What is a display mode, and where does it come from?Monitors advertise a list of display modes they support to the device they are connected to through their Extended Display Identification Data (EDID).
A mode consists of a resolution at a refresh rate (with number of colours). For example: 1920x1080 at 60 Hz.
The EDID will also include the detailed timing the monitor expects to receive the video signal at. Not all monitors use the default timings recommended by the VESA standard.
What is the preferred mode?There is one mode in the EDID that the monitor will call the preferred mode. This is usually the highest resolution at the highest refresh rate supported by a display, but this is not compulsory.
This preferred resolution is especially useful in LCD/LED/OLED panels where, unlike CRT displays, pixels are discrete. If the resolution sent differs from the native resolution of such panel, then the image will not appear clearly. The native resolution is the number of physical display units/dots (pixels) the panel has in each dimension (horizontal x vertical).
What is an inferred mode?An inferred mode is a mode added by the graphics adapter that it believes is also supported: from the list of display modes advertised by the monitor, it can be deduced (= inferred), that the monitor also supports additional modes because the graphics card has information of other modes that have been tested as working on other monitors advertising similar modes.
For instance, if the monitor supports 1920x1080 at 60 Hz, it is likely to also support 1920x1080 at 30 Hz, even if that mode is not listed in the EDID, because it has been tested as working on many other displays and there is no obvious technical reason for that mode not to work.
However, an inferred mode may not work as it is not official tested or specified by the display manufacturer on that particular model.
Why does anyone add inferred modes?This is not specific to DisplayLink. Any graphics adapter will add inferred modes. The main reason is to be able to offer more choices to the end user.
It is also most useful, necessary even, when a display doesn't offer any mode close to the adapter maximum specification.
For instance, a 4K monitor may only advertise 3840x2160 (also called "4K") and 1920x1080 (also called "Full HD"), but the graphics adapter may be able to achieve 2560x1440.
If the graphics adapter doesn't offer any inferred modes, then the end user will only be able to select 1920x1080, a much lower resolution that the full system may be able to run at.
Therefore, instead of artificially limiting the resolutions available, inferred modes are added. The end user can select them, and if they don't work, their computer will revert to the previous setting within seconds, as designed by the OS vendor.
An inferred mode added by DisplayLink will not be chosen at first connect, the user must manually select it. DisplayLink will only ever use a mode existing in the EDID at first connect to ensure that the user has a working display.