[PATCH v1] libcamera: libipa: camera_sensor_helper: Use `variant` instead of `union`

[Laurent Pinchart]

Hi Barnabás,

Thank you for the patch.

On Thu, Dec 05, 2024 at 12:27:05PM +0000, Barnabás Pőcze wrote:
> Use an `std::variant` to store the analogue gain instead of a bare union + tag.

Sounds like the right tool for the job.

> Signed-off-by: Barnabás Pőcze <pobrn at protonmail.com>

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

> ---
>  src/ipa/libipa/camera_sensor_helper.cpp | 198 ++++++++----------------
>  src/ipa/libipa/camera_sensor_helper.h   |  18 +--
>  2 files changed, 70 insertions(+), 146 deletions(-)
> 
> diff --git a/src/ipa/libipa/camera_sensor_helper.cpp b/src/ipa/libipa/camera_sensor_helper.cpp
> index a76e5e75..ac3d712b 100644
> --- a/src/ipa/libipa/camera_sensor_helper.cpp
> +++ b/src/ipa/libipa/camera_sensor_helper.cpp
> @@ -87,21 +87,18 @@ namespace ipa {
>   */
>  uint32_t CameraSensorHelper::gainCode(double gain) const
>  {
> -	const AnalogueGainConstants &k = gainConstants_;
> +	if (auto *l = std::get_if<AnalogueGainLinear>(&gain_)) {
> +		ASSERT(l->m0 == 0 || l->m1 == 0);
>  
> -	switch (gainType_) {
> -	case AnalogueGainLinear:
> -		ASSERT(k.linear.m0 == 0 || k.linear.m1 == 0);
> -
> -		return (k.linear.c0 - k.linear.c1 * gain) /
> -		       (k.linear.m1 * gain - k.linear.m0);
> -
> -	case AnalogueGainExponential:
> -		ASSERT(k.exp.a != 0 && k.exp.m != 0);
> -
> -		return std::log2(gain / k.exp.a) / k.exp.m;
> +		return (l->c0 - l->c1 * gain) /
> +		       (l->m1 * gain - l->m0);
> +	}
> +	else if (auto *e = std::get_if<AnalogueGainExp>(&gain_)) {
> +		ASSERT(e->a != 0 && e->m != 0);
>  
> -	default:
> +		return std::log2(gain / e->a) / e->m;
> +	}
> +	else {
>  		ASSERT(false);
>  		return 0;
>  	}
> @@ -119,38 +116,28 @@ uint32_t CameraSensorHelper::gainCode(double gain) const
>   */
>  double CameraSensorHelper::gain(uint32_t gainCode) const
>  {
> -	const AnalogueGainConstants &k = gainConstants_;
>  	double gain = static_cast<double>(gainCode);
>  
> -	switch (gainType_) {
> -	case AnalogueGainLinear:
> -		ASSERT(k.linear.m0 == 0 || k.linear.m1 == 0);
> -
> -		return (k.linear.m0 * gain + k.linear.c0) /
> -		       (k.linear.m1 * gain + k.linear.c1);
> +	if (auto *l = std::get_if<AnalogueGainLinear>(&gain_)) {
> +		ASSERT(l->m0 == 0 || l->m1 == 0);
>  
> -	case AnalogueGainExponential:
> -		ASSERT(k.exp.a != 0 && k.exp.m != 0);
> -
> -		return k.exp.a * std::exp2(k.exp.m * gain);
> +		return (l->m0 * gain + l->c0) /
> +		       (l->m1 * gain + l->c1);
> +	}
> +	else if (auto *e = std::get_if<AnalogueGainExp>(&gain_)) {
> +		ASSERT(e->a != 0 && e->m != 0);
>  
> -	default:
> +		return e->a * std::exp2(e->m * gain);
> +	}
> +	else {
>  		ASSERT(false);
>  		return 0.0;
>  	}
>  }
>  
>  /**
> - * \enum CameraSensorHelper::AnalogueGainType
> - * \brief The gain calculation modes as defined by the MIPI CCS
> - *
> - * Describes the image sensor analogue gain capabilities.
> - * Two modes are possible, depending on the sensor: Linear and Exponential.
> - */
> -
> -/**
> - * \var CameraSensorHelper::AnalogueGainLinear
> - * \brief Gain is computed using linear gain estimation
> + * \struct CameraSensorHelper::AnalogueGainLinear
> + * \brief Analogue gain constants for the linear gain model
>   *
>   * The relationship between the integer gain parameter and the resulting gain
>   * multiplier is given by the following equation:
> @@ -165,11 +152,27 @@ double CameraSensorHelper::gain(uint32_t gainCode) const
>   * The full Gain equation therefore reduces to either:
>   *
>   * \f$gain=\frac{c0}{m1x+c1}\f$ or \f$\frac{m0x+c0}{c1}\f$
> + *
> + * \var CameraSensorHelper::AnalogueGainLinear::m0
> + * \brief Constant used in the linear gain coding/decoding
> + *
> + * \note Either m0 or m1 shall be zero.
> + *
> + * \var CameraSensorHelper::AnalogueGainLinear::c0
> + * \brief Constant used in the linear gain coding/decoding
> + *
> + * \var CameraSensorHelper::AnalogueGainLinear::m1
> + * \brief Constant used in the linear gain coding/decoding
> + *
