[{"id":36751,"web_url":"https://patchwork.libcamera.org/comment/36751/","msgid":"","date":"2025-11-07T16:12:38","subject":"Re: [PATCH] libcamera: software_isp: Fix gamma table when CCM is\n\tused","submitter":{"id":242,"url":"https://patchwork.libcamera.org/api/people/242/","name":"Hans de Goede","email":"johannes.goede@oss.qualcomm.com"},"content":"Hi,\n\n\nOn 7-Nov-25 4:20 PM, Milan Zamazal wrote:\n> Software CPU ISP computes a gamma lookup table. The table incorporates\n> black level and contrast. All entries in the table below the black\n> level are set to 0. This is not necessarily correct all the time.\n> \n> Let's consider this case: The CCM is\n> \n> [1 0 0]\n> [0 1 0]\n> [0 0 0]\n> \n> and contrast is set to zero. The gamma table has all the entries above\n> the black level set to 186 (due to zero contrast) and all the entries\n> below the black level set to 0. CCM is applied before gamma, a\n> non-black level pixel has the blue component set to 0 with the CCM\n> above. Now, when the gamma lookup is applied, the red and green\n> components are set to 186, while the blue component is set to 0. The\n> resulting pixel is then yellow rather than grey (as it should be with\n> zero contrast).\n> \n> There are two ways to fix this: Either clamping pixel colour channels to\n> the black level in debayering or setting the below black level entries\n> in the gamma lookup table to the lowest value of the gamma table rather\n> than 0. Both should have the same effect. Let's opt for the latter for\n> its simplicity.\n> \n> Signed-off-by: Milan Zamazal \n\nThanks, patch looks good to me:\n\nReviewed-by: Hans de Goede \n\nRegards,\n\nHans\n\n\n> ---\n> src/ipa/simple/algorithms/lut.cpp | 9 ++++++++-\n> 1 file changed, 8 insertions(+), 1 deletion(-)\n> \n> diff --git a/src/ipa/simple/algorithms/lut.cpp b/src/ipa/simple/algorithms/lut.cpp\n> index d1d5f7271..d4a79e101 100644\n> --- a/src/ipa/simple/algorithms/lut.cpp\n> +++ b/src/ipa/simple/algorithms/lut.cpp\n> @@ -61,7 +61,6 @@ void Lut::updateGammaTable(IPAContext &context)\n> \tconst unsigned int blackIndex = blackLevel * gammaTable.size() / 256;\n> \tconst auto contrast = context.activeState.knobs.contrast.value_or(1.0);\n> \n> -\tstd::fill(gammaTable.begin(), gammaTable.begin() + blackIndex, 0);\n> \tconst float divisor = gammaTable.size() - blackIndex - 1.0;\n> \tfor (unsigned int i = blackIndex; i < gammaTable.size(); i++) {\n> \t\tdouble normalized = (i - blackIndex) / divisor;\n> @@ -75,6 +74,14 @@ void Lut::updateGammaTable(IPAContext &context)\n> \t\tgammaTable[i] = UINT8_MAX *\n> \t\t\t\tstd::pow(normalized, context.configuration.gamma);\n> \t}\n> +\t/*\n> +\t * Due to CCM operations, the table lookup may reach indices below the black\n> +\t * level. Let's set the table values below black level to the minimum\n> +\t * non-black value to prevent problems when the minimum value is\n> +\t * significantly non-zero (for example, when the image should be all grey).\n> +\t */\n> +\tstd::fill(gammaTable.begin(), gammaTable.begin() + blackIndex,\n> +\t\t gammaTable[blackIndex]);\n> \n> \tcontext.activeState.gamma.blackLevel = blackLevel;\n> \tcontext.activeState.gamma.contrast = contrast;","headers":{"Return-Path":"","X-Original-To":"parsemail@patchwork.libcamera.org","Delivered-To":"parsemail@patchwork.libcamera.org","Received":["from lancelot.ideasonboard.com (lancelot.ideasonboard.com\n\t[92.243.16.209])\n\tby patchwork.libcamera.org (Postfix) with ESMTPS id E1C05BDE4C\n\tfor ;\n\tFri, 7 Nov 2025 16:12:44 +0000 (UTC)","from lancelot.ideasonboard.com (localhost [IPv6:::1])\n\tby lancelot.ideasonboard.com (Postfix) with ESMTP id 3103460A80;\n\tFri, 7 Nov 2025 17:12:44 +0100 (CET)","from mx0b-0031df01.pphosted.com (mx0b-0031df01.pphosted.com\n\t[205.220.180.131])\n\tby lancelot.ideasonboard.com (Postfix) with ESMTPS id 9B676608CF\n\tfor ;\n\tFri, 7 Nov 2025 17:12:42 +0100 (CET)","from pps.filterd (m0279870.ppops.net [127.0.0.1])\n\tby mx0a-0031df01.pphosted.com (8.18.1.11/8.18.1.11) with ESMTP id\n\t5A7B2TP73040981 for ;\n\tFri, 7 Nov 2025 16:12:41 GMT","from mail-qt1-f197.google.com (mail-qt1-f197.google.com\n\t[209.85.160.197])\n\tby mx0a-0031df01.pphosted.com (PPS) with ESMTPS id 4a9fj08t0c-1\n\t(version=TLSv1.3 cipher=TLS_AES_128_GCM_SHA256 bits=128 verify=NOT)\n\tfor ;\n\tFri, 07 Nov 2025 16:12:41 +0000 (GMT)","by mail-qt1-f197.google.com with SMTP id\n\td75a77b69052e-4ed6ceab125so26181461cf.1\n\tfor ;\n\tFri, 07 Nov 2025 08:12:41 -0800 (PST)","from ?IPV6:2001:1c00:c32:7800:5bfa:a036:83f0:f9ec?\n\t(2001-1c00-0c32-7800-5bfa-a036-83f0-f9ec.cable.dynamic.v6.ziggo.nl.\n\t[2001:1c00:c32:7800:5bfa:a036:83f0:f9ec])\n\tby smtp.gmail.com with ESMTPSA id\n\ta640c23a62f3a-b72bf407a01sm277776466b.22.2025.11.07.08.12.39\n\t(version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128);\n\tFri, 07 Nov 2025 08:12:39 -0800 (PST)"],"Authentication-Results":"lancelot.ideasonboard.com; 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