JP3758809B2 - Point light source device and target for photogrammetry using the point light source device - Google Patents

Description

【００２１】
図５のフローチャートに沿って２枚の画像から平面図を得るステップを説明する。これらのステップは、例えば外部のコンピュータ（図示しない）により行なわれる。
【００２２】
まず処理がスタートすると、ステップＳ１０２で（１）式における未知変量、即ち基準座標系における第２のカメラ位置（Ｘｏ，Ｙｏ，Ｚｏ）、および光軸Ｏ２の光軸Ｏ１に対する回転角（α，β，γ）は０でない適当な数値が与えられる。ステップＳ１０４では、前述したように基準点Ｐ１の２枚の画像における像点ｐ11、ｐ21がペアに指定され、それぞれの写真座標系で表される（図２参照）。基準点Ｐ２、Ｐ３についても同様に像点のペアｐ12とｐ22、ｐ13とｐ23が指定される。
【００２３】
次にステップＳ１０６において、初期値を１とする変数ｋが与えられる。ステップＳ１０８では、２枚の画像に共通して写る任意の物点、例えば図１に示す立方体の頂点Ｑｋ（ｋ＝１）を決定する。そして物点Ｑ１の画像１（図２(a) 参照）における像点をｑ11、画像２（図２(b) 参照）における像点をｑ21とし、この２点をペアに指定する。
【００２４】
ステップＳ１１０において、共線方程式を例えば逐次近似解法などの手法を用いて解き、基準点Ｐｉ（ｉ= １〜３）の３次元座標（ＰＸｉ，ＰＹｉ，ＰＺｉ）、および物点Ｑ１の３次元座標（ＱＸ１，ＱＹ１，ＱＺ１）を求める。逐次近似解法とは、前述の共線方程式において未知変量Ｘｏ、Ｙｏ、Ｚｏ、α、β、γに初期値を与え、この初期値の周りにテーラー展開して線形化し、最小二乗法により未知変量の補正量を求める手法である。この演算により未知変量のより誤差の少ない近似値が求められる。
【００２５】
上述のように基準座標系における基準点Ｐｉ（ｉ= １〜３）の３次元座標（ＰＸｉ，ＰＹｉ，ＰＺｉ）は、２つの写真座標ｐ1i（ｐｘ1i，ｐｙ1i）、ｐ2i（ｐｘ2i，ｐｙ2i）から変換されると同時に、Ｘｏ、Ｙｏ、Ｚｏ、α、β、γの近似値が求められる。また物点Ｑ１も、２つの写真座標ｑ11（ｑｘ11，ｑｙ11）、ｑ21（ｑｘ21，ｑｙ21）から、３次元の基準座標（ＱＸ１，ＱＹ１，ＱＺ１）に変換される。
【００２６】
ステップＳ１１２では、座標値による距離を実際の距離に補正するための補正倍率ｍを求める。この演算には既知の長さ、例えば基準点Ｐ１とＰ２との距離が用いられる。Ｐ１とＰ２の実際の距離はターゲット１０の一辺の長さＬであることから、基準座標系（Ｘ，Ｙ，Ｚ）におけるＰ１とＰ２の距離Ｌ’（図３参照）とＬとの間には次の関係式が成り立つ。
【００２７】
Ｌ＝Ｌ’×ｍ （ｍ：補正倍率）
【００２８】
ステップＳ１１４では、上式で求められた補正倍率ｍを用いて実際の長さにスケーリングされる。
【００２９】
ステップＳ１１６では、図４に示すようにＰ１とＰ２を結ぶ直線をＸ軸とし、基準形状を含む平面ＰｓをＸ−Ｚ平面とする３次元座標系（Ｘ’，Ｙ’，Ｚ’）が設定され、基準点Ｐ１を原点として基準点Ｐ２、Ｐ３、および物点Ｑ１が基準座標系から座標変換される。なお、原点は基準形状を含む面内であれば、任意の点でも構わない。この座標変換は、例えばベクトル変換などを用いて行なわれる。
【００３０】
ステップＳ１１８では図示しないモニタなどに、例えばＸ−Ｚ平面図として基準点Ｐ１〜Ｐ３とともに物点Ｑ１が図示される。なお、特にＸ−Ｚ平面図に限定されることはなく、Ｘ−Ｙ平面図あるいは立体斜視図でもよい。
【００３１】
ステップＳ１２０ではペア指定を継続するか否か、即ちさらに別の物点の３次元座標を求めるか否かを判定する。ペア指定を継続しない場合は処理が終了する。さらにペア指定を行なう場合はステップＳ１２２においてｋが１つカウントされ、ステップＳ１０８から再実行される。
【００３２】
このように任意の物点Ｑｋの数、即ちｋの回数分だけステップＳ１０８からステップＳ１２２まで繰り返し行なわれ、２枚の画像から基準点から形成される基準平面を基に作図される。なお物点Ｑｋの数ｋは、Ｘｏ、Ｙｏ、Ｚｏ、α、β、γを誤差の少ない値に近似するために最低２つ（基準点の３点と合わせて５点）必要であり、２つ以上が好ましい。
【００３３】
図６から図９には、第１実施形態である点光源装置が写真測量用ターゲットと共に示される。
図６はターゲット１０を分解して示す斜視図である。ターゲット１０は幅３〜５ｃｍ、高さ５ｃｍの木材が１辺１ｍの正三角形になるように形成された枠材１２と、この枠材１２の底を閉密する三角形の底板１４と、枠材１２上に載置される三角形の遮光板１６を備えている。
【００３４】
遮光板１６は１辺１ｍ、厚さ２〜３ｍｍの正三角形の板であり、光を遮断する鋼材から成る。遮光板１６の各頂点近傍には、約１０ｍｍ程度の円形穴２０、２２、２４がそれぞれ形成される。円形穴の口径は、５ｍ離れて見たときに擬似的に点として視認される大きさ、即ち１０ｍｍ以下が好ましい。円形穴２０、２２、２４の中心をそれぞれ基準点Ｐ１とすると、この３点Ｐ１は測量の基準形状となる正確な正三角形を形成する。なお基準形状は正三角形に限定されることはなく、一定の形状を有していればよい。例えば四角形でもよく、大きさも限定されない。また遮光板１６の材料は鋼材に限ることなく、木材などでもよい。
【００３５】
円形穴２０、２２、２４の下方の底板１４には、それぞれ点光源装置３０、３２、３４が設けられる。各点光源装置３０、３２、３４は円環状に配置された複数の高輝度ＬＥＤ（発光ダイオード）を備えており、対応する円形穴２０、２２、２４から光が射出することにより、基準点Ｐ１が強調される。
【００３６】
遮光板１６の中央には、３つの点光源装置３０、３２、３４を駆動する光源駆動部２６が埋め込まれる。光源駆動部２６は、受光量によって抵抗が変化するＣｄＳ素子（図示しない）を備え、周囲の光量に応じて各点光源装置３０、３２、３４の光量を制御する。
【００３７】
図７は点光源装置３０を底板１４側から見た平面図であり、図８は図６のＩ−Ｉ線に沿った断面における点光源装置を示す。
点光源装置３０は、１２個のＬＥＤ３０１〜３１２を備えている。１２個のＬＥＤ３０１〜３１２は底板１４に環状に等間隔に配置されており、また底板１４から円形穴２０に向かって斜め上方に光を射出するように設けられている。
【００３８】
図９はＬＥＤの指向特性を示す図である。ＬＥＤは指向性を有しており、指向軸ＢＬに沿って、図中実線で示される光度が分布している。なお指向軸ＢＬは、本明細書中においては、ＬＥＤの発光点Ｂ１を起点とし、所定平面上で発光点Ｂ１から所定距離はなれた発光点周囲の最も放射強度の大きい点Ｂ２（以下ピーク点という）を結ぶ直線として定義する。図９では、発光点Ｂ１を原点に放射状に延びた目盛りが、指向軸ＢＬに対して時計回り及び反時計回りに９０°の範囲で１０°刻みに記される。一方、径方向の目盛りは指向軸ＢＬの放射強度を１とした時の放射強度比を示し、０．１刻みに記される。例えば、指向軸ＢＬから１０°成す角の点における放射強度は、ピーク点の放射強度に対して約０．７倍である。
【００３９】
指向軸ＢＬに対する光の広がり角は、例えば半値角および半値全角で示される。放射強度比が０．５の時の、指向軸ＢＬに対する光の広がり角を半値角という。指向軸ＢＬの左右の半値角を合わせた角、即ち図中矢印で示す角ηを半値全角という。図９において、実線と放射強度比０．５の太目盛り線との交点をＢ３とし、この交点Ｂ３と発光点Ｂ１とを結ぶ一点破線と、指向軸Ｂｌとの成す角が半値角である。図９に示すＬＥＤの半値角は約１３°であるから、半値全角ηは約２６°である。
【００４０】
図８に特によく示すように、ＬＥＤ３０１〜３１２から射出された光は円形穴２０を通って外部に向うが、このとき円形穴２０が絞りの役目を果たし、射出光の輝度が最も高くなるように絞られる。円形穴２０で絞られた光の広がる角度φは、ＬＥＤの半値全角、例えば約３０°（図中、実線で示す）であることが望ましい。
