How to use cron + python to regularly adjust screen brightness?

Question

I want to use cron and this script (http://askubuntu.com/questions/23593/use-webcam-to-sense-lighting-condition-and-adjust-screen-brightness):

import opencv
import opencv.highgui
import time
import commands


def get_image():
    image = opencv.highgui.cvQueryFrame(camera)
    return opencv.adaptors.Ipl2PIL(image)

camera = opencv.highgui.cvCreateCameraCapture(-1)

while 1:
    image = get_image()
    image.thumbnail((32, 24, ))
    image = tuple(ord(i) for i in image.tostring())
    x = int((int((max(image) / 256.0) * 10) + 1) ** 0.5 / 3 * 10)
    cmd = ("sudo su -c 'echo " + str(x) +
        " > /sys/devices/virtual/backlight/acpi_video0/brightness'")
    status, output = commands.getstatusoutput(cmd)
    assert status is 0

crontab: /30*** sudo python /home/username/screen.py

However, there are two issues:

First, can the while-loop be terminated after 5 sec or so? Second, could anyone please try to improve the script so that lower brightness-levels can be set using the script? Maybe the way the 256 brightness-levels are mapped to the ones that can be set for the screen would need to be changed.

Thanks

Answer 1

In older kernels there was brightness control file somewhere in /proc, but I think that it was the same functionality as /sys in your code snippet. In this /procfile there was several levels of brightness that you could use and probably the same is in that mechanism. Try to cat /sys/devices/virtual/backlight/acpi_video0/brightness and check if there is information about brightness levels that you may use.

If you remove loop whole code will be executed once so it will be possible to set brightness only once per half hour period. There is little disadvantage of this solution - if you cover camera at the moment of brightness checking, brightness will be changed and next reading will be after half hour so you end up with completely dark screen for half hour.

To avoid this situation you could replace while loop by for loop (0 to 5 for example) with 5 seconds sleep with it. In loop you can only check brightness five times and after loop you could calculate average brightness and set it.

EDIT: Code with average from 25 seconds:

import opencv
import opencv.highgui
import time
import commands
from time import sleep

def get_image():
    image = opencv.highgui.cvQueryFrame(camera)
    return opencv.adaptors.Ipl2PIL(image)

camera = opencv.highgui.cvCreateCameraCapture(-1)

x = []

for i in range(5):
    image = get_image()
    image.thumbnail((32, 24, ))
    image = tuple(ord(i) for i in image.tostring())
    x.append(int((int((max(image) / 256.0) * 10) + 1) ** 0.5 / 3 * 10))
    sleep(5)

sum = 0
for i in x:
    sum = sum + i

avg = sum / len(x)
cmd = ("sudo su -c 'echo " + str(avg) + " > /sys/devices/virtual/backlight/acpi_video0/brightness'")
status, output = commands.getstatusoutput(cmd)
assert status is 0

Unfortunately I don't have option to change backlight (old kernel or something missing in kernel) and I don't have camera to check if it is working...