[PATCH v3 2/8] Documentation: design: ae: Document the design for AE controls
Paul Elder
paul.elder at ideasonboard.com
Wed Nov 13 14:12:50 CET 2024
Document the design and rationale for the AE-related controls.
Also add documentation for the controls.
Signed-off-by: Paul Elder <paul.elder at ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo at jmondi.org>
---
Changes in v3:
- merge the control documentation into the same document (including the
patch)
- this is because it was a bit unwieldy to put it in
control_ids.cpp.in, now that it's used generically to generate
control ids of all namespaces
---
Documentation/design/ae.rst | 348 ++++++++++++++++++++++++++++++++++++
1 file changed, 348 insertions(+)
create mode 100644 Documentation/design/ae.rst
diff --git a/Documentation/design/ae.rst b/Documentation/design/ae.rst
new file mode 100644
index 000000000000..f121afecea5a
--- /dev/null
+++ b/Documentation/design/ae.rst
@@ -0,0 +1,348 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Design of Exposure and Gain controls
+====================================
+
+This document explains the design and rationale of the controls related to
+exposure and gain. This includes the all-encompassing auto-exposure (AE), the
+manual exposure control, and the manual gain control.
+
+Description of the problem
+--------------------------
+
+Sub controls
+^^^^^^^^^^^^
+
+There are more than one control that make up exposure: exposure, gain, and
+aperture (though for now we will not consider aperture). We already had
+individual controls for setting the values of manual exposure and manual gain,
+but for switching between auto mode and manual mode we only had a high-level
+boolean AeEnable control that would set *both* exposure and gain to auto mode
+or manual mode; we had no way to set one to auto and the other to manual.
+
+So, we need to introduce two new controls to act as "levers" to indicate
+individually for exposure and gain if the value would come from AEGC or if it
+would come from the manual control value.
+
+Aperture priority
+^^^^^^^^^^^^^^^^^
+
+We eventually will support aperture, and so whatever our solution is for having
+only some controls on auto and the others on manual needs to be extensible.
+
+Flickering when going from auto to manual
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When a manual exposure or gain value is requested by the application, it costs
+a few frames worth of time for them to take effect. This means that during a
+transition from auto to manual, there would be flickering in the control values
+and the transition won't be smooth.
+
+Take for instance the following flow, where we start on auto exposure (which
+for the purposes of the example increments by 1 each frame) and we want to
+switch seamlessly to manual exposure, which involves copying the exposure value
+computed by the auto exposure algorithm:
+
+::
+
+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
+ | N | | N+1 | | N+2 | | N+3 | | N+4 | | N+5 | | N+6 |
+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
+
+ Mode requested: Auto Auto Auto Manual Manual Manual Manual
+ Exp requested: N/A N/A N/A 2 2 2 2
+ Set in Frame: N+2 N+3 N+4 N+5 N+6 N+7 N+8
+
+ Mode used: Auto Auto Auto Auto Auto Manual Manual
+ Exp used: 0 1 2 3 4 2 2
+
+As we can see, after frame N+2 completes, we copy the exposure value that was
+used for frame N+2 (which was computed by AE algorithm), and queue that value
+into request N+3 with manual mode on. However, as it takes two frames for the
+exposure to be set, the exposure still changes since it is set by AE, and we
+get a flicker in the exposure during the switch from auto to manual.
+
+A solution is to *not submit* any exposure value when manual mode is enabled,
+and wait until the manual mode as been "applied" before copying the exposure
+value:
+
+::
+
+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
+ | N | | N+1 | | N+2 | | N+3 | | N+4 | | N+5 | | N+6 |
+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
+
+ Mode requested: Auto Auto Auto Manual Manual Manual Manual
+ Exp requested: N/A N/A N/A None None None 5
+ Set in Frame: N+2 N+3 N+4 N+5 N+6 N+7 N+8
+
+ Mode used: Auto Auto Auto Auto Auto Manual Manual
+ Exp used: 0 1 2 3 4 5 5
+
+In practice, this works. However, libcamera has a policy where once a control
+is submitted, its value is saved and does not need to be resubmitted (even
+though this isn't implemented yet). So if the manual exposure value was set
+while auto mode was on, in theory the value would be saved, so when manual mode
+is enabled, the exposure value that was previously set would immediately be
+used. Clearly this solution isn't correct, but it can serve as the basis for a
+proper solution, with some more rigorous rules.
+
+Existing solutions
+------------------
+
+Raspberry Pi
+^^^^^^^^^^^^
+
+The raspberry pi IPA got around the lack of individual AeEnable controls for
+exposure and gain by using magic values. When AeEnable was false, if one of the
+manual control values was set to 0 then the value computed by AEGC would be
+used for just that control. This solution isn't desirable, as it prevents
+that magic value from being used as a valid value.