【００４１】
ＬＥＤの数は特に限定されないが、どの方向から見ても光が視認できる数があればよい。例えば射出光の広がり角度φが３０°であれば、１２個以上設ければ全方向から視認できる。ＬＥＤ３０１〜３１２は、３６０°のどの方向からでも視認できるようにするために調整されて配置される。
【００４２】
ＬＥＤ３０１の指向軸をＡ１（図中一点鎖線で示される）とすると、指向軸Ａ１は円形穴２０の中心軸上に位置する基準点Ｐ１を通っている。同様にＬＥＤ３０２の指向軸Ａ２、ＬＥＤ３０３の指向軸Ａ３、（以下同様）も基準点Ｐ１を通っている。即ち、１２個のＬＥＤの各指向軸は、基準点Ｐ１で交差している。また、各指向軸は遮光板１６に対して角度θを成している。角度θは、５ｍ離れた場所から水平面に置かれた基準点が視認しやすい角度、即ち約１５°から２０°が好ましいが、特に限定されない。
【００４３】
点光源３２、３４は点光源３０と同じ構成である。なお点光源３２のＬＥＤは、対応する点光源３０のＬＥＤの番号に１２を加算して示し、点光源３４のＬＥＤは、対応する点光源３０のＬＥＤの番号に２４を加算して示す。
【００４４】
各ＬＥＤの指向軸の角度θによって決定される基準点の高さは、３つの基準点Ｐ１において等しければよく、それぞれの基準点Ｐ１において点光源装置の１２個のＬＥＤ指向軸が交差していればよい。また点光源装置３０、３２、３４の中心、即ち基準点Ｐ１が正確な正三角形を形成していればよい。従って、枠１４や遮光板１６の形状は正確でなくてもよく、精度を要求されない。
【００４５】
図９は点光源装置と光源駆動部との電気的構成を示す図である。
本実施形態の点光源装置では、光源の駆動電圧を制御することにより、各ＬＥＤの発光光量が制御される。ＬＥＤの発光光量は、同一電流でもＬＥＤ毎に異なるため、光量を一定にするために制限抵抗をそれぞれの発光光量に応じて調整し、光量が均一になるようにしている。従って、全体の駆動電圧を制御して各々のＬＥＤに流れる電流を変化させると、発光光量は各ＬＥＤにおいて同様に変化する。
【００４６】
光源駆動部２６は、電源２６２と、スイッチ２６４と、直列に接続された抵抗２６１と光伝導素子２６６と、固定抵抗２６３と、トランジスタ２６８とを備える。光伝導素子２６６は例えば可視光を検出するＣｄＳ（硫化カドミウム）から成り、ターゲット１０が設置された場所が暗くなる、即ち周囲の光量が小さくなると抵抗値が下がり、明るくなると抵抗値は増大する。ＬＥＤ３０１〜３１２は、それぞれ対応した制限抵抗４０１〜４１２と直列に連結され、各ＬＥＤと制限抵抗とが１セットにされる。そして、１２セットが並列に連結される。即ち、トランジスタ２６８から出力された電流は、３６個のＬＥＤ３０１〜３３６に同じ電流量で流れる。
【００４７】
なお図示しないが、他の発光量の制御手段として、個々のＬＥＤに流す電流値を制御してもよい。例えば、個々のＬＥＤに直列に連結された抵抗の値を変化させてもよいし、抵抗部を電流源に置き換えて制御してもよい。
【００４８】
次に回路の動作を説明する。
スイッチ２６４をＯＮにすると、駆動電流が回路内を流れ、各ＬＥＤ３０１〜３３６が一斉に点灯する。その後、例えばターゲット１０の周囲が電源ＯＮ直後より暗くなると、光伝導素子２６６の抵抗が下がり、光伝導素子２６６と抵抗２６１との分圧比が変化する。抵抗２６１の電圧は下がるので、トランジスタ２６８のベース電圧も下がる。ベース電圧が低下すると、ＬＥＤを駆動する駆動電圧が低くなり、ＬＥＤに流れる電流も減少するので、従ってＬＥＤの発光量は押さえられる。
【００４９】
即ち、周囲が暗くなるとＬＥＤの輝度は小さくなり、夜間や曇り等の環境でターゲットを使用する場合は、消費電流が少ないので昼間の晴れの時に使用する場合に比べ長時間使用でき、電池が節約できる。逆に、周囲が明るくなるとＬＥＤの輝度は大きくなるので、例えば日中の測量時においても、容易に識別できる正確な基準点が得られる。
【００５０】
本実施形態の点光源装置を用いたターゲットでは、複数のＬＥＤにより、基準点が明るく強調され、また円形穴が絞りになっているので、基準点が更に強調される。また、周囲の明るさによって各ＬＥＤの明るさを制御するので、昼間の明るいところでも基準点が視認しやすく、暗いところでの省電力が図られ、電池を有効に活用できる。
【００５１】
ＬＥＤはその指向軸が斜め上方に向かうように配置されるので、射出光が人の目の高さにおいて視認しやすく、環状に配置することにより、ターゲットをどの方角から見ても基準点が容易に視認できる。また各ＬＥＤの指向軸が１点、即ち基準点を通っているので正確な基準点が得られる。
【００５２】
上述のように、本実施形態のターゲットを用いて測量を行なうと、周囲の光り条件に左右されることなく、常に正確な基準点が得られるので、演算値の誤差は少なく、実像に近い平面図が容易に作成できる。
【００５３】
図１０から図１６は、点光源装置の他の実施形態を示す図である。点光源装置３０、３２、３４（図６参照）は明るい場所での撮影でも識別が容易であり、またどの方向から撮影しても視認できる構成であればよい。なおこれら実施形態において、第１実施形態と実質的に同一の部材については同番号が付している。
【００５４】
図１０から図１２は、点光源装置の第２実施形態を示す。第２実施形態は円形穴の下方に円筒部材４０を設けたこと以外は第１実施形態と同様の構成であり、他の構成はここでは詳述しない。図１０は部分断面図であり、図１１は円筒部材の斜視図、図１２は１つのＬＥＤを円筒部材と共に示す部分平面図である。
【００５５】
中空の円筒部材４０は例えば透明のガラスを材料とする。円筒部材４０の内径は、遮光板１６の円形穴２０の径と一致しており、円筒部材４０は遮光板１６の直下に設けられ、遮光板１６と底板１４と枠材１２によりターゲット内が閉密される。従ってターゲット内部あるいはＬＥＤ３０１〜３３６に対する防塵、防水が可能になる。
【００５６】
図１２によく示すように、ＬＥＤ３１０から射出した光は、円筒部材４０の曲面により屈折し、実際の半値全角より左右に角度λずつ拡大する。以下、角度λを拡散量という。即ち、円筒部材４０を設けたことにより、ＬＥＤの半値全角は２λ分だけ広くなり、視認性が高まる。また、例えば半値全角３０°のＬＥＤを１２個配置した場合、３６０°のどの方向からでも視認できるが、死角ができないようにＬＥＤの環状配置を微調整しなければならない。しかし、光拡散効果を有する第２実施形態においては、この微調整が容易になる。
【００５７】
なお拡散量λは、円筒部材４０の径および材料固有の屈折率により決定されるが、例えば屈折率１．４７のパイレックスガラスを用いた１０ｍｍの直径の円筒部材であれば、拡散量λは約２〜３°であり、視認性を高める十分な効果が得られる。
【００５８】
図１３は、点光源装置の第３実施形態を示す部分断面図である。第３実施形態は、円形穴の上方にカバーを設けたこと以外は第１実施形態と同様の構成であり、他の構成はここでは詳述しない。カバー４２は円形穴２０の上方に設けられ、下方側が開口したカップ型である。下方の開口部の内径と円形穴２０の径とが一致し、上に行くにしたがって径が漸減している。第３実施形態においても、第２実施形態と同様、防水・防塵効果が高められ、またカバー側面による拡散効果が得られ、視認性が高まる。
【００５９】
図１４は、点光源装置の第４実施形態を示す部分断面図である。円形穴の上方にボールレンズを設けたこと以外は第１実施形態と同様の構成である。ボールレンズ４４は蛍光物質が混入したアクリル樹脂から形成され、１２個のＬＥＤ３０１〜３１２から入射した光を周囲に拡散する。各ＬＥＤの指向軸がこのボールレンズ４４の中心を通るように、即ち、ボールレンズ４４の中心が基準点になるように、ＬＥＤ３０１〜３１２は配置される。第４実施形態においても、第２実施形態と同様、防水・防塵効果が高められる。またボールレンズ４４による拡散効果および蛍光物質による発光により、視認性が高まる。
【００６０】
