[SOLVED] 24 bit depth on some apps with NVIDIA 340.24

Refractor · 07-23-2014, 02:33 PM

Hey everyone!
I've got a Slack64 14.1, upgraded to current and then installed the multilib packages on a Lenovo Y510P without hybrid graphics running on a single NVIDIA GT 755M (used to be two of those but I removed one because Linux and SLI aren't playing nice on this laptop and Lenovo won't cooperate

).
Installed nvidia-{kernel,driver} from SBo and it's working. There's something bugging me though: KDM, the KDE wallpaper and Google Chrome are using 24-bit depth instead of 32-bit depth. The result is poor colors and terrible gradients on wallpapers, web pages, etc. I've tried querying windows with xwininfo and here's what I got:

Code:

xwininfo: Window id: 0x2200199 "plasma-desktop"

  Absolute upper-left X:  0
  Absolute upper-left Y:  0
  Relative upper-left X:  0
  Relative upper-left Y:  0
  Width: 1920
  Height: 1080
  Depth: 24
  Visual: 0x21
  Visual Class: TrueColor
  Border width: 0
  Class: InputOutput
  Colormap: 0x20 (installed)
  Bit Gravity State: NorthWestGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +0+0  -0+0  -0-0  +0-0
  -geometry 1920x1080+0+0

Code:

xwininfo: Window id: 0x5600042 "axtroz : xwininfo – Konsole"

  Absolute upper-left X:  416
  Absolute upper-left Y:  104
  Relative upper-left X:  0
  Relative upper-left Y:  0
  Width: 1276
  Height: 669
  Depth: 32
  Visual: 0x23
  Visual Class: TrueColor
  Border width: 0
  Class: InputOutput
  Colormap: 0x5600001 (not installed)
  Bit Gravity State: NorthWestGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +416+104  -228+104  -228-307  +416-307
  -geometry 1276x669+414+81

I did my fair share of googling but found nothing.
If I connect an external monitor and reboot, I get 24-bit depth on the primary laptop monitor and 32-bit depth on the external one. If I revert the drivers back to 337.12 (which I used before the -current upgrade) it's working just fine. I tried reinstalling the whole OS and using the 340.24 drivers via NVIDIA's installer on a pure 64-bit 14.1 system but it yields the same results.

Does anyone have any idea how to chase down this issue as it's really annoying ?
Thanks in advance!

keefaz · 07-23-2014, 06:10 PM

24 bits depth means 16 millions differents colors, that should be enough to produce nice gradients :/

Quote:

The human eye can discriminate up to ten million colors

http://en.wikipedia.org/wiki/Color_depth

I bet the apps displaying 32bits depth use the extra bits for alpha channel (compositing to create transparency)

Could you post some gradient pictures from both apps?

Refractor · 07-25-2014, 06:02 PM

The problem is that I'm getting 256k (16bit) colors + 8bit alpha channel and the gradients are terrible. However, I made a screenshot and sent it to a friend - he sees the gradients just fine, which means that all the color information is being stored on the file on disk and it just can't display correctly.
I've just downgraded back to 14.1 from current, the kernel from 3.14.12 to 3.10.17-2 and the nvidia drivers to 337.12 and all works. When I get time I'll try and chase down the issue, might be just me doing something wrong though I can't recall just now.
I'm marking it as solved and will reopen when/if I stumble upon the issue again.

Cheers,

Refractor · 08-20-2014, 12:53 AM

I managed to solve it.
It seems it has something to do with the display itself, because if I attach another display on the HDMI or VGA port and put a gradient across both displays, they're smooth on the external and 'choppy' on the laptop display. What I found is that with the 340.24 (and above) driver, in the nvidia-settings utility there's a "Controls" section for my laptop display. There, in "Dithering Controls" I have "Depth" set to "Auto" and the system reports that the depth is 8pbc. The other available option was 6pbc and when I set it to that, everything smoothed out instantly.
I still don't know why it's smooth with the smaller bit depth but it works, so hopefully this information will help someone with the same issue.