Add doc for system health guidelines for AOD clock faces.

# Clock Plugins

## Introduction

The clock appearing on the lock screen and always on display (AOD) can be

customized via the ClockPlugin plugin interface.

## System Health

Clocks are high risk for battery consumption and screen burn-in because they

modify the UI of AOD.

To reduce battery consumption, it is recommended to

target a maximum on-pixel-ratio (OPR) of 5%. Clocks that are composed of

large blocks of color that cause the OPR to exceed 5% should be avoided.

To prevent screen burn-in, clocks should not be composed of large solid

blocks of color, and the clock should be moved around the screen to

distribute the on pixels across a large number of pixels. Software

burn-in testing is a good starting point to assess the pixel shifting

(clock movement) scheme and shape of the clock.

### Software Burn-In Test

The goal is to look for bright spots in the luminosity average over a period of

time. It is difficult to define a threshold where burn-in will occur. It is,

therefore, recommended to compare against an element on AOD that is known not

to cause problems.

For clock face that contain color, it is recommended to use an all white

version of the face. Since white has the highest luminosity, this version of

the clock face represents the worst case scenario.

To start, generate a sequence of screenshots for each minute over a 12 hr interval.

```

serial = '84TY004MS' # serial number for the device

count = 1

t = datetime.datetime(2019, 1, 1)

stop = t + datetime.timedelta(hours=12)

if not os.path.exists(OUTPUT_FOLDER):

  raise RuntimeError('output folder "%s" does not exist' % OUTPUT_FOLDER)

while t <= stop:

  os.system("adb -s %s shell 'date %s ; am broadcast -a android.intent.action.TIME_SET'" % (serial, t.strftime('%m%d%H%M%Y.%S')))

  os.system('adb -s %s shell screencap -p > %s/screencap_%06d.png' % (serial, OUTPUT_FOLDER, count))

  t += datetime.timedelta(minutes=1)

  count += 1

```

Average the luminosity of the screenshots.

```

#!python

import numpy

import scipy.ndimage

from imageio import imread, imwrite

import matplotlib.pylab as plt

import os

import os.path

def images(path):

  return [os.path.join(path, name) for name in os.listdir(path) if name.endswith('.png')]

def average(images):

  AVG = None

  for name in images:

    IM = scipy.ndimage.imread(name, mode='L')

    A = numpy.array(IM, dtype=numpy.double)

    if AVG is None:

      AVG = A

    else:

      AVG += A

  AVG /= len(images)

  return numpy.array(AVG, dtype=numpy.uint8)

def main(path):

  ims = images(path)

  if len(ims) == 0:

    raise ValueError("folder '%s' doesn't contain any png files" % path)

  AVG = average(ims)

  imwrite('average.png', AVG)

  plt.imshow(AVG)

  plt.show()

if __name__=='__main__':

  import sys

  main(sys.argv[1])

```

Look for bright spots in the luminosity average. If bright spots are found,

action should be taken to change the shape of the clock face or increase the

amount of pixel shifting.