図１５は、点光源装置の第５実施形態を示す部分断面図である。円形穴の上方に円錐プリズム部材を設けたこと以外は第１実施形態と同様の構成である。円錐プリズム部材５０はカップ状の外壁５２と、カップの底面から突出する円錐プリズム５４とが一体的に形成された透明のガラス部材である。円錐プリズム部材５０は開口部が下方になるように円形穴２０上に載置され、円形穴２０をカバーする。各ＬＥＤから射出された光は円錐プリズム５４の側面により反射され、外壁５２を通って外部へ拡散する。円錐プリズム部材５０からの射出光軸と水平面とが成す角θは、円錐プリズム５２の頂角の大きさと各ＬＥＤの配置方向により決定される。第５実施形態においても、第２実施形態と同様、防水・防塵効果が高められる。また円錐プリズム部材５０による拡散効果により視認性が高まる。
【００６１】
図１６は、点光源装置の第６実施形態を示す部分断面図である。円形穴に円錐プリズムを設けたこととＬＥＤの配置方向以外は第１実施形態と同様の構成である。各ＬＥＤ３０１〜３１２は遮光板１６から斜め下方に向かって環状に配置される。円錐プリズム６０は下方に突出した円錐形のプリズム６２と、一部が円形穴２０から上方に突出した円錐台形のプリズム６４とを備える。
【００６２】
指向軸が一点破線で示されるように、ＬＥＤ３０４から出射した光はまずプリズム６４の下方側面に入射し、プリズム６２の側面６２ａにより内部反射する。次にプリズム６４の側面６４ａにより内部反射した光は、側面６４ａの円形穴２０から突出した部分から射出する。この時、射出方向はＬＥＤの入射方向とほぼ反対になる。２つのプリズム６２、６４の頂角は内面反射時に全反射をするように設定される。第６実施形態においても、第２実施形態と同様、防水・防塵効果が高められる。
【００６３】
以上のように本発明の各実施形態によれば、複数の光源を環状に配置させることにより基準点を強調させるので、基準点が容易に識別できる。従って基準点の正確な位置が得られるので、水平面を基準に画像を幾何学演算により求めることにより、演算値の信頼性を高めることができる。
【００６４】
【発明の効果】
以上のように本発明によれば、視認性の高い点光源装置およびターゲットが得られる。
【図面の簡単な説明】
【図１】本発明の実施形態である写真測量用ターゲットと被写体とカメラとの位置関係を示す斜視図である。
【図２】第１及び第２のカメラ位置から撮影したときの画像を示す図である。
【図３】基準点とその像点と撮影レンズの後側主点位置との位置関係を３次元座標で示す図である。
【図４】基準形状を含む平面に基づく３次元座標を示す図である。
【図５】２枚の画像から被写体の平面図を得るステップを示すフローチャートである。
【図６】図１に示す写真測量用ターゲットを分解して示す拡大斜視図である。
【図７】図１に示す写真測量用ターゲットの点光源装置を示す平面図である。
【図８】図１に示す写真測量用ターゲットの点光源装置を示す部分断面図である。
【図９】図８に示す点光源装置をＬＥＤの指向特性を示す図である。
【図１０】図１に示す写真測量用ターゲットの回路構成を示すブロック図である。
【図１１】本発明の第２実施形態である点光源装置を示す断面図である。
【図１２】図１１に示す点光源装置の円筒部材を示す斜視図である。
【図１３】図１１に示す点光源装置のＬＥＤと円筒部材を共に示す部分平面図である。
【図１４】本発明の第３実施形態である点光源装置を示す断面図である。
【図１５】本発明の第４実施形態である点光源装置を示す断面図である。
【図１６】本発明の第５実施形態である点光源装置を示す断面図である。
【図１７】本発明の第６実施形態である点光源装置を示す断面図である。
【符号の説明】
１０ ターゲット
１２ 枠材
１４ 底板
１６ 遮光板
２０、２２、２４ 円形穴
３０、３２、３４ 点光源装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a point light source device for a photogrammetry target used as a reference for length and angle at the time of photographing, for example, in photogrammetry.
[0002]
[Prior art]
In conventional photogrammetry performed in traffic accident investigations, for example, a subject is photographed by a camera using a silver salt film or an electronic still camera, and the three-dimensional coordinates of the subject are obtained by calculation from the two-dimensional coordinates of the subject in the recorded image. It is done.
[0003]
In such photogrammetry, for example, conical marks (hereinafter referred to as cones) are installed at three locations, and photographing including these cones is performed. Then, when calculating the actual coordinates using the recorded image, the calculation is performed using the tip of each cone as a reference point and the reference plane defined by these reference points as a pseudo horizontal plane. Plotting is performed based on the value.
[0004]
[Problems to be solved by the invention]
However, the reference point that is the tip of the cone is difficult to identify depending on the direction, particularly when surveying is performed at night or the like, causing a problem that an error occurs in the coordinate value and accurate drawing cannot be performed.
[0005]
The present invention has been made in view of such problems, and an object thereof is to provide a photogrammetric target in which the reference point can be easily identified.
[0006]
[Means for Solving the Problems]
The point light source device according to the present invention has directivity, and includes a plurality of light sources that emit light in a predetermined angle range with respect to a directional axis, and a diaphragm provided on a light emitting side of the light source, A plurality of light sources are arranged so that the directivity axes of the plurality of light sources intersect at one point.
