This document describes how powerd manages the display's backlight brightness. For the vast majority of devices this is the internal display of the device, but for a small set of devices (e.g., Chromeboxes) this may be an external display.
powerd internally deals with two types of brightness values:
A user-visible brightness percentage, represented as a value between 0% and 100%.
A hardware-specific brightness level, used by the drivers and kernel.
The two values do not have a linear relationship. Instead, given a hardware-specific minimum-visible level and maximum level, the mapping from a user-visible percentage percent to the hardware level level is calculated as follows:
fraction = (percent - 6.25) / (100 - 6.25) linear_level = fraction² level = min_visible_level + linear_level * (max_level - min_visible_level)
See policy::InternalBacklightController::PercentToLevel() for the full implementation.
In this document, we refer to the hardware level as a hardware level, and the brightness non-linear brightness percent. The intermediate calculation linear level is also discussed: this roughly corresponds to the fraction of the display is driven at, where zero is “minimum visible level” and one is “full brightness”.
By default, powerd will automatically choose a backlight brightness based on the current power source (AC / battery), and the level of ambient light (direct sunlight / something dimmer than that). When the one of these changes, powerd will transition to a new brightness level.
The automatically-chosen non-linear brightness percentages are as follows:
| Power source | Direct sunlight (>= 400 lux) | Normal ambient light |
|---|---|---|
| AC | 100% (100% linear) | 80% (~62% linear) |
| Battery | 80% (~62% linear) | 63% (~37% linear) |
Devices that lack ambient light sensors just use the “normal ambient light” levels listed above. Note that these levels may be set differently for different devices.
In the past, powerd made continuous adjustments to the screen brightness based on the ambient light level. This was distracting to users and also generally ineffective: the majority of indoor environments occupy the bottom end of the range reported by our ambient light sensors. Due to the coarse readings within this range, the automatically-chosen brightness levels were frequently undesirable. We decided to switch to just two levels: one that would work well in most indoor environments, and a very-bright level for outdoor environments.
Some devices, such as the Pixelbook, are equipped with a better ambient light sensor. Such devices have a more finely-tuned configuration consisting of seven levels as follows:
| Situation | Lux Step Down | Lux Step Up | Brightness UI | Brightness Linear |
|---|---|---|---|---|
| Low light | N/A | 90 | 36.14% | 10.75% |
| Indoor - normal | 40 | 250 | 47.62% | 20.00% |
| Indoor - bright | 180 | 360 | 60.57% | 34.00% |
| Outdoor - dark | 250 | 500 | 71.65% | 49.00% |
| Outdoor - overcast | 350 | 1700 | 85.83% | 72.24% |
| Outdoor - clear sky | 1100 | 7000 | 93.27% | 86.25% |
| Direct sunlight | 5000 | N/A | 100.00% | 100.00% |
Additionally, simple exponential smoothing is applied to the raw values read from the ambient light sensor. This acts as a low-pass filter to remove noise from the data, to avoid adjusting the brightness too frequently in some lighting conditions such as an overhead source of warm white LED lighting. issue 826968
In M88, a small number of devices were moved to an ML-based brightness control logic. This logic is not hosted in powerd directly; rather, the logic is in Ash, which calculates a desired backlight level and communicates the result to powerd via a D-Bus API.
Before the user has touched a brightness key, the brightness will update automatically based on ambient light or when AC power is connected or disconnected. However, when the user presses the brightness-up or brightness-down keys, powerd animates to the requested level and stops making further ambient-light-triggered or power-source-triggered automated adjustments until the system is rebooted.
A single user-configured brightness is tracked for both AC and battery power; once the user has adjusted the brightness via the brightness keys, the brightness remains at that level until the next time the system boots. (Prior to M36, separate user-configured levels were maintained for AC and battery power -- see issue 360042.) There are 16 user-selectable brightness steps, divided evenly between the full non-linear percentage-based range (i.e. each button press moves the brightness by 100 / 16 = 6.25%). The brightness popup that appears when a button is pressed actually contains a draggable slider that can be used to select a brightness percentage that doesn't match one of the pre-defined steps.
In the past, the previous user-configured level was restored at boot, but this was deliberately removed. A given device is frequently used in many different environments: dark rooms, well-lit rooms with lots of ambient light, etc. We decided that always booting with a reasonable default brightness was preferable to sometimes restoring a blindingly-high brightness when booting in a dark room or restoring an extremely-dim brightness when booting in a bright room.
When the user is inactive for an extended period of time, the screen is dimmed to 10% of its maximum level (computed linearly) and then turned off. The screen is turned back on in response to user activity (which is interpreted broadly: keyboard or touchpad activity, power source change, external display being connected or disconnected, etc.).
Users may reduce the backlight brightness to 0% using the brightness-down (F6) key; this may be desirable to conserve battery power while streaming music. The backlight is automatically increased to a low-but-visible level when user input is observed after the brightness has been manually set to 0%.
At boot, the panel backlight's brightness is set to 40% (computed linearly) of its maximum level by the boot-splash Upstart job. This happens before the boot splash animation is displayed by frecon.
After powerd starts, it chooses an initial backlight brightness based on the power source (either AC or battery) and the ambient light level, as described in the “Automatic brightness changes” section.
As of M35, Chromeboxes' brightness keys (or F6 and F7 keys) attempt to use DDC/CI to increase or decrease external displays' brightness (issue 315371).