[PATCH v3 2/8] Documentation: design: ae: Document the design for AE controls
Stefan Klug
stefan.klug at ideasonboard.com
Thu Nov 14 15:36:41 CET 2024
Hi Paul,
Thank you for the patch.
On Wed, Nov 13, 2024 at 10:12:50PM +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 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.
That is a lot of stuff to go through. I like the model. One thing that
came to my mind: What is the reason to use a enum for the ExposureTimeMode
control instead of a bool? Are there further extensions planned? GenICam
devices often have an additional mode "Once" that runs the control loop
until converged and then snaps back to off/manual.
But overall I believe we should get that it.
Reviewed-by: Stefan Klug <stefan.klug at ideasonboard.com>
Cheers,
Stefan
> --
> 2.39.2
>
More information about the libcamera-devel
mailing list