[0007]
The photogrammetry target according to the present invention is a photogrammetry target used for photogrammetry for obtaining the coordinates of a subject with respect to an arbitrary origin based on a recorded image, and includes at least three reference points that define a reference plane; A plurality of point light source devices corresponding to the reference points and emphasizing each of the reference points, the point light source devices having directivity, and a plurality of light beams emitted in a predetermined angle range with respect to the directional axis A light source and a diaphragm provided on the light emission side with respect to the light source are provided, and the directional axes of the plurality of light sources are arranged so as to intersect at a reference point.
[0008]
In the point light source device, preferably, the light intensity from the light source gradually increases as the light emitted from the light source approaches the directivity axis, and the light intensity at the directivity axis is the highest.
[0009]
In the point light source device, preferably, a plurality of light sources are arranged in a ring shape. More preferably, the light emitted from the light source through the stop through the point passing through the point where the respective directional axes intersect and perpendicular to the surface on which the point light source device is placed can be viewed from all directions around the axis.
[0010]
In the point light source device, preferably, light detection means for detecting the amount of ambient light is provided, and the amount of light emitted from the light source is controlled in accordance with the amount of light detected from the light detection means.
[0011]
In the point light source device, the light emitted from the light source is preferably narrowed so that the luminance of the light source becomes the highest.
[0012]
In the point light source device, the diaphragm is preferably a circular hole formed in the light shielding plate.
[0013]
In the point light source device, preferably, a waterproof and dustproof transparent member is provided in the vicinity of the diaphragm. More preferably, the transparent member is a cylindrical member provided between the light source and the diaphragm, and diffuses the light of the point light source.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of a photogrammetric target according to the present invention will be described below with reference to the accompanying drawings. Note that the camera used in this embodiment is an electronic still camera using an image sensor, and the captured image is recorded on a recording medium electrically or magnetically.
[0015]
FIG. 1 is a diagram illustrating a positional relationship among a target 10 according to an embodiment of the present invention, a cube 102 that is a subject, and a camera 100. The camera 100 is photographed from two directions so that both the cube 102 and the target 10 can be seen. The first and second camera positions are indicated by rear principal point positions M1 and M2, respectively, and the optical axis directions are indicated by O1 and O2, respectively. The first camera position M1 is indicated by a solid line, and the second camera position M2 is indicated by a broken line.
