[libcamera-devel,v1.1,2/3] Documentation: design: ae: Document the design for AE controls
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Message ID 20220518145353.996277-1-paul.elder@ideasonboard.com
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Paul Elder May 18, 2022, 2:53 p.m. UTC
Document the design and rationale for the AE-related controls.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Changes in v1.1:
- fix compilation by adding it in a toc entry in index.rst

 Documentation/design/ae.rst | 260 ++++++++++++++++++++++++++++++++++++
 Documentation/index.rst     |   1 +
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diff --git a/Documentation/design/ae.rst b/Documentation/design/ae.rst
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+.. 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
+                +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
+                | 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.
+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: Disabled        |                                        V
+                       |                                       |\
+                       |                                       | \
+                       |  /----------------------------------> | 1|  ExposureTime
+                       |  |    +-------------+  exposure time  |  | -------------->
+                       \--)--> |             | --------------> | 0|
+    ExposureTime          |    |             |                 | /
+  ------------------------+--> |             |                 |/
+                               |             |                       AeState
+                               |     AEGC    | ----------------------------------->
+    AnalogueGain               |             |
+  ------------------------+--> |             |                 |\
+                          |    |             |                 | \
+                       /--)--> |             | --------------> | 0|  AnalogueGain
+                       |  |    +-------------+  analogue gain  |  | -------------->
+                       |  \----------------------------------> | 1|
+                       |                                       | /
+                       |                                       |/
+                       |                                        ^
+    AnalogueGainMode   |                                        | AnalogueGainMode
+  ---------------------+----------------------------------------+----------------->
+    0: Auto
+    1: Disabled
+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*.
+    - Disabled (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
+Inactive. 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
+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.
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 0ee10044..3b8cc285 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -22,3 +22,4 @@ 
    Environment variables <environment_variables>
    Sensor driver requirements <sensor_driver_requirements>
    Lens driver requirements <lens_driver_requirements>
+   Auto Exposure and Gain Design <design/ae>