[v4,2/8] Documentation: design: ae: Document the design for AE controls

22174 diff mbox series

Document the design and rationale for the AE-related controls.
Also add documentation for the controls.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>

---
Changes in v4:
- fix wording
- fix compilation
- remove locked state in explanation

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 | 320 ++++++++++++++++++++++++++++++++++++
 Documentation/index.rst     |   4 +-
 Documentation/meson.build   |   1 +
 3 files changed, 324 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/design/ae.rst

Hi Paul,

Thank you for the patch.

On Thu, Dec 05, 2024 at 08:22:35PM +0900, Paul Elder wrote:
> Document the design and rationale for the AE-related controls.
> Also add documentation for the controls.
> 
> Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
> Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
> Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>
> 
> ---
> Changes in v4:
> - fix wording
> - fix compilation
> - remove locked state in explanation
> 
> 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 | 320 ++++++++++++++++++++++++++++++++++++
>  Documentation/index.rst     |   4 +-
>  Documentation/meson.build   |   1 +
>  3 files changed, 324 insertions(+), 1 deletion(-)
>  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..69bf6dfd3e02
> --- /dev/null
> +++ b/Documentation/design/ae.rst
> @@ -0,0 +1,320 @@
> +.. 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 time, gain, and

s/make up exposure/make up total exposure/

> +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 may need to 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. 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 gets around the lack of individual AeEnable controls for
> +exposure and gain by using magic values. When AeEnable is 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 is false, the Raspberry Pi
> +AEGC simply stops updating the values to be set, without restoring the
> +previously set manual exposure time and gain. This works, but is 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 reported 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.

Drop the indentation, move the '-' to the left-most column.

> +
> +The AeState control reports the state of the unified AEGC block. If both
> +ExposureTimeMode and AnalogueGainMode are set to manual 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 manual mode: the manual control value is not retained if
> +it is set during auto mode. This means that if manual 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    Manual  Manual  Manual  Manual  Manual  Manual  Manual  Manual
> + 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+11    N+11

I think the last N+11 value should be N+12.

> +
> + Mode used:       Auto    Auto    Auto    Manual  Manual  Manual  Manual  Manual  Manual
> + 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 manual mode has been
> +entered, 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
> +---------------------------
> +
> +As descibed above, libcamera offers the following controls related to exposure

s/descibed/described/

> +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.
> +
> +When the respective mode is set to auto, the respective value that is computed
> +by the AEGC algorithm is applied to the image sensor. Any value that is
> +supplied in the manual ExposureTime/AnalogueGain control is ignored and not
> +retained when the respective mode control is auto. Another way to understand

s/  when the respective mode control is auto// (already stated in the
first sentence)

> +this is that when the mode transitions from auto to manual, the internally
> +stored control value is overwritten with the last value computed by the auto
> +algorithm.
> +
> +This means that when we transition from auto to manual without supplying the
> +corresponding manual control value, the last value that was set by the AEGC

s/the corresponding/a/

> +algorithm will be used. This can be used to do a flickerless transition from

Maybe s/will be used/will keep being used/

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

> +auto to manual as described earlier. If the camera started out in manual mode
> +and no corresponding value has been supplied yet, then a best-effort default
> +value shall be set.
> +
> +The manual control value can be set in the same request as setting the mode to
> +auto if the desired manual control value is already known.
> +
> +Transitioning from manual to auto shall be implicitly flickerless, as the AEGC
> +algorithms are expected to start running from the last manual value.
> +
> +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.
> diff --git a/Documentation/index.rst b/Documentation/index.rst
> index bea406608477..251112fbdf5f 100644
> --- a/Documentation/index.rst
> +++ b/Documentation/index.rst
> @@ -23,7 +23,9 @@
>     SoftwareISP Benchmarking <software-isp-benchmarking>
>     Tracing guide <guides/tracing>
>  
> +   Design document: AE <design/ae>
> +
>  .. toctree::
>     :hidden:
>  
> -   introduction
> \ No newline at end of file
> +   introduction
> diff --git a/Documentation/meson.build b/Documentation/meson.build
> index 36ffae239353..6158320e177c 100644
> --- a/Documentation/meson.build
> +++ b/Documentation/meson.build
> @@ -128,6 +128,7 @@ if sphinx.found()
>          'coding-style.rst',
>          'conf.py',
>          'contributing.rst',
> +        'design/ae.rst',
>          'documentation-contents.rst',
>          'environment_variables.rst',
>          'feature_requirements.rst',