+
+To get around the flickering issue, when AeEnable was false, the raspberry pi
+AEGC would simply stop updating the values to be set. As mentioned above, since
+the value retention mechanism hasn't actually been implemented yet, this
+worked. But, it's not a proper solution.
+
+Android
+^^^^^^^
+
+The Android HAL specification requires that exposure and gain (sensitivity)
+must both be manual or both be auto. It cannot be that one is manual while the
+other is auto, so they simply don't support sub controls.
+
+For the flickering issue, the Android HAL has an AeLock control. To transition
+from auto to manual, the application would keep AE on auto, and turn on the
+lock. Once the lock has propagated through, then the value can be copied from
+the result into the request and the lock disabled and the mode set to manual.
+
+The problem with this solution is, besides the extra complexity, that it is
+ambiguous what happens if there is a state transition from manual to locked
+(even though it's a state transition that doesn't make sense). If locked is
+defined to "use the last automatically computed values" then it could use the
+values from the last time it AE was set to auto, or it would be undefined if AE
+was never auto (eg. it started out as manual), or if AE is implemented to run
+in the background it could just use the current values that are computed. If
+locked is defined to "use the last value that was set" there would be less
+ambiguity. Still, it's better if we can make it impossible to execute this
+nonsensical state transition, and if we can reduce the complexity of having
+this extra control or extra setting on a lever.
+
+Summary of goals
+----------------
+
+ - We need a lock of some sort, to instruct the AEGC to not update output
+ results
+
+ - We need manual modes, to override the values computed by the AEGC
+
+ - We need to support seamless transitions from auto to manual, and do so
+ without flickering
+
+ - We need custom minimum values for the manual controls; that is, no magic
+ values for enabling/disabling auto
+
+ - All of these need to be done with AE sub-controls (exposure time, analogue
+ gain) and be extensible to aperture in the future
+
+Our solution
+------------
+
+A diagram of our solution:
+
+::
+
+ +----------------------------+-------------+------------------+-----------------+
+ | INPUT | ALGORITHM | RESULT | OUTPUT |
+ +----------------------------+-------------+------------------+-----------------+
+
+ ExposureTimeMode ExposureTimeMode
+ ---------------------+----------------------------------------+----------------->
+ 0: Auto | |
+ 1: Manual | V
+ | |\
+ | | \
+ | /----------------------------------> | 1| ExposureTime
+ | | +-------------+ exposure time | | -------------->
+ \--)--> | | --------------> | 0|
+ ExposureTime | | | | /
+ ------------------------+--> | | |/
+ | | AeState
+ | AEGC | ----------------------------------->
+ AnalogueGain | |
+ ------------------------+--> | | |\
+ | | | | \
+ /--)--> | | --------------> | 0| AnalogueGain
+ | | +-------------+ analogue gain | | -------------->
+ | \----------------------------------> | 1|
+ | | /
+ | |/
+ | ^
+ AnalogueGainMode | | AnalogueGainMode
+ ---------------------+----------------------------------------+----------------->
+ 0: Auto
+ 1: Manual
+
+
+The diagram is divided in four sections horizontally:
+
+ - Input: The values received from the request controls
+
+ - Algorithm: The algorithm itself
+
+ - Result: The values calculated by the algorithm
+
+ - Output: The values that sent in result metadata and applied to the device
+
+The four input controls are divided between manual values (ExposureTime and
+AnalogueGain), and operation modes (ExposureTimeMode and AnalogueGainMode). The
+former are the manual values, the latter control how they're applied. The two
+modes are independent from each other, and each can take one of two values:
+
+ - Auto (0): The AGC computes the value normally. The AGC result is applied
+ to the output. The manual value is ignored *and is not retained*.
+
+ - Manual (1): The AGC uses the manual value internally. The corresponding
+ manual control from the request is applied to the output. The AGC result
+ is ignored.
+
+The AeState control reports the state of the unified AEGC block. If both
+ExposureTimeMode and AnalogueGainMode are set to disabled then it will report
+Idle. If at least one of the two is set to auto, then AeState will report
+if the AEGC has Converged or not (Searching). This control replaces the old
+AeLocked control, as it was insufficient for reporting the AE state.
+
+There is a caveat to the disabled mode: the manual control value is not
+retained if it is set during auto mode. This means that if the disabled mode is
+entered without also setting the manual value, then it will enter a state
+similar to "locked", where the last automatically computed value while the mode
+was auto will be used. Once the manual value is set, then that will be used and
+retained as usual.
+
+This simulates an auto -> locked -> manual or auto -> manual state transition,
+and makes it impossible to do the nonsensical manual -> locked state
+transition.
+
+We specifically do not have a "master AE control" like the old AeEnable. This
+is because we have the individual mode controls, and if we had a master AE
+control it would be a "control that sets other controls", which could easily
+get out of control.
