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Thu, 5 Mar 2026 15:16:24 -0500 (EST) X-Mailer: MessagingEngine.com Webmail Interface From: Javier Tia Date: Thu, 05 Mar 2026 14:10:37 -0600 Subject: [PATCH v3 1/3] ipa: simple: agc: Replace bang-bang controller with proportional MIME-Version: 1.0 Message-Id: <20260305-agc-proportional-v3-1-25abc1bfacca@jetm.me> To: libcamera-devel@lists.libcamera.org Cc: Javier Tia , Milan Zamazal X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=openpgp-sha256; l=5190; i=floss@jetm.me; h=from:subject:message-id; bh=EYZd/7BZtmvDdIVohTa3jmhfVl5Dkcqp/2W+9Tnztlk=; b=owEB7QES/pANAwAKAbXuwwuoZ3cfAcsmYgBpqeM8XJfEdd+Ud6sv3+9/kpCBzHFiZu8oHziGX kZNhvDLVrqJAbMEAAEKAB0WIQSbE7ILzw7eI0VKk8m17sMLqGd3HwUCaanjPAAKCRC17sMLqGd3 HzMTC/9wEzJjsQCPfEHrCwJQ4nZNieDcTIZfntkBl6Y8uniXsUaj36F6UIzS2JkOBido9dA5uC/ Y9YeIOK/TSOG6qYW4Rlnf6S2r/tku9wFFswEW1Zno44ici8a1KYoHlX2X8QReJjSOYXY2ZvBM7Y AknVDgUq4ChwYSxQ1v11V6MMfE4OVwetsNL+ul98pcEoxrTUfe5MZmcZ3JhNZB4kCZTr1FjoSI1 SgaSShnjqv5Vf/VvFGTx9CQWciDgJzUnQcHnyhpU8To/duF4wfu9ptHTNlsOyQFpawjC89xmC5e yRfZADQtbnb1B+EohLGqmXDXqZ0iQzKneWXMmV7eJ4teq8+ZG01cVnWp3elOW5u+lixkAX+4T1D NhUF9MM8aSILlNiF3ExrfmvHB1QlMKw1f9nSaIKBIBoDTjYEIba9On67HdgF/d2UM9dHTumXL0q ls5xLya7e4On9w3/IXmux+LFMj2KWA4DLdEvWEkyiTE10g0Pn9ymK+2S5IkMmn8Jx0jkc= X-Developer-Key: i=floss@jetm.me; a=openpgp; fpr=9B13B20BCF0EDE23454A93C9B5EEC30BA867771F In-Reply-To: <20260305-agc-proportional-v3-0-25abc1bfacca@jetm.me> References: <20260305-agc-proportional-v3-0-25abc1bfacca@jetm.me> X-BeenThere: libcamera-devel@lists.libcamera.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: libcamera-devel-bounces@lists.libcamera.org Sender: "libcamera-devel" The AGC's updateExposure() uses a fixed ~10% step per frame regardless of how far the current exposure is from optimal. With a hysteresis dead band of only +/-4%, the controller overshoots when the correct value falls within one step, causing visible brightness oscillation (flicker). Replace the fixed-step bang-bang controller with a proportional one where the correction factor scales linearly with the MSV error: factor = 1.0 + clamp(error * 0.04, -0.15, +0.15) At maximum error (~2.5), this gives the same ~10% step as before. Near the target, steps shrink to <1%, eliminating overshoot. The existing hysteresis (kExposureSatisfactory) still prevents hunting on noise. Tested on OV2740 behind Intel IPU6 ISYS (ThinkPad X1 Carbon Gen 10) where the old controller produced continuous brightness flicker. The proportional controller converges in ~3 seconds from cold start with no visible oscillation. Signed-off-by: Javier Tia Reviewed-by: Milan Zamazal --- src/ipa/simple/algorithms/agc.cpp | 73 ++++++++++++++++++++++++++------------- 1 file changed, 49 insertions(+), 24 deletions(-) diff --git a/src/ipa/simple/algorithms/agc.cpp b/src/ipa/simple/algorithms/agc.cpp index 2f7e040c..a13a7552 100644 --- a/src/ipa/simple/algorithms/agc.cpp +++ b/src/ipa/simple/algorithms/agc.cpp @@ -7,6 +7,8 @@ #include "agc.h" +#include +#include #include #include @@ -37,52 +39,74 @@ static constexpr float kExposureOptimal = kExposureBinsCount / 2.0; */ static constexpr float kExposureSatisfactory = 0.2; +/* + * Proportional gain