[0016]
As will be described later, the target 10 has three reference points P1, P2, and P3 located at the vertices of an equilateral triangle, and has a shape defined by these reference points P1, P2, and P3 (indicated by hatching in the figure). Is referred to as a reference shape in this specification. In this embodiment, the reference shape is a regular triangle having a length L.
[0017]
FIGS. 2 (a) and 2 (b) are images taken from two camera positions M1 and M2, respectively. In the image 1 shown in FIG. 2A, a first photographic coordinate system (x1, y1), which is a two-dimensional orthogonal coordinate system with the imaging center c1 as the origin, is set on the image. The image point of the reference point P1 in the first photographic coordinate system is indicated by p11 (px11, py11). Similarly, the reference points P2 and P3 correspond to the image points p12 (px12, py12) and p13 (px13, py13), respectively. Also in the image 2 of FIG. 2B, the image points of the reference points P1 to P3 in the second photographic coordinate system (x1, y1) are p21 (px21, py21), p22 (px22, py22), p23 ( px23, py23).
[0018]
FIG. 3 is a diagram three-dimensionally showing the positional relationship between the camera, two images, and the target. In order to obtain the cubic three-dimensional coordinates from the two images shown in FIG. 2, it is necessary to set a certain three-dimensional reference coordinate system and determine the positions of the two images in the reference coordinate system. is there. A right-handed three-dimensional orthogonal coordinate system (X, Y, Z) with the first camera position M1 as the origin and the optical axis O1 direction as the Z-axis is defined as a reference coordinate system, and the position of the second camera position M2 is defined. This is represented by the reference coordinates. That is, the second camera position M2 is indicated by a displacement amount (Xo, Yo, Zo) with respect to the first camera position and a rotation angle (α, β, γ) with respect to the optical axis O1.
[0019]
For the three-dimensional coordinates (PXi, PYi, PZi) of the reference point Pi (i = 1 to 3) in the reference coordinate system, for example, the reference point, its image point, and the rear principal point position of the photographing lens are in a straight line. It is obtained using a collinear equation (Equation (1)) using a certain thing. Note that C in the equation (1) is a principal point distance, that is, a focal length, and is the same in the two images. In FIG. 3, the principal point distance C is the distance between the rear principal point position M1 of the photographing lens and the imaging center c1, or the distance between the rear principal point position M2 of the photographing lens and the imaging center c2.
[0020]
[Expression 1]

[0021]
The step of obtaining a plan view from two images will be described along the flowchart of FIG. These steps are performed by, for example, an external computer (not shown).
[0022]
First, when the process starts, in step S102, the unknown variable in the equation (1), that is, the second camera position (Xo, Yo, Zo) in the reference coordinate system, and the rotation angle (α, β) of the optical axis O2 with respect to the optical axis O1. , Γ) is given a suitable non-zero numerical value. In step S104, as described above, the image points p11 and p21 in the two images of the reference point P1 are designated as a pair and are represented by their respective photographic coordinate systems (see FIG. 2). Similarly, for the reference points P2 and P3, pairs of image points p12 and p22 and p13 and p23 are designated.
[0023]
Next, in step S106, a variable k having an initial value of 1 is given. In step S108, an arbitrary object point appearing in common in the two images, for example, the vertex Qk (k = 1) of the cube shown in FIG. 1 is determined. The image point of the object point Q1 in image 1 (see FIG. 2 (a)) is designated as q11, and the image point in image 2 (see FIG. 2 (b)) is designated as q21, and these two points are designated as a pair.
[0024]
In step S110, the collinear equation is solved using a technique such as a successive approximation method, and the three-dimensional coordinates (PXi, PYi, PZi) of the reference point Pi (i = 1 to 3) and the three-dimensional coordinates of the object point Q1. (QX1, QY1, QZ1) is obtained. In the successive approximation method, initial values are given to unknown variables Xo, Yo, Zo, α, β, and γ in the above-mentioned collinear equation, and the Taylor expansion is performed around these initial values to linearize them. This is a method for obtaining the correction amount. By this calculation, an approximate value with less error of the unknown variable is obtained.
[0025]
As described above, the three-dimensional coordinates (PXi, PYi, PZi) of the reference point Pi (i = 1 to 3) in the reference coordinate system are converted from the two photographic coordinates p1i (px1i, py1i) and p2i (px2i, py2i). At the same time, approximate values of Xo, Yo, Zo, α, β, and γ are obtained. The object point Q1 is also converted from the two photographic coordinates q11 (qx11, qy11) and q21 (qx21, qy21) to the three-dimensional reference coordinates (QX1, QY1, QZ1).
[0026]
In step S112, a correction magnification m for correcting the distance based on the coordinate values to the actual distance is obtained. This calculation uses a known length, for example, the distance between the reference points P1 and P2. Since the actual distance between P1 and P2 is the length L of one side of the target 10, between the distance L ′ (see FIG. 3) between P1 and P2 in the reference coordinate system (X, Y, Z) and L. The following relational expression holds.
[0027]
L = L ′ × m (m: correction magnification)
[0028]
In step S114, scaling is performed to the actual length using the correction magnification m obtained by the above equation.
[0029]
In step S116, a three-dimensional coordinate system (X ′, Y ′, Z ′) is set in which the straight line connecting P1 and P2 is the X axis and the plane Ps including the reference shape is the XZ plane as shown in FIG. Then, the reference points P2, P3 and the object point Q1 are coordinate-transformed from the reference coordinate system with the reference point P1 as the origin. The origin may be an arbitrary point as long as it is within the plane including the reference shape. This coordinate conversion is performed using, for example, vector conversion.
[0030]
In step S118, the object point Q1 is shown together with the reference points P1 to P3, for example, as an XZ plan view on a monitor (not shown). Note that the present invention is not particularly limited to the XZ plan view, and may be an XY plan view or a three-dimensional perspective view.
[0031]
In step S120, it is determined whether or not the pair designation is continued, that is, whether or not three-dimensional coordinates of another object point are to be obtained. If the pair designation is not continued, the process ends. When further pair designation is performed, one k is counted in step S122, and the process is re-executed from step S108.
