Dithering

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.

Diagnose

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 :

        xrandr --prop

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.