Some flat panels use only 6 bits per color channel, and are not able to display the 8-bit colors the video card outputs. But it is possible to simulate 8-bit colors with a 6-bit screen by turning on and off quickly the pixels. This process is known as dithering.
If you can see stripes on this image , then you don't have 8 bit colors. 64 stripes means your driver, video card or monitor is only able to render 6-bit colors.
Enable Dithering in Nouveau
Select the right output
This command :
will output something like that :
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192 HDMI-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 353mm x 198mm EDID: 00ffffffffffff004ca34d3100000000 00120103802314780a87f594574f8c27 27505400000001010101010101010101 0101010101011d3680a070381e403020 250061c6100000190000000f00000000 00000000002091026400000000fe0053 414d53554e470a2020202020000000fe 00313630485430332d3030310a200060 dithering: Off supported: Off On scaling mode: Full supported: None Full Center Full aspect 1920x1080 60.0*+ 60.0 1680x1050 60.0 1400x1050 60.0 1280x1024 59.9 1280x960 59.9 1152x864 60.0 1024x768 59.9 800x600 59.9 640x480 59.4 720x400 59.6 640x400 60.0 640x350 59.8 (null)-1 disconnected (normal left inverted right x axis y axis) dithering: Off supported: Off On scaling mode: Full supported: None Full Center Full aspect VGA-1 disconnected (normal left inverted right x axis y axis) scaling mode: None supported: None Full Center Full aspect
Turn dithering on or off
We can see that dithering is off for HDMI-1, which seems to be the main monitor. Let's turn it on :
xrandr --output HDMI-1 --set dithering On
If dithering was the cause of the color depth problem, then the stripes on the attached image should have disappeared.