[0032]
As described above, the process is repeatedly performed from step S108 to step S122 by the number of arbitrary object points Qk, that is, k times, and drawing is performed based on the reference plane formed from the reference points from the two images. Note that the number k of the object points Qk is required to be at least two (five points, including the three reference points) in order to approximate Xo, Yo, Zo, α, β, and γ to values with less error. One or more are preferred.
[0033]
The point light source device which is 1st Embodiment is shown with FIGS. 6-9 with the target for photogrammetry.
FIG. 6 is an exploded perspective view showing the target 10. The target 10 has a frame member 12 formed so that a timber having a width of 3 to 5 cm and a height of 5 cm is a regular triangle having a side of 1 m, a triangular base plate 14 that closes the bottom of the frame member 12, and a frame member 12 is provided with a triangular light shielding plate 16 placed on the surface 12.
[0034]
The light shielding plate 16 is a regular triangular plate having a side of 1 m and a thickness of 2 to 3 mm, and is made of a steel material that blocks light. In the vicinity of each vertex of the light shielding plate 16, circular holes 20, 22, 24 of about 10 mm are formed. The diameter of the circular hole is preferably a size visually recognized as a point when viewed at a distance of 5 m, that is, 10 mm or less. Assuming that the centers of the circular holes 20, 22, and 24 are the reference points P1, these three points P1 form an accurate equilateral triangle that serves as a reference shape for surveying. Note that the reference shape is not limited to an equilateral triangle, and may have a certain shape. For example, it may be a rectangle and the size is not limited. The material of the light shielding plate 16 is not limited to steel, and may be wood.
[0035]
Point light source devices 30, 32, and 34 are provided on the bottom plate 14 below the circular holes 20, 22, and 24, respectively. Each point light source device 30, 32, 34 includes a plurality of high-intensity LEDs (light-emitting diodes) arranged in an annular shape, and light is emitted from the corresponding circular holes 20, 22, 24, whereby the reference point P1. Is emphasized.
[0036]
A light source driving unit 26 for driving the three point light source devices 30, 32 and 34 is embedded in the center of the light shielding plate 16. The light source driving unit 26 includes a CdS element (not shown) whose resistance varies depending on the amount of received light, and controls the light amount of each point light source device 30, 32, 34 in accordance with the ambient light amount.
[0037]
FIG. 7 is a plan view of the point light source device 30 as viewed from the bottom plate 14 side, and FIG. 8 shows the point light source device in a cross section taken along line II in FIG.
The point light source device 30 includes 12 LEDs 301 to 312. The twelve LEDs 301 to 312 are annularly arranged on the bottom plate 14 at equal intervals, and are provided so as to emit light obliquely upward from the bottom plate 14 toward the circular hole 20.
[0038]
FIG. 9 is a diagram showing the directivity characteristics of the LED. The LED has directivity, and the light intensity indicated by the solid line in the drawing is distributed along the directivity axis BL. In this specification, the directional axis BL is a point B2 (hereinafter referred to as a peak point) having the highest radiant intensity around the light emitting point that is a predetermined distance from the light emitting point B1 on a predetermined plane, starting from the light emitting point B1 of the LED. ). In FIG. 9, the scale extending radially from the light emitting point B1 as the origin is marked in increments of 10 ° in the range of 90 ° clockwise and counterclockwise with respect to the directivity axis BL. On the other hand, the scale in the radial direction indicates the ratio of radiation intensity when the radiation intensity of the directivity axis BL is 1, and is written in increments of 0.1. For example, the radiation intensity at a point formed at an angle of 10 ° from the directivity axis BL is about 0.7 times the radiation intensity at the peak point.
[0039]
The light divergence angle with respect to the directivity axis BL is represented by, for example, a half-value angle and a full-value half-value angle. The spread angle of light with respect to the directivity axis BL when the radiation intensity ratio is 0.5 is called a half-value angle. The angle obtained by combining the left and right half-value angles of the directivity axis BL, that is, the angle η indicated by the arrow in the figure is referred to as half-value full angle. In FIG. 9, the intersection point between the solid line and the thick scale line with the radiation intensity ratio of 0.5 is B3, and the angle formed by the one-point broken line connecting the intersection point B3 and the light emission point B1 and the directivity axis Bl is a half-value angle. Since the half-value angle of the LED shown in FIG. 9 is about 13 °, the full-value half-value η is about 26 °.
[0040]
As shown particularly well in FIG. 8, the light emitted from the LEDs 301 to 312 travels outward through the circular hole 20. At this time, the circular hole 20 serves as an aperture so that the luminance of the emitted light becomes the highest. It is narrowed down to. The angle φ at which light narrowed down by the circular hole 20 spreads is desirably a full width at half maximum of the LED, for example, about 30 ° (indicated by a solid line in the figure).
[0041]
The number of LEDs is not particularly limited, but it is sufficient that the number of LEDs is visible from any direction. For example, when the spread angle φ of the emitted light is 30 °, it can be visually recognized from all directions if 12 or more are provided. The LEDs 301 to 312 are adjusted and arranged so as to be visible from any direction of 360 °.
[0042]
Assuming that the directivity axis of the LED 301 is A1 (indicated by a one-dot chain line in the drawing), the directivity axis A1 passes through a reference point P1 located on the central axis of the circular hole 20. Similarly, the directivity axis A2 of the LED 302, the directivity axis A3 of the LED 303 (and so on) pass through the reference point P1. That is, the directivity axes of the 12 LEDs intersect at the reference point P1. Each directional axis forms an angle θ with respect to the light shielding plate 16. The angle θ is preferably an angle at which a reference point placed on a horizontal plane from a place 5 m away is easily visible, that is, about 15 ° to 20 °, but is not particularly limited.
[0043]
The point light sources 32 and 34 have the same configuration as the point light source 30. The LED of the point light source 32 is indicated by adding 12 to the LED number of the corresponding point light source 30, and the LED of the point light source 34 is indicated by adding 24 to the LED number of the corresponding point light source 30.
[0044]
The heights of the reference points determined by the angle θ of the directivity axis of each LED need only be equal at the three reference points P1, and the 12 LED directivity axes of the point light source device may intersect at each reference point P1. That's fine. Moreover, the center of the point light source devices 30, 32, and 34, that is, the reference point P1 only needs to form an accurate equilateral triangle. Therefore, the shapes of the frame 14 and the light shielding plate 16 may not be accurate, and accuracy is not required.
