JP4478981B2 - Color noise reduction method and color imaging apparatus - Google Patents

Color noise reduction method and color imaging apparatus Download PDF

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JP4478981B2
JP4478981B2 JP2004354685A JP2004354685A JP4478981B2 JP 4478981 B2 JP4478981 B2 JP 4478981B2 JP 2004354685 A JP2004354685 A JP 2004354685A JP 2004354685 A JP2004354685 A JP 2004354685A JP 4478981 B2 JP4478981 B2 JP 4478981B2
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忠 杉木
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本発明は、カラー画像信号から色にじみなく色信号の雑音を低減する手法と装置に関し、特にカラー撮像装置の色信号雑音低減手法に関する。   The present invention relates to a method and apparatus for reducing color signal noise from a color image signal without causing color blur, and more particularly to a color signal noise reduction method for a color imaging apparatus.
近年、ディジタルスチルカメラなどのカラー固体撮像装置の小型化が著しい。これには、固体撮像素子の小型化が大きく寄与している。この固体撮像素子の小型化は、半導体微細加工技術の進展により、マイクロレンズの改善による集光率の向上や電荷電圧変換率の向上などにより、感度を維持しながら画素の微細化を実現してきた。 In recent years, color solid-state imaging devices such as digital still cameras have been remarkably miniaturized. To this end, downsizing of the solid-state image sensor greatly contributes. The downsizing of this solid-state image sensor has realized the miniaturization of pixels while maintaining the sensitivity by improving the microlens and improving the condensing rate and the charge-voltage conversion rate by the progress of semiconductor microfabrication technology. .
しかしながら、電荷電圧変換率に頼る感度向上では、信号電荷の数自体は低下しているため、量子性の雑音であるショット雑音や暗電流と呼ばれるフォトダイオードのリーク電流の雑音は増加してしまい、固体撮像素子のS/Nが低下し、特に色雑音の増加は画像の艶やかさを損なわせ画質低下の主要因の1つとなっている。 However, in the sensitivity improvement that relies on the charge-voltage conversion rate, the number of signal charges itself has decreased, so the noise of photodiode leakage current called shot noise or dark current, which is quantum noise, increases. The S / N of the solid-state imaging device is lowered, and particularly, the increase in color noise is one of the main causes of image quality degradation because the glossiness of the image is impaired.
このS/Nの低下を補うため、近年のカラービデオカメラでは、フレームメモリーを使い、入力画像信号の静止している領域では残像を多く、動きのある領域では残像を弱く制御することで雑音を低減する巡回型の雑音低減回路を搭載しているものもある。この雑音低減方法は、入射光量のゆらぎである光ショット雑音のようにランダムに変動する雑音には効果があるが、暗電流のばらつきのような固定パターン雑音については低減効果はなく、また撮影で1つの画像信号しか得られないスチルカメラには使用できない。 To compensate for this reduction in S / N, recent color video cameras use frame memory to control the noise by controlling afterimages in areas where the input image signal is stationary and weakly in areas where there is motion. Some are equipped with a cyclic noise reduction circuit that reduces noise. This noise reduction method is effective for noise that fluctuates randomly, such as light shot noise, which is a fluctuation in the amount of incident light, but it does not reduce fixed pattern noise such as variations in dark current. It cannot be used for a still camera that can obtain only one image signal.
1つの画像信号だけで雑音を低減する手法としては、特許文献1の領域成長法を用いた雑音低減手法がある。これは、注目画素の信号値に対し、注目画素を起点として隣接画素の信号値が所定のレベル差以内にある画素を含む領域に順次拡大して近隣領域を求め、求めた近隣領域の信号値の平均を注目画素の雑音を除去した信号値とするものである。各画素に対してこの操作を繰り返すことで、画像の雑音除去ができる。また、信号レベル差の大きな画素は平均値の算出には入らないため、画像エッジ部での色にじみは発生しない。
米国特許第 6731806号
As a technique for reducing noise with only one image signal, there is a noise reduction technique using the region growing method of Patent Document 1. This is because the signal value of the target pixel starts from the target pixel and the neighboring pixel signal value is sequentially expanded to a region including pixels within a predetermined level difference to obtain the neighboring region. Is the signal value from which the noise of the pixel of interest has been removed. By repeating this operation for each pixel, image noise can be removed. In addition, since pixels having a large signal level difference are not included in the calculation of the average value, color bleeding at the image edge portion does not occur.
