[Sugar-devel] Grayscale screen implementation query

James Cameron quozl at laptop.org
Tue Jun 4 05:30:26 EDT 2019

Thanks for asking.

A kernel configuration file is used to select colour or grayscale
mode in the driver, and thus the display controller ASIC.  This is in
the kernel sources.

The keyboard shim detects brightness control keys and manages the
backlight power.  When the backlight is turned off, the grayscale mode
is enabled.  When the backlight is turned on, the grayscale mode is
disabled.  This is in the keyboard shim sources.

On the XO-1.75 and XO-4 models, a reverse biased red LED in the same
SMT package as the green storage LED is used as an ambient light
sensor.  The power management daemon (powerd) detects high ambient
light and turns off the backlight and turns on the grayscale mode.
The backlight is turned off to save power, and increase running time
on battery.  This is in the powerd sources.

However, without any change to backlight brightness or mode switching,
full sunlight does appear as greyscale, because the human eye can't
discern the small amount of colour emitted by the backlight compared
to the large amount of white light reflected from the sun.

On Tue, Jun 04, 2019 at 12:48:38AM +0530, Sumit Srivastava wrote:
> Hi, all!
> Here's a query that has been directed at me on the internet, however I don't
> have any idea about the grayscale implementation of the screen in XO-1. How
> does it switch from colour to grayscale and on what basis is it decided?
> >The original query:
> {"generic Josh number 96" asks
> "Do you know if the display is switching modes with the greyscale thing? Very
> curious how the screen works"}

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James Cameron

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