[0045]
FIG. 9 is a diagram illustrating an electrical configuration of the point light source device and the light source driving unit.
In the point light source device of this embodiment, the amount of light emitted from each LED is controlled by controlling the driving voltage of the light source. Since the amount of light emitted by the LED differs for each LED even with the same current, the limiting resistance is adjusted according to the amount of light emitted to make the amount of light constant so that the amount of light is uniform. Therefore, when the overall drive voltage is controlled to change the current flowing through each LED, the amount of emitted light similarly changes in each LED.
[0046]
The light source driving unit 26 includes a power source 262, a switch 264, a resistor 261, a photoconductive element 266 connected in series, a fixed resistor 263, and a transistor 268. The photoconductive element 266 is made of, for example, CdS (cadmium sulfide) that detects visible light. The location where the target 10 is installed becomes dark, that is, the resistance value decreases as the amount of ambient light decreases, and the resistance value increases as the brightness increases. The LEDs 301 to 312 are connected in series with the corresponding limiting resistors 401 to 412, respectively, and each LED and the limiting resistor are made into one set. And 12 sets are connected in parallel. That is, the current output from the transistor 268 flows through the 36 LEDs 301 to 336 with the same amount of current.
[0047]
Although not shown in the figure, the current value flowing through each LED may be controlled as another means for controlling the amount of emitted light. For example, the value of a resistor connected in series to each LED may be changed, or the resistance unit may be replaced with a current source for control.
[0048]
Next, the operation of the circuit will be described.
When the switch 264 is turned on, the drive current flows in the circuit, and the LEDs 301 to 336 are turned on all at once. Thereafter, for example, when the periphery of the target 10 becomes darker than immediately after the power is turned on, the resistance of the photoconductive element 266 decreases, and the voltage dividing ratio between the photoconductive element 266 and the resistor 261 changes. Since the voltage of the resistor 261 decreases, the base voltage of the transistor 268 also decreases. When the base voltage is lowered, the driving voltage for driving the LED is lowered and the current flowing through the LED is also reduced, so that the light emission amount of the LED is suppressed.
[0049]
In other words, when the surroundings become darker, the brightness of the LED decreases, and when using the target in an environment such as nighttime or cloudy, the current consumption is low, so it can be used for a long time compared to when it is sunny in the daytime, saving battery it can. On the contrary, since the brightness of the LED increases as the surroundings become brighter, an accurate reference point that can be easily identified can be obtained, for example, during daytime surveying.
[0050]
In the target using the point light source device of the present embodiment, the reference point is brightly emphasized by the plurality of LEDs, and the circular hole is a stop, so that the reference point is further emphasized. In addition, since the brightness of each LED is controlled by the ambient brightness, the reference point can be easily seen even in a bright place in the daytime, power can be saved in a dark place, and the battery can be used effectively.
[0051]
Since the LED is arranged so that its directional axis is obliquely upward, the emitted light is easily visible at the height of the human eye, and by arranging it in an annular shape, the reference point can be easily seen from any direction Visible to. Further, since the directivity axis of each LED passes through one point, that is, the reference point, an accurate reference point can be obtained.
[0052]
As described above, when surveying is performed using the target of the present embodiment, an accurate reference point is always obtained without being influenced by the surrounding light conditions, so that there is little error in the calculated value and a plane close to a real image. Illustrations can be created easily.
[0053]
10 to 16 are views showing other embodiments of the point light source device. The point light source devices 30, 32, and 34 (see FIG. 6) may be configured so that they can be easily identified even when taken in a bright place and can be visually recognized from any direction. In these embodiments, the same reference numerals are given to substantially the same members as those in the first embodiment.
[0054]
10 to 12 show a second embodiment of the point light source device. The second embodiment has the same configuration as that of the first embodiment except that the cylindrical member 40 is provided below the circular hole, and other configurations will not be described in detail here. 10 is a partial sectional view, FIG. 11 is a perspective view of a cylindrical member, and FIG. 12 is a partial plan view showing one LED together with the cylindrical member.
[0055]
The hollow cylindrical member 40 is made of, for example, transparent glass. The inner diameter of the cylindrical member 40 coincides with the diameter of the circular hole 20 of the light shielding plate 16. The cylindrical member 40 is provided immediately below the light shielding plate 16, and the inside of the target is closed by the light shielding plate 16, the bottom plate 14, and the frame material 12. It is dense. Accordingly, dustproofing and waterproofing inside the target or the LEDs 301 to 336 are possible.
[0056]
As well shown in FIG. 12, the light emitted from the LED 310 is refracted by the curved surface of the cylindrical member 40 and expands by an angle λ to the left and right from the actual full width at half maximum. Hereinafter, the angle λ is referred to as a diffusion amount. That is, by providing the cylindrical member 40, the full width at half maximum of the LED is widened by 2λ, and visibility is improved. Further, for example, when twelve LEDs having a full width at half maximum of 30 ° are arranged, the LEDs can be viewed from any direction of 360 °, but the annular arrangement of the LEDs must be finely adjusted so as not to cause a blind spot. However, in the second embodiment having a light diffusion effect, this fine adjustment becomes easy.
[0057]
The diffusion amount λ is determined by the diameter of the cylindrical member 40 and the refractive index inherent to the material. For example, if the cylindrical member has a diameter of 10 mm using Pyrex glass having a refractive index of 1.47, the diffusion amount λ is about It is 2 to 3 °, and a sufficient effect of improving the visibility is obtained.
[0058]
FIG. 13 is a partial cross-sectional view showing a third embodiment of the point light source device. The third embodiment has the same configuration as that of the first embodiment except that a cover is provided above the circular hole, and other configurations will not be described in detail here. The cover 42 is a cup type provided above the circular hole 20 and opened on the lower side. The inner diameter of the lower opening coincides with the diameter of the circular hole 20, and the diameter gradually decreases toward the top. In the third embodiment, as in the second embodiment, the waterproof / dustproof effect is enhanced, and the diffusion effect by the side surface of the cover is obtained, thereby improving the visibility.
[0059]
FIG. 14 is a partial cross-sectional view showing a fourth embodiment of the point light source device. The configuration is the same as that of the first embodiment except that a ball lens is provided above the circular hole. The ball lens 44 is formed of an acrylic resin mixed with a fluorescent material, and diffuses light incident from the 12 LEDs 301 to 312 to the surroundings. The LEDs 301 to 312 are arranged so that the directivity axis of each LED passes through the center of the ball lens 44, that is, the center of the ball lens 44 is a reference point. Also in the fourth embodiment, the waterproof and dustproof effect is enhanced as in the second embodiment. Visibility is enhanced by the diffusion effect by the ball lens 44 and the light emission by the fluorescent material.
