifo files. Graeme Bartlett (talk) 22:02, 8 June 2010 (UTC)

Select DVD Video and drag your source files into the VIDEO_TS folder. If you have non-compliant files then you must use Nero Vision to convert them to VOB etc. If you don't have that, try using ConvertXtoDVD software which will eliminate the need to use Nero for creating DVDs. Sandman30s (talk) 07:57, 9 June 2010 (UTC)

All Windows desktop icons disappeared

4-year old HP desktop running Windows XP home SP3. All the desktop icons have disappeared; desktop background is still there. Everything else seems to work, including icons in the task bar. When I try to set up a new icon by dragging it onto the desktop, on releasing the mouse key it disappears. When I did this a second time with the WP icon from the Firefox address bar, a message box said this icon was already in "the folder" and did I want to replace it. Any ideas? JohnCD (talk) 08:20, 8 June 2010 (UTC)

I assume you've tried the classic turning-it-off-and-waiting-and-then-turning-it-on-again? ╟─TreasuryTag►sheriff─╢ 09:01, 8 June 2010 (UTC)

Yes, I did a restart and then a complete turn off overnight and restart from cold. And I am even-as-we-speak running a virus scan. JohnCD (talk) 09:49, 8 June 2010 (UTC)

I do not remember the old Windows XP system very well, but in Windows Vista and Windows 7, you can right-click the desktop, select "View" and then toggle "View desktop icons" on/off. --Andreas Rejbrand (talk) 11:53, 8 June 2010 (UTC)

That did it, thanks! It wasn't quite like that, but under the "Arrange icons by" submenu there was a "Show desktop icons" item which had somehow unchecked itself. Thanks again! JohnCD (talk) 13:00, 8 June 2010 (UTC)

Thinking about animations.

For an assignment, I've been told to explain what should be taken into account with regards to making animations for use on the web (probably .gif files, maybe some Flash animations, it's not specified), with particular reference to accessing the Internet from PDA's, mobile phones, etc. I figure image size (on such a small screen) and file size (on such slow connections on low-power machines) are particular relevant; anything else? I don't need an explanation, I can probably figure it out just as soon as I know what it is I might talk about. Thanks in advance - Vimescarrot (talk) 09:17, 8 June 2010 (UTC)

The frame rate is important. The higher the frame rate, the higher the CPU load. Mobile devices have less-powerful CPUs than desktop and laptop devices. (Of course, there are also older, less powerful machines — e.g., Pentium IIIs — still surfing the net, too.) Also important is the number of things on the "stage" of the movie. If you just have one thing on the stage moving around against a white background, then the CPU won't have to draw as much. But if you have many actors (what we call sprites) moving around, then the CPU load will go up. These are issues to consider both with flash and animated GIFs. I have made both animated GIFs and flash animations that bog down high-performance desktops.

Also consider the implications of adding music to your animation. Music tastes vary quite a bit, so you should add a volume slider or off button in case they don't like it.--Best Dog Ever (talk) 09:54, 8 June 2010 (UTC)

That helps. Thanks Vimescarrot (talk) 11:15, 8 June 2010 (UTC)

Excel indirect formula (not Excel indirect cell location)

Is it possible to have a range of cells in excel all refer to a FORMULA in some other cell and evaluate that. I don't mean the INDIRECT function that returns the data pointed to by a cell address in another cell.

 EG:
A1={=B1+C1} or {"B1+C1"} depending upon how it works
A2={=FANCYFUNCTION(A1)} which evaluates as {=B2+C2}
A3={=FANCYFUNCTION(A1)} which evaluates as {=B3+C3}
 then I change A1 to be
A1={=B2*5} or {"B2*5"} depending upon how it works and now I get
A2={=FANCYFUNCTION(A1)} which evaluates as {=B3*5}
A3={=FANCYFUNCTION(A1)} which evaluates as {=B4*5}

-- SGBailey (talk) 15:33, 8 June 2010 (UTC)

I'm only used to the Google Documents spreadsheet, but I doubt that Excel has what you want either. Programming languages with first-class functions do what you want, though. Paul (Stansifer) 18:38, 8 June 2010 (UTC)

You're best off using a macro for that sort of thing. (Has Google spreadsheets anything like macros?, it sounds unlikely to me) Dmcq (talk) 08:27, 9 June 2010 (UTC)

LCD monitor vs LCD TV

Why are LCD monitors more expensive than LCD TVs? --212.120.247.67 (talk) 19:34, 8 June 2010 (UTC)

Well, I don't know about the economics of it, but generally speaking LCD monitors have higher resolutions than LCD televisions, and have more variability in terms of refresh rate. I'm sure that must have some kind of effect on how they pack the transistors in there and so forth. --Mr.98 (talk) 23:28, 8 June 2010 (UTC)

As much as anything, I'd wager it has to do with economies of scale, and as Mr 98 said, the density of pixels. Riffraffselbow (talk) 13:33, 9 June 2010 (UTC)

You can certainly find a given resolution TV to be more expensive than the same PC monitor, and vice versa, it's all about the fatures (and to some extent, yes, pixel density). TVs tend to use lower fidelity display technology (such as twisted nematic), since overall quality is not as important due to the user typically being farther away. At the same time any LCD TV will include an ATSC/QAM tuner which, at full resolution is a fairly sophisticated device. Monitors will probably include fewer inputs overall but they will be of higher quality (DVI, Displayport, HDMI, RGB) compared to most TVs which sport HDMI and a few others (if you're lucky). With all that being said, now that digital output from PCs (DVI or HDMI) are ubiquitous, and digital input on TVs is the same, why not just get an LCD tv for your next monitor if it will save you some money? IMHO there are some very nice 32" units out there now that are well priced, perform the same as a monitor (same resolution, refresh, etc) and are far cheaper than even a 24" monitor. --144.191.148.3 (talk) 19:21, 9 June 2010 (UTC)

ssh authorized_keys

I need to maintain a central /etc/ssh/authorized_keys file on one machine, as opposed to having per-user authorized_keys files in ~/.ssh/authorized_keys. For forced commands, this is easily done by prepending the public key with

command="test \"$USER\" = \"allowed_user_name_here\" && /forced/command/here". 