+
+With this solution, the earlier example would become:
+
+::
+
+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
+ | N+2 | | N+3 | | N+4 | | N+5 | | N+6 | | N+7 | | N+8 | | N+9 | | N+10|
+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
+ Mode requested: Auto Disab Disab Disab Disab Disab Disab Disab Disab
+ Exp requested: N/A None None None None 10 None 10 10
+ Set in Frame: N+4 N+5 N+6 N+7 N+8 N+9 N+10 N+10 N+10
+
+ Mode used: Auto Auto Auto Disab Disab Disab Disab Disab Disab
+ Exp used: 2 3 4 5 5 5 5 10 10
+
+This example is extended by a few frames to exhibit the simulated "locked"
+state. At frame N+5 the application has confirmed that the auto mode has been
+disabled, but does not provide a manual value until request N+7. Thus, the
+value that is used in requests N+5 and N+6 (where the mode is disabled), comes
+from the last value that was used when the mode was auto, which comes from
+frame N+4.
+
+Then, in N+7, a manual value of 10 is supplied. It takes until frame N+9 for
+the exposure to be applied. N+8 does not supply a manual value, but the last
+supplied value is retained, so a manual value of 10 is still used and set in
+frame N+10.
+
+Although this behavior is the same as what we had with waiting for the manual
+mode to propagate (in the section "Description of the problem"), this time it
+is correct as we have defined specifically that if a manual value was specified
+while the mode was auto, it will not be retained.
+
+Description of the controls
+===========================
+
+ libcamera offers the following controls related to exposure and gain:
+
+ - AnalogueGain
+
+ - AnalogueGainMode
+
+ - ExposureTime
+
+ - ExposureTimeMode
+
+ - AeState
+
+ Auto-exposure and auto-gain can be enabled and disabled separately using the
+ ExposureTimeMode and AnalogueGainMode controls respectively. There is no
+ overarching AeEnable control.
+
+ For each of exposure and gain, we can model it with three states: auto,
+ locked, and manual. Note that AnalogueGainMode and ExposureTimeMode only
+ have two values, as the locked state is simulated.
+
+ ::
+
+ /---------------------------------\
+ | |
+ V |
+ +--------+ +--------+
+ | | ---------------------> | |
+ | Auto | +--------+ | Manual |
+ | | ----> | Locked | ----> | |
+ +--------+ +--------+ +--------+
+ ^ |
+ | |
+ \----------------/
+
+ Notice from the state diagram that locked to manual is a one-way state
+ change, as the reverse direction is nonsensical (see the design document for
+ more details on this topic).
+
+ The exposure/gain is in the Auto state when
+ ExposureTimeMode/AnalogueGainMode is set to Auto. In this state, the value
+ that is computed by the AE algorithm is applied to the image sensor. Any
+ value that is supplied in the ExposureTime/AnalogueGain control is ignored
+ and is not retained.
+
+ If ExposureTimeMode/AnalogueGainMode is set to Manual, it can put us in
+ either the Locked or Manual state. The difference is in if
+ ExposureTime/AnalogueGain has been supplied. If it has not yet been
+ supplied, then we are in the Locked state. If it has been supplied, then we
+ are in the Manual state.
+
+ In both the Locked state and the Manual state the exposure/gain value does
+ not come from the AE algorithm. In the Locked state the value comes from the
+ last value computed by the AE algorithm while the state was Auto, or if the
+ state was never Auto (e.g. we started in Locked, or the camera doesn't
+ support Auto), then the value should be a best-effort default value. In the
+ Manual state the value comes from the value supplied in the
+ ExposureTime/AnalogueGain control.
+
+ To transition from the Locked state to the Manual state, a value needs to be
+ submitted in ExposureTime/AnalogueGain. Once the state has transitioned to
+ Manual, then this value will be retained, and doesn't need to be resubmitted
+ if it doesn't change.
+
+ To transition to the Auto state, simply set
+ ExposureTimeMode/AnalogueGainMode to Auto.
+
+
+ The AeState metadata reports the state of the AE algorithm. As AE cannot
+ compute exposure and gain separately, the state of the AE component is
+ unified. There are three states: Idle, Searching, and Converged.
+
+ The state shall be Idle if both ExposureTimeMode and AnalogueGainMode
+ are set to Manual. If the camera only supports one of the two controls,
+ then the state shall be Idle if that one control is set to Manual. If
+ the camera does not support Manual for at least one of the two controls,
+ then the state will never be Idle, as AE will always be running.
+
+ The state shall be Searching if at least one of exposure or gain calculated
+ by the AE algorithm is used (that is, at least one of the two modes is Auto),
+ *and* the value(s) have not converged yet.
+
+ The state shall be Converged if at least one of exposure or gain calculated
+ by the AE algorithm is used (that is, at least one of the two modes is Auto),
+ *and* the value(s) have converged.
--
2.39.2
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