xBRZ (https://sourceforge.net/projects/xbrz/) scaler is integrated into DOSBox-X.

To enable, use
    scaler=xbrz
or
    scaler=xbrz_bilinear
option in [render] section.

The difference between two options is in the post-scaling resize method. 
xBRZ scaler uses fixed scale factors, 2X to 6X, which are usually not exactly
matching the display window. So nearest size xBRZ scaler output is then 
resized (upscaled/dowscaled) using either nearest neighbor ('xbrz') or 
bilinear ('xbrz_bilinear') algorithm to match the window size.

It is highly recommended to use 'direct3d' or any of the 'opengl' output modes
with xBRZ because bilinear post-scaler performance is terrible and nearest
neighbor post-scaler does not provide good results.

In 'direct3d' / 'opengl' output modes, there is no difference between
'xbrz' and 'xbrz_bilinear' variants, because software post-scaler is not used
and output is post-scaled by the output interface/hardware.

Differences from reference implementation on xBRZ site / in Daum build:

- DOSBox-X implementation supports 'surface', 'direct3d' and 'opengl' outputs
  (including opengl variants 'openglnb' and 'openglhq')
- Windowed mode is fully working with xBRZ scaler enabled
- You can combine xBRZ with 'direct3d' mode post-scalers to get i.e. TV effect
- Enabling xBRZ scaler does not disable 'aspect' option, it is honored

Things to notice:

- The scaler is very CPU intensive so it will run with decent speed for
  games and demos only on high-end CPUs. Keep this in mind. You can try
  to correct performance for high output resolutions by using either
  'xbrz fixed scale factor' or 'xbrz max scale factor' options to limit
  xBRZ scale factor (lower factors improve performance), of course this
  reduces image quality for higher output resolutions proportionally.

- Using 'direct3d' or 'opengl' mode actually improves performance a lot
  because it gets rid of the software post-scaler pass.

- xBRZ scaler code uses parallel processing internally. There is also
  'xbrz slice' option that affects parallelism, its default (16) gives
  good results on 4-8 core CPUs.

- When xBRZ scaler is enabled, internal DOSBox scaler options are disabled
  because any pre-scaling will break xBRZ algorithm. For the same very reason,
  'doublescan' option is always turned off internally when xBRZ is enabled.

- When using xBRZ in 'surface' output mode and window/screen sizes less than
  2x of the original DOS resolution, 'xbrz' scaler can degrade quality instead
  of improving it, use 'direct3d'/'opengl' outputs or 'xbrz_bilinear' scaler
  variant to avoid this.

Caveats / issues / things to do / incomplete:

- In case video adapter interface uses non-standard color scheme / byte order,
  colors with xBRZ scaler enabled would most probably be garbled.
  [YET TO REPRODUCE ANYWHERE]