Windows 8 is designed to provide a fast and fluid user experience.
Prior to Windows 8, brightness changes were typically initiated by the end user through either a UI or hot keys. Windows 8 is optimized to automatically adjust the brightness based on the outside lighting condition as measured by the Ambient Light Sensor (ALS). In such a scenario, the brightness level is adjusted without direct user input. Traditionally, all brightness changes from one level to another have been done as a step change. While this is functional, it can be visually jarring and not a fluid user experience.
Windows 8 enables the Windows Display Driver Model (WDDM) graphics driver to implement a smooth transition of the brightness from one level to another. This is supported by the DXGK_BRIGHTNESS_INTERFACE_2 and is a mandatory requirement for WDDM 1.2 drivers. The WDDM driver must implement this interface as defined by the Windows Hardware Certification requirement System.Client.BrightnessControls.SmoothBrightness.
For this feature, the driver must make many incremental steps from the start level to the target level, without depending on the embedded controller for smoothness. To achieve the smoothness, the driver will need to customize the number of steps and the time interval at each step based on the panel characteristics of the integrated panel. However, the transition must complete in less than 500 ms. At this time, Microsoft has not defined any particular curve to use for the smooth transition, and the OEM and the independent hardware vendor (IHV) must work together to determine the optimal curve.
For the complete list of requirements, refer to the Windows Hardware Certification Requirements.