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This is my first post here. I'm at the beginning of the road called Raspberry Pi.

Is there any way of changing the gamma/contrast/brightness of monitor via Raspberry Pi?

My Samsung 960BF doesn't have any hardware adjustment, only a power button.

I tried using xbacklight and got error

"No outputs have backlight property".

gddccontrol also didn't work. Error:

I/O warning: failed to load external entity "/home/pi/.dccontrol/monitorlist"  
   Document not parsed successfully
   Probing for available monitors"

And nothing is happening.

I connected my monitor through DVI-HDMI cable. Maybe this is why it doesn't work?

I'm using Raspberry Pi B+ V1.2 with Raspbian.

tarzanno
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1 Answers1

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One tool I have used successfully (not on a pi though), is xcalib from the package of the same name. You can use it to change the gamma, eg:

xcalib -gc 1.1 -a

You can repeat this. To cancel use

xcalib -clear

Another useful option is making the whole screen reverse-video:

 xcalib -invert -a

It is intended to work with icc profiles, but I have only used the above commands.

meuh
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  • Thanks for reply. I tried this, but nothing happens, end after executing command: xcalib -b 99 there is an error: "Warning: Unable to read file ' ' " – tarzanno Jul 08 '15 at 13:44
  • You would need `-b 99 -a` but I wouldnt do that, the screen will be all white. – meuh Jul 11 '15 at 04:34
  • Unfortunately, it doesn't work either. I tried 'xcalib -b 99 -a', 'sudo xcalib -b 99 -a' , and 'xcalib -b 10 -a' to make sure my monitor is not set to max brightness already. There is no error and nothing is happening. I'm afraid that I will need to learn i2c and control the monitor by that... – tarzanno Jul 11 '15 at 11:47
  • Perhaps your cable is not passing through the i2c data line. Look in your `/var/log/Xorg.0.log` or similar log file to see if the display's eeprom was found (look for `EDID`). – meuh Jul 11 '15 at 12:12
  • Nothing like that exists in this file. There is only a line telling that Rpi is using default monitor and : "no device specified for default monitor section". Must I buy a new HDMI-DVI cable, or there is another way? – tarzanno Jul 11 '15 at 12:31
  • This [wikipedia](https://en.wikipedia.org/wiki/Digital_Visual_Interface) article on DVI says pins 6 and 7 carry the DDC (i2c) signal. And for [hdmi](https://en.wikipedia.org/wiki/HDMI) it is 15 and 16. You could use a multimeter to see if your cable passes these signals, or if the pins even exist. – meuh Jul 11 '15 at 12:52
  • I used multimeter and it seems to be a voltage on 6 and 7 pin, so there must be a software issue. I checked and my monitor has DDC 2B. – tarzanno Jul 11 '15 at 13:40
  • Also, I tried 'sudo i2cdetect -y 1' and there are no ports detected... very strange. EDIT: after adding 'device_tree=' to '/boot/config.txt', I managed to detect some ports with i2cdetect. There is 'UU' visible on B 10 and B 30. Despite this, I still can't change anything with xcalib. – tarzanno Jul 11 '15 at 14:08
  • xcalib as I understand it only works within the X server. To chat with the monitor there is `ddccontrol`. – meuh Jul 11 '15 at 14:11
  • Now, in Xorg.0.log I can see the monitor device `Allwiner A10/A13 FBDEV` . it seems like i2c is now working, but is busy (UU). After typing `ddccontrol -p` I get an error `pcilib: can't find any working access method`. – tarzanno Jul 11 '15 at 14:23
  • There is also `(...) monitorspcilib: Can't open /proc/bus/pci` And there is no `pci` folder there as Raspberry doesn't have a pci bus. There is no way of using ddccontrol then? – tarzanno Jul 11 '15 at 14:36