This article applies to versions prior to v. 5.1.40.

When the resolution of the remote monitor is higher than 1920x1080, NoMachine version 4 or later doesn't use H.264 hardware-accelerated decoding  on Windows clients. If available, H.264 software decoding (based on the libav library) will be used, otherwise the encoding method falls back to VP8. 

This limit has been set to avoid possible screen corruption if the graphical video card doesn't support resolutions higher than Full HD (1920x1080). According to DXVA (DirectX Video Acceleration) specifications, the Windows API for hardware-accelerated decoding, all the GPUs have to decode up to Full HD resolution (1920x1080). Higher resolutions can be supported or not depending on the video card capabilities.

The implementation of this Feature Request, available since v. 5.1.40, allows users to configure the client for using the hardware accelerated decoding on Windows when the resolution of the remote screen is higher than 1920x1080:

https://www.nomachine.com/FR09L02830

To enable it, edit the player.cfg configuration file and set:

<option key="Switch to software decoding on Windows if resolution is bigger than 1920x1080" value="true" />