> + * \note Either m0 or m1 shall be zero.
> + *
> + * \var CameraSensorHelper::AnalogueGainLinear::c1
> + * \brief Constant used in the linear gain coding/decoding
>   */
>  
>  /**
> - * \var CameraSensorHelper::AnalogueGainExponential
> - * \brief Gain is expressed using an exponential model
> + * \struct CameraSensorHelper::AnalogueGainExp
> + * \brief Analogue gain constants for the exponential gain model
>   *
>   * The relationship between the integer gain parameter and the resulting gain
>   * multiplier is given by the following equation:
> @@ -185,54 +188,14 @@ double CameraSensorHelper::gain(uint32_t gainCode) const
>   *
>   * When the gain is expressed in dB, 'a' is equal to 1 and 'm' to
>   * \f$log_{2}{10^{\frac{1}{20}}}\f$.
> - */
> -
> -/**
> - * \struct CameraSensorHelper::AnalogueGainLinearConstants
> - * \brief Analogue gain constants for the linear gain model
> - *
> - * \var CameraSensorHelper::AnalogueGainLinearConstants::m0
> - * \brief Constant used in the linear gain coding/decoding
> - *
> - * \note Either m0 or m1 shall be zero.
> - *
> - * \var CameraSensorHelper::AnalogueGainLinearConstants::c0
> - * \brief Constant used in the linear gain coding/decoding
> - *
> - * \var CameraSensorHelper::AnalogueGainLinearConstants::m1
> - * \brief Constant used in the linear gain coding/decoding
> - *
> - * \note Either m0 or m1 shall be zero.
> - *
> - * \var CameraSensorHelper::AnalogueGainLinearConstants::c1
> - * \brief Constant used in the linear gain coding/decoding
> - */
> -
> -/**
> - * \struct CameraSensorHelper::AnalogueGainExpConstants
> - * \brief Analogue gain constants for the exponential gain model
>   *
> - * \var CameraSensorHelper::AnalogueGainExpConstants::a
> + * \var CameraSensorHelper::AnalogueGainExp::a
>   * \brief Constant used in the exponential gain coding/decoding
>   *
> - * \var CameraSensorHelper::AnalogueGainExpConstants::m
> + * \var CameraSensorHelper::AnalogueGainExp::m
>   * \brief Constant used in the exponential gain coding/decoding
>   */
>  
> -/**
> - * \struct CameraSensorHelper::AnalogueGainConstants
> - * \brief Analogue gain model constants
> - *
> - * This union stores the constants used to calculate the analogue gain. The
> - * CameraSensorHelper::gainType_ variable selects which union member is valid.
> - *
> - * \var CameraSensorHelper::AnalogueGainConstants::linear
> - * \brief Constants for the linear gain model
> - *
> - * \var CameraSensorHelper::AnalogueGainConstants::exp
> - * \brief Constants for the exponential gain model
> - */
> -
>  /**
>   * \var CameraSensorHelper::blackLevel_
>   * \brief The black level of the sensor
> @@ -240,12 +203,7 @@ double CameraSensorHelper::gain(uint32_t gainCode) const
>   */
>  
>  /**
> - * \var CameraSensorHelper::gainType_
> - * \brief The analogue gain model type
> - */
> -
> -/**
> - * \var CameraSensorHelper::gainConstants_
> + * \var CameraSensorHelper::gain_
>   * \brief The analogue gain parameters used for calculation
>   *
>   * The analogue gain is calculated through a formula, and its parameters are
> @@ -526,8 +484,7 @@ public:
>  	{
>  		/* From datasheet: 64 at 10bits. */
>  		blackLevel_ = 4096;
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 100, 0, 0, 1024 };
> +		gain_ = AnalogueGainLinear{ 100, 0, 0, 1024 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("gc05a2", CameraSensorHelperGc05a2)
> @@ -539,8 +496,7 @@ public:
>  	{
>  		/* From datasheet: 64 at 10bits. */
>  		blackLevel_ = 4096;
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 100, 0, 0, 1024 };
> +		gain_ = AnalogueGainLinear{ 100, 0, 0, 1024 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("gc08a3", CameraSensorHelperGc08a3)
> @@ -552,8 +508,7 @@ public:
>  	{
>  		/* From datasheet: 64 at 10bits. */
>  		blackLevel_ = 4096;
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 0, 512, -1, 512 };
> +		gain_ = AnalogueGainLinear{ 0, 512, -1, 512 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("imx214", CameraSensorHelperImx214)
> @@ -565,8 +520,7 @@ public:
>  	{
