@Denis
If you have access to the video signal (i.e., via DVI output), which is only 8bpc and <=1920x1080@60Hz, you could use Epiphan's DVI2USB3.0 frame grabber (if somebody knows of a different USB grabber capable of 8bpc, I am all ears). I use this for verifying that there is actually no dithering, but so far only with Windows and Linux. Epiphan dropped support for Mac (the latest driver available is for MacOS 10.10 & 10.11), but one can always use a second PC for doing the grabbing. Obviously, measurements done with such a grabber can only tell what the graphics card delivers, leaving the monitor out of the equation.
Marc
If you have access to the video signal (i.e., via DVI output), which is only 8bpc and <=1920x1080@60Hz, you could use Epiphan's DVI2USB3.0 frame grabber (if somebody knows of a different USB grabber capable of 8bpc, I am all ears). I use this for verifying that there is actually no dithering, but so far only with Windows and Linux. Epiphan dropped support for Mac (the latest driver available is for MacOS 10.10 & 10.11), but one can always use a second PC for doing the grabbing. Obviously, measurements done with such a grabber can only tell what the graphics card delivers, leaving the monitor out of the equation.
Marc