Scaling is the process of resizing a digital image. This function takes effects when the configured display mode not matches the display device's native mode.

A CRT monitor has an infinitely variable pixel size limited only by the maximum scan frequecy the monitor can handle. But an LCD display, TV or other flat panel display has a fixed pixel size determinded by a fixed raster. That fixed pixel size is referred as the native resolution (or native mode). It cannot be changed to match the signal being displayed as a CRT monitor can, meaning that optimal display quality can be reached only when the signal input matches the native mode.

In order to let the output image occupy the full screen when the configured mode is smaller than the native mode (scaling up) or avoid panning when the configured mode is larger than the native mode (scaling down), we could use the following settings.

Option "DISPLAY_3DScaling_LCD" "TRUE"                         # Enable 3D Scaling for LCD

Option "DISPLAY_3DScaling_HDMI" "TRUE"                       # Enable 3D Scaling for HDMI 

Option "DISPLAY_3DScaling_TV" "TRUE"                            # Enable 3D Scaling for TV/HDTV

The option belongs to Section "Device".