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I noticed that with the Froyo "desk clock" application, when you put it in low light mode, it forces the back-light far darker than you can manually make it in the settings.

My question is: is there an application or a non-root hack that can make the back-light really dim just like in the desk clock application? As that clock can set brightness lower without rooting, there should be a way to do that without rooting.

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I don't know anything about the particulars of this situation, but I'd just like to point out that just because a system app can do something does not mean that other non-root apps can. Apps signed with the system-keys have access to far more permissions and APIs than other apps, enabling manufacturers to do many more things than a typical 3rd party developer. For example, some phones can take screenshots with a simple key combination, but you need root to accomplish this from a 3rd party app. –  eldarerathis May 31 '11 at 17:34
    
I just noticed this same thing last night when I was setting my clock. –  Daniel May 31 '11 at 22:21
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3 Answers

up vote 2 down vote accepted

The Clock app is setting flags on its own fullscreen window that override the system default screen brightness, but this only affects that app's window.

See the source for DeskClock.java

 551             winParams.flags |= WindowManager.LayoutParams.FLAG_FULLSCREEN;
 552             winParams.dimAmount = DIM_BEHIND_AMOUNT_DIMMED;
 553             winParams.buttonBrightness = WindowManager.LayoutParams.BRIGHTNESS_OVERRIDE_OFF;

The flags used are documented on the Android Developer site, in particular:

float BRIGHTNESS_OVERRIDE_OFF Value for screenBrightness and buttonBrightness indicating that the screen or button backlight brightness should be set to the lowest value when this window is in front.

So, the DeskClock is not using anything special to dim the screen, but it can only affect its own window.

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Does it mean that low brightness of clock has no effect on battery? I mean lower battery usage –  LifeH2O Jun 7 '11 at 18:37
    
According to the accepted answer for this question, it will help battery usage for OLED, AMOLED, and Super AMOLED, but not LCD screens. –  TomG Jun 8 '11 at 1:58
    
Does that code indicate that an application with such ability (dim beyond original limit for all apps) can be built? –  LifeH2O Jun 19 '11 at 3:24
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Try Screen Filter.

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If you want to read books in low light, use Foxit Reader, it can dim the screen nearly as clock.

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