However, I need to allow interactive logins, too, and what happens when I place a public key in /etc/ssh/authorized_keys is that whoever owns the corresponding private key can log on as any user known to the system, including root. Obviously, real security doesn't work that way. So how can I make sure that each public key in the central /etc/ssh/authorized_keys file is valid for one specific user only, without blocking that user from running interactive sessions? -- 109.193.27.65 (talk) 22:15, 8 June 2010 (UTC)

It doesn't really look like the authorized_keys file is designed to work that way. They specifically say the file shouldn't be accessible by anyone other than the intended user. Why do you need a central authorized_keys file? Oh, hmm.. could you just make the /forced/command/here for each key be "/bin/login $USER" (or the relevant username)? Indeterminate (talk) 04:09, 9 June 2010 (UTC)

I need it in a central location because the machine is basically a master copy of several clones, where /home is swapped out with the home partition needed for that specific clone, so I would have to implement some sort of detection mechanism that checks if a new /home has been mounted, and follows up with a copy/merge into the individual ~/.ssh/ folder. Also, I do not want regular users to be able to add their own additional keys.

The way I solved it for now is not using a central file, but a central path and individual file names:

AuthorizedKeysFile      /etc/ssh/%u_authorized_keys

Though I'm not sure I'm going to leave it that way. Maybe

AuthorizedKeysFile      /etc/ssh/authorized_keys.d/%u 

would make more sense.

What both solutions have in common is that the files are stored in a central location and that ordinary users can't change them because of insufficient write permissions. -- 109.193.27.65 (talk) 08:11, 9 June 2010 (UTC)

Vertical and Horizontal retrace

Quote from the "Principles of analogue television" article:

"A CRT television displays an image by scanning a beam of electrons across the screen in a pattern of horizontal lines known as a raster. At the end of each line the beam returns to the start of the next line; at the end of the last line it returns to the top of the screen."

Why wasn't analogue television designed in a boustrophedon-style signal, where the first line is traced from left to right, and the second from right to left, and when the whole screen is drawn, it starts to draw the next frame from bottom to top. This should make both horizontal and vertical retrace unnecessary. But of course I have great respect for those scientists who developed television had some reason for not doing like this. So my questions is, what's wrong with my thinking? —Preceding unsigned comment added by 83.226.129.174 (talk) 22:53, 8 June 2010 (UTC)

It is not necessary to complicate the design further by making it trace right to left to right to left... etc... It is complicated because a television must stay in sync or the picture will be flipped left-to-right or top-to-bottom. You can add safeguards for the flipping, but that makes it more complicated. The "retrace" is trivial. It is nothing more than a change in voltage on the magnetic controller. Like flipping a switch, it is nearly instantaneous to jump from one point on the screen to another. -- kainaw™ 23:07, 8 June 2010 (UTC)

I don't really know for sure, but I'd guess it's to do with graceful handling of timing drift and a failure of horizontal lock. With the l->r only scheme, if horizontal lock is lost the image will roll slowly left or rightward; that did occasionally happen with the old analog tvs of my youth. So a picture like:

  _abcdefgh_
  _ABCDEFGH_

(where _ is the horizontal blanking gap) would roll to become:

  __abcdefgh
  __ABCDEFGH

But with an alternating scheme, a differential of even 1% will cause utter loss of vertical coherence (that is, a vertical bar would become two fuzzy vertical bars) and essentially destroy the picture. So:

  _00001000_  
  _00001000_  

would become

  __00001000  
  00001000__  

Given that, when analog standards like PAL and NTSC were developed, they were targeting devices built from wobbly and very analog components, tolerance to such drifts has to be a major consideration. -- Finlay McWalter • Talk 23:16, 8 June 2010 (UTC)

Correcting my last, losing hlock on an alternating-direction scheme would completely diagonalise the picture, making it instantly garbage. -- Finlay McWalter • Talk 23:19, 8 June 2010 (UTC)

Doing it one direction only improves the registration between lines. You might notice with a printer that if you ask for high quality rather than fast it will print each line in one direction only, and that's using a stepper motor which is digital. You'd never have got a decent picture with the analogue electronics. Dmcq (talk) 08:20, 9 June 2010 (UTC)

The fast retrace from right to left is not trivial. It requires a fast change in current through the magnetic deflection coils. Due to their inductance, that corresponds to a spike of high voltage. That provides the source of high voltage that a CRT needs. If the OP's bi-directional scan were used, TVs with CRTs would be more expensive due to their need for a separate EHT supply. Using the existing line scan rates, such TVs would emit the unpleasant noise of a 7812.5Hz sawtooth. Cuddlyable3 (talk) 20:23, 11 June 2010 (UTC)