for exposure/gain adjustment. Maps the MSV error to a + * multiplicative correction factor: + * + * factor = 1.0 + kExpProportionalGain * error + * + * With kExpProportionalGain = 0.04: + * - max error ~2.5 -> factor 1.10 (~10% step, same as before) + * - error 1.0 -> factor 1.04 (~4% step) + * - error 0.3 -> factor 1.012 (~1.2% step) + * + * This replaces the fixed 10% bang-bang step with a proportional correction + * that converges smoothly and avoids overshooting near the target. + */ +static constexpr float kExpProportionalGain = 0.04; + +/* + * Maximum multiplicative step per frame, to bound the correction when the + * scene changes dramatically. + */ +static constexpr float kExpMaxStep = 0.15; + Agc::Agc() { } void Agc::updateExposure(IPAContext &context, IPAFrameContext &frameContext, double exposureMSV) { - /* - * kExpDenominator of 10 gives ~10% increment/decrement; - * kExpDenominator of 5 - about ~20% - */ - static constexpr uint8_t kExpDenominator = 10; - static constexpr uint8_t kExpNumeratorUp = kExpDenominator + 1; - static constexpr uint8_t kExpNumeratorDown = kExpDenominator - 1; - int32_t &exposure = frameContext.sensor.exposure; double &again = frameContext.sensor.gain; - if (exposureMSV < kExposureOptimal - kExposureSatisfactory) { + double error = kExposureOptimal - exposureMSV; + + if (std::abs(error) <= kExposureSatisfactory) + return; + + /* + * Compute a proportional correction factor. The sign of the error + * determines the direction: positive error means too dark (increase), + * negative means too bright (decrease). + */ + float step = std::clamp(static_cast(error) * kExpProportionalGain, + -kExpMaxStep, kExpMaxStep); + float factor = 1.0f + step; + + if (factor > 1.0f) { + /* Scene too dark: increase exposure first, then gain. */ if (exposure < context.configuration.agc.exposureMax) { - int32_t next = exposure * kExpNumeratorUp / kExpDenominator; - if (next - exposure < 1) - exposure += 1; - else - exposure = next; + int32_t next = static_cast(exposure * factor); + exposure = std::max(next, exposure + 1); } else { - double next = again * kExpNumeratorUp / kExpDenominator; + double next = again * factor; if (next - again < context.configuration.agc.againMinStep) again += context.configuration.agc.againMinStep; else again = next; } - } - - if (exposureMSV > kExposureOptimal + kExposureSatisfactory) { + } else { + /* Scene too bright: decrease gain first, then exposure. */ if (again > context.configuration.agc.again10) { - double next = again * kExpNumeratorDown / kExpDenominator; + double next = again * factor; if (again - next < context.configuration.agc.againMinStep) again -= context.configuration.agc.againMinStep; else again = next; } else { - int32_t next = exposure * kExpNumeratorDown / kExpDenominator; - if (exposure - next < 1) - exposure -= 1; - else - exposure = next; + int32_t next = static_cast(exposure * factor); + exposure = std::min(next, exposure - 1); } } @@ -96,6 +120,7 @@ void Agc::updateExposure(IPAContext &context, IPAFrameContext &frameContext, dou LOG(IPASoftExposure, Debug) << "exposureMSV " << exposureMSV + << " error " << error << " factor " << factor << " exp " << exposure << " again " << again; }