>  		/* From datasheet: 64 at 10bits. */
>  		blackLevel_ = 4096;
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 0, 256, -1, 256 };
> +		gain_ = AnalogueGainLinear{ 0, 256, -1, 256 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("imx219", CameraSensorHelperImx219)
> @@ -578,8 +532,7 @@ public:
>  	{
>  		/* From datasheet: 0x40 at 10bits. */
>  		blackLevel_ = 4096;
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 0, 512, -1, 512 };
> +		gain_ = AnalogueGainLinear{ 0, 512, -1, 512 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("imx258", CameraSensorHelperImx258)
> @@ -591,8 +544,7 @@ public:
>  	{
>  		/* From datasheet: 0x32 at 10bits. */
>  		blackLevel_ = 3200;
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 0, 2048, -1, 2048 };
> +		gain_ = AnalogueGainLinear{ 0, 2048, -1, 2048 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("imx283", CameraSensorHelperImx283)
> @@ -604,8 +556,7 @@ public:
>  	{
>  		/* From datasheet: 0xf0 at 12bits. */
>  		blackLevel_ = 3840;
> -		gainType_ = AnalogueGainExponential;
> -		gainConstants_.exp = { 1.0, expGainDb(0.3) };
> +		gain_ = AnalogueGainExp{ 1.0, expGainDb(0.3) };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("imx290", CameraSensorHelperImx290)
> @@ -615,8 +566,7 @@ class CameraSensorHelperImx296 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperImx296()
>  	{
> -		gainType_ = AnalogueGainExponential;
> -		gainConstants_.exp = { 1.0, expGainDb(0.1) };
> +		gain_ = AnalogueGainExp{ 1.0, expGainDb(0.1) };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("imx296", CameraSensorHelperImx296)
> @@ -633,8 +583,7 @@ public:
>  	{
>  		/* From datasheet: 0x32 at 10bits. */
>  		blackLevel_ = 3200;
> -		gainType_ = AnalogueGainExponential;
> -		gainConstants_.exp = { 1.0, expGainDb(0.3) };
> +		gain_ = AnalogueGainExp{ 1.0, expGainDb(0.3) };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("imx335", CameraSensorHelperImx335)
> @@ -644,8 +593,7 @@ class CameraSensorHelperImx415 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperImx415()
>  	{
> -		gainType_ = AnalogueGainExponential;
> -		gainConstants_.exp = { 1.0, expGainDb(0.3) };
> +		gain_ = AnalogueGainExp{ 1.0, expGainDb(0.3) };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("imx415", CameraSensorHelperImx415)
> @@ -660,8 +608,7 @@ class CameraSensorHelperImx477 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperImx477()
>  	{
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 0, 1024, -1, 1024 };
> +		gain_ = AnalogueGainLinear{ 0, 1024, -1, 1024 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("imx477", CameraSensorHelperImx477)
> @@ -675,8 +622,7 @@ public:
>  		 * The Sensor Manual doesn't appear to document the gain model.
>  		 * This has been validated with some empirical testing only.
>  		 */
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 128 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 128 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov2685", CameraSensorHelperOv2685)
> @@ -686,8 +632,7 @@ class CameraSensorHelperOv2740 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperOv2740()
>  	{
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 128 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 128 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov2740", CameraSensorHelperOv2740)
> @@ -699,8 +644,7 @@ public:
>  	{
>  		/* From datasheet: 0x40 at 12bits. */
>  		blackLevel_ = 1024;
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 128 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 128 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov4689", CameraSensorHelperOv4689)
> @@ -712,8 +656,7 @@ public:
>  	{
>  		/* From datasheet: 0x10 at 10bits. */
>  		blackLevel_ = 1024;
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 16 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 16 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov5640", CameraSensorHelperOv5640)
> @@ -723,8 +666,7 @@ class CameraSensorHelperOv5647 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperOv5647()
