[android-developers] Is it possible to lock screen without turning off display

I had came across several apps

Lock Screen Widget - Lock screen and turn off display by tapping on
screen button. User will need to press hardware power button to make
the screen visible again.

Tap Tap App - Lock screen and turn off display by tapping on screen
button. Proximity sensor still activate after screen turned off. User
can double tab across proximity sensor to make the screen visible
again.

I was wondering, is it possible for the following app to be developed,
without using any unofficial API from Android SDK

To lock a screen
1. Press a on screen button to lock the screen.
2. The screen remains visible, but get much less bright to preserve
power.

To unlock a screen
1. Slide the touch screen to unlock.
2. Brightness of the screen back to normal.

So far, from my observation is that, to lock the screen, the screen
must be turned off.

Is there any way we can lock screen without turning off display?

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