Scenario-based backlight power optimization
All portable Windows 8 devices are expected to deliver extended battery life. This is particularly true for thin and lightweight touch-based tablet devices. The display backlight is one of the greatest consumers of power in these devices. The amount of power consumed by the display is often directly proportional to the brightness level produced by the backlight.
The most intuitive option to improve battery life would be to reduce screen brightness. However, in many cases this would negatively impact the user experience; it could make text difficult to read or video dark to watch. However, it is possible to reduce screen backlight to improve battery life without having a significant impact on the user experience. This can be achieved by enhancing the contrasts of individual pixels to compensate for the lower backlight level of the display. The Windows hardware ecosystem has innovated in this area and designed algorithms to consume less battery power by optimizing the backlight level in certain scenarios. This is called scenario-based backlight power optimization.
In some scenarios, such an optimization can be enabled without having a significant negative impact on the user experience. The key characteristic of such a scenario is a series of rapidly changing frames. This is typically seen in scenarios such as video playback or game playing. To not compromise user experience in other scenarios, Windows 8 offers a scenario-based policy on which such optimization can be enabled. The amount of optimization can be customized by the implementer. However, the optimization must balance delivering on the rich visual experience against the desired battery life. The OEM must work with the independent hardware vendor (IHV) to define the optimization level and ensure that the end user experience is not impacted.
This is supported by the Windows Display Driver Model (WDDM) DXGK_BRIGHTNESS_INTERFACE_2 and must be implemented by WDDM 1.2 drivers. In brief, Windows sets the current scenario by using the DxgkDdiSetBacklightOptimization function. The WDDM driver is required to honor the intent of the scenario. Using the constant values of the DXGK_BACKLIGHT_OPTIMIZATION_LEVEL enumeration, scenarios range from "DxgkBacklightOptimizationDisable," in which the driver is required to completely disable all backlight optimizations, to "DxgkBacklightOptimizationDesktop," "DxgkBacklightOptimizationDynamic," and "DxgkBacklightOptimizationDimmed," where the driver is required to enable backlight optimizations at various aggressiveness levels.
For more details, see DXGK_BACKLIGHT_OPTIMIZATION_LEVEL and the System.Client.BrightnessControls.BacklightOptimization requirement in the Windows Hardware Certification Requirements.
Similar to smooth brightness control, the optimization is dependent on the panel characteristics, and the IHV must work with the OEM to determine the ideal optimization. Scenario-based backlight optimization must also be tested with adaptive brightness enabled. It must be tested under different lighting conditions and brightness levels. The smooth brightness must continue to work even when scenario-based backlight optimization is enabled.