>  	{
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 16 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 16 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov5647", CameraSensorHelperOv5647)
> @@ -734,8 +676,7 @@ class CameraSensorHelperOv5670 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperOv5670()
>  	{
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 128 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 128 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov5670", CameraSensorHelperOv5670)
> @@ -747,8 +688,7 @@ public:
>  	{
>  		/* From Linux kernel driver: 0x40 at 10bits. */
>  		blackLevel_ = 4096;
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 128 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 128 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov5675", CameraSensorHelperOv5675)
> @@ -758,8 +698,7 @@ class CameraSensorHelperOv5693 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperOv5693()
>  	{
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 16 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 16 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov5693", CameraSensorHelperOv5693)
> @@ -769,8 +708,7 @@ class CameraSensorHelperOv64a40 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperOv64a40()
>  	{
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 128 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 128 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov64a40", CameraSensorHelperOv64a40)
> @@ -780,15 +718,13 @@ class CameraSensorHelperOv8858 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperOv8858()
>  	{
> -		gainType_ = AnalogueGainLinear;
> -
>  		/*
>  		 * \todo Validate the selected 1/128 step value as it differs
>  		 * from what the sensor manual describes.
>  		 *
>  		 * See: https://patchwork.linuxtv.org/project/linux-media/patch/20221106171129.166892-2-nicholas@rothemail.net/#142267
>  		 */
> -		gainConstants_.linear = { 1, 0, 0, 128 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 128 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov8858", CameraSensorHelperOv8858)
> @@ -798,8 +734,7 @@ class CameraSensorHelperOv8865 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperOv8865()
>  	{
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 128 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 128 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov8865", CameraSensorHelperOv8865)
> @@ -809,8 +744,7 @@ class CameraSensorHelperOv13858 : public CameraSensorHelper
>  public:
>  	CameraSensorHelperOv13858()
>  	{
> -		gainType_ = AnalogueGainLinear;
> -		gainConstants_.linear = { 1, 0, 0, 128 };
> +		gain_ = AnalogueGainLinear{ 1, 0, 0, 128 };
>  	}
>  };
>  REGISTER_CAMERA_SENSOR_HELPER("ov13858", CameraSensorHelperOv13858)
> diff --git a/src/ipa/libipa/camera_sensor_helper.h b/src/ipa/libipa/camera_sensor_helper.h
> index 75868205..a9300a64 100644
> --- a/src/ipa/libipa/camera_sensor_helper.h
> +++ b/src/ipa/libipa/camera_sensor_helper.h
> @@ -11,6 +11,7 @@
>  #include <optional>
>  #include <stdint.h>
>  #include <string>
> +#include <variant>
>  #include <vector>
>  
>  #include <libcamera/base/class.h>
> @@ -30,31 +31,20 @@ public:
>  	virtual double gain(uint32_t gainCode) const;
>  
>  protected:
> -	enum AnalogueGainType {
> -		AnalogueGainLinear,
> -		AnalogueGainExponential,
> -	};
> -
> -	struct AnalogueGainLinearConstants {
> +	struct AnalogueGainLinear {
>  		int16_t m0;
>  		int16_t c0;
>  		int16_t m1;
>  		int16_t c1;
>  	};
>  
> -	struct AnalogueGainExpConstants {
> +	struct AnalogueGainExp {
>  		double a;
>  		double m;
>  	};
>  
> -	union AnalogueGainConstants {
> -		AnalogueGainLinearConstants linear;
> -		AnalogueGainExpConstants exp;
> -	};
> -
>  	std::optional<int16_t> blackLevel_;
> -	AnalogueGainType gainType_;
> -	AnalogueGainConstants gainConstants_;
> +	std::variant<std::monostate, AnalogueGainLinear, AnalogueGainExp> gain_;
>  
>  private:
>  	LIBCAMERA_DISABLE_COPY_AND_MOVE(CameraSensorHelper)

-- 
Regards,

Laurent Pinchart