[0060]
FIG. 15 is a partial cross-sectional view showing a fifth embodiment of the point light source device. The configuration is the same as that of the first embodiment except that a conical prism member is provided above the circular hole. The conical prism member 50 is a transparent glass member in which a cup-shaped outer wall 52 and a conical prism 54 protruding from the bottom surface of the cup are integrally formed. The conical prism member 50 is placed on the circular hole 20 so that the opening is downward, and covers the circular hole 20. The light emitted from each LED is reflected by the side surface of the conical prism 54 and diffuses to the outside through the outer wall 52. The angle θ formed between the optical axis emitted from the conical prism member 50 and the horizontal plane is determined by the size of the apex angle of the conical prism 52 and the arrangement direction of each LED. Also in the fifth embodiment, the waterproof / dustproof effect is enhanced as in the second embodiment. Further, the visibility is enhanced by the diffusion effect of the conical prism member 50.
[0061]
FIG. 16 is a partial sectional view showing a sixth embodiment of the point light source device. The configuration is the same as that of the first embodiment except that a conical prism is provided in the circular hole and the LED arrangement direction. Each of the LEDs 301 to 312 is annularly arranged from the light shielding plate 16 obliquely downward. The conical prism 60 includes a conical prism 62 protruding downward and a truncated cone prism 64 partially protruding upward from the circular hole 20.
[0062]
As indicated by the dashed line, the light emitted from the LED 304 first enters the lower side surface of the prism 64 and is internally reflected by the side surface 62 a of the prism 62. Next, the light internally reflected by the side surface 64a of the prism 64 is emitted from the portion protruding from the circular hole 20 of the side surface 64a. At this time, the emission direction is almost opposite to the incident direction of the LED. The apex angles of the two prisms 62 and 64 are set so as to perform total reflection during internal reflection. Also in the sixth embodiment, the waterproof and dustproof effect is enhanced as in the second embodiment.
[0063]
As described above, according to each embodiment of the present invention, since the reference point is emphasized by arranging a plurality of light sources in a ring shape, the reference point can be easily identified. Therefore, since the exact position of the reference point can be obtained, the reliability of the calculated value can be improved by obtaining the image by geometric calculation with reference to the horizontal plane.
[0064]
【The invention's effect】
As described above, according to the present invention, a point light source device and a target with high visibility can be obtained.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a positional relationship among a photogrammetry target, a subject, and a camera according to an embodiment of the present invention.
FIG. 2 is a diagram illustrating an image taken from first and second camera positions.
FIG. 3 is a diagram illustrating a positional relationship between a reference point, an image point thereof, and a rear principal point position of the photographing lens in three-dimensional coordinates.
FIG. 4 is a diagram illustrating three-dimensional coordinates based on a plane including a reference shape.
FIG. 5 is a flowchart showing steps for obtaining a plan view of a subject from two images.
6 is an enlarged perspective view showing the photogrammetry target shown in FIG. 1 in an exploded manner. FIG.
7 is a plan view showing a point light source device of the photogrammetric target shown in FIG. 1. FIG.
8 is a partial cross-sectional view showing a point light source device of the photogrammetric target shown in FIG. 1. FIG.
9 is a diagram showing directivity characteristics of LEDs in the point light source device shown in FIG.
10 is a block diagram showing a circuit configuration of the photogrammetric target shown in FIG. 1. FIG.
FIG. 11 is a cross-sectional view showing a point light source device according to a second embodiment of the present invention.
12 is a perspective view showing a cylindrical member of the point light source device shown in FIG. 11. FIG.
13 is a partial plan view showing both the LED and the cylindrical member of the point light source device shown in FIG.
FIG. 14 is a cross-sectional view showing a point light source device according to a third embodiment of the present invention.
FIG. 15 is a cross-sectional view showing a point light source device according to a fourth embodiment of the present invention.
FIG. 16 is a cross-sectional view showing a point light source device according to a fifth embodiment of the present invention.
FIG. 17 is a cross-sectional view showing a point light source device according to a sixth embodiment of the present invention.
[Explanation of symbols]
10 Target
12 Frame material
14 Bottom plate
16 Shading plate
20, 22, 24 round holes
30, 32, 34 Point light source device

Claims (10)

A plurality of light sources having directivity and emitting light in a predetermined angle range with respect to a directivity axis; and a diaphragm provided on a light emitting side of the light source,
The point light source device, wherein the plurality of light sources are arranged so that the directivity axes of the plurality of light sources intersect at one point. A photogrammetry target used for photogrammetry to determine the coordinates of a subject relative to an arbitrary origin based on a recorded image,
At least three reference points defining a reference plane;
At least three point light source devices corresponding to the reference points and emphasizing the reference points,
This point light source device includes a plurality of light sources that have directivity and emit light in a predetermined angle range with respect to a directional axis, and a diaphragm provided on the light emitting side with respect to the light source,
A photogrammetric target, wherein the directional axes of the plurality of light sources are arranged so as to intersect at a reference point. 3. The point light source device according to claim 1, wherein the light intensity is gradually increased as the light emitted from the light source is closer to the directivity axis, and the light intensity at the directivity axis is the highest. The point light source device according to claim 3, wherein the plurality of light sources are arranged in a ring shape. The light emitted from the light source through the diaphragm can be viewed from all directions around the axis with respect to the axis perpendicular to the surface on which the point light source device is placed through the points where the directivity axes intersect. The point light source device according to claim 4. The point light source device according to claim 3, wherein a light detection unit that detects a surrounding light amount is provided, and an emission light amount of the light source is controlled according to a light amount detected from the light detection unit. 4. The photogrammetric target according to claim 3, wherein the aperture stops the light emitted from the light source so that the luminance of the light source is the highest. The point light source device according to claim 7, wherein the diaphragm is a circular hole formed in a light shielding plate. The photogrammetric target according to claim 7, wherein a waterproof and dustproof transparent member is provided in the vicinity of the diaphragm. The point light source device according to claim 9, wherein the transparent member is a cylindrical member provided between the light source and the diaphragm, and diffuses light of the point light source.

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