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I installed Windows 7 on Boot Camp on my new iMac 27'' (ATI videocard) and everything was good until recently I noticed that the default Windows 7 background (then one on the background on login) looked jerky.

When I say jerky I mean the kind of jerky you get if you can't display enough colours, and instead of nice fading shades you just get stripes and jerky patterns. I am on native resolution but even if I go down to 1920x1080 I get the same.

This might have happened after a firmware update but as I don't use Windows very often I am not too sure it's what caused it.

Oh, and when I am playing games everything looks OK (as in not jerky!).

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i don't think jerky is the word you're looking for. do you mean something like this upload.wikimedia.org/wikipedia/commons/b/be/… – Jay May 17 '10 at 22:21
    
yes - not as bad but that's the idea - which word would u suggest? – JohnIdol May 17 '10 at 22:39
    
When gradients get compression artifacts, they're said to be "banded". When a bitmap gets enlarged beyond its native resolution, it's said to be "jaggy". Are you seeing jaggies, or banding, or both? – Spiff May 18 '10 at 3:20
    
I think I am seeing banding. I get that at very high resolutions the wallpapers could become banded because of the stretch - but I do not get why it was not happening before then but just recently. – JohnIdol May 18 '10 at 16:35
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You mentioned a firmware upgrade. What was this firmware an upgrade for, the Monitor or something else? – Daisetsu Jun 1 '10 at 21:18

Does this happen in windowed applications (i.e., opening a photo)? This is often a sign that your monitor is running at a low depth of color. Banding generally isn't noticeable with 24- or 32-bit color depth, though with 16-bit depth is might be noticeable depending on content, and 8-bit or less will be extremely noticeable with todays applications/themes/pictures.

You can check/set the color depth for windows by going to the control panel -> Display -> Screen Resolution, click "Advanced Settings" -> Monitor Tab. At the bottom there should be a dropdown labelled "Colors:". What is this set to? If it is not set to "True color (32-bit)", try changing it to that.

Fullscreen games probably won't show the banding because they set the color depth when they take over the screen.

If you do not have 32-bit color available (perhaps only 16-bit or less) in the dropdown, I'd look for an update to your graphics drivers (either from ATI or Apple/Bootcamp). If 32-bit color is available and already selected, and other applications show images fine (it's just the login background), I would check that the file hasn't been modified (It's at C:\Windows\Web\Wallpaper\Windows\img0.jpg for me) - Right-click -> Properties -> Details -> Image -> Bit Depth (Should be "24").

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that dropdown already says true color (32bit) :( – JohnIdol Jun 4 '10 at 19:57
    
@JohnIdol do other images show the banding when you open them in an image viewer? – Darth Android Jun 4 '10 at 20:10
up vote 0 down vote accepted

This is gone after recent MacOS/Windows 7 updates - not sure exactly which!

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