U.S. Pat. No. 6,731,806
フレームメモリーを使用した巡回型の雑音低減では、固定パターン雑音や単一画像の雑音は低減できないという問題点がある。また、領域成長法を用いた雑音低減手法では、各画素に対してそれぞれ信号値が近い近隣領域を決定し平均値を算出するので、膨大な計算量になるという問題点がある。
In cyclic noise reduction using a frame memory, there is a problem that fixed pattern noise and single image noise cannot be reduced. In addition, the noise reduction method using the region growing method has a problem in that it requires a huge amount of calculation because neighboring regions having close signal values for each pixel are determined and an average value is calculated.
上記課題を解決するために、本発明では入力されたカラー画像信号に含まれる明るさ信号を一次微分した値の絶対値をとり勾配信号を生成する。この生成された勾配信号値が隣接する画素の半数以上の勾配信号値より大きい画素を領域境界画素として抽出する。入力されたカラー画像信号に含まれる複数の色差信号をこの領域境界画素で分断された2次元平滑化処理を施してそれぞれ平滑化色差信号を生成する。入力画像の色差信号値と対応する生成された平滑化色差信号値をそれぞれ画素毎に比較し、近い場合には平滑化色差信号を主に、離れている場合には入力画像の色差信号を主に合成し出力色差信号とする。 In order to solve the above-described problem, in the present invention, a gradient signal is generated by taking an absolute value of a value obtained by first-order differentiation of a brightness signal included in an input color image signal. Pixels with the generated gradient signal value larger than the gradient signal value of half or more of adjacent pixels are extracted as region boundary pixels. A plurality of color difference signals included in the input color image signal are subjected to a two-dimensional smoothing process divided by the region boundary pixels to generate smoothed color difference signals. The chrominance signal value of the input image and the corresponding generated smoothed chrominance signal value are compared for each pixel, and the smoothed chrominance signal is mainly displayed when close, and the chrominance signal of the input image is mainly displayed when separated. To produce an output color difference signal.
色差信号に領域境界画素で分断された2次元平滑化処理を施して生成するため、平滑化色差信号は画像エッジ部で色にじみを生じない。このため、広範囲の色差信号を使って平滑化でき、揺らぎの少ない平滑化色差信号が生成できる。平滑化色差信号値と対応する入力画像の色差信号値を画素毎に比較し合成比率を変えて合成することで、色雑音による差異と認められる信号値が近い場合には主に平滑化色差信号を出力することで色雑音を抑圧する。入力画像の雑音が大きい時には、雑音の影響で領域境界画素が分断され、平滑化色差信号の一部に色にじみが生じたとしても、入力画像の色差信号値との差が大きく主に入力画像の色差信号を出力するので、色にじみは発生しない。 Since the color difference signal is generated by subjecting the color difference signal to two-dimensional smoothing divided at the region boundary pixels, the smoothed color difference signal does not cause color blur at the image edge portion. Therefore, smoothing can be performed using a wide range of color difference signals, and a smoothed color difference signal with little fluctuation can be generated. By comparing the color difference signal value of the input image corresponding to the smoothed color difference signal value for each pixel and changing the composition ratio, the smoothed color difference signal is mainly used when the signal value recognized as a difference due to color noise is close. Is used to suppress color noise. When the noise of the input image is large, the region boundary pixels are divided due to the noise, and even if color blurring occurs in a part of the smoothed color difference signal, the difference from the color difference signal value of the input image is mainly large. Since the color difference signal is output, color bleeding does not occur.
また、カラービデオカメラに使用する場合には、平滑化色差信号の生成に時間が必要なので、入力画像に含まれる色差信号と平滑化色差信号は別のフレームまたはフィールド画像なので、画像中で動きのある部分では色差信号値と平滑化色差信号値の差が大きく、主に入力画像の色差信号を出力するので表示色の時間遅れは発生せず、画像中の静止部分では色差信号値と平滑化色差信号値の差が小さく、画像内で平滑化された色差信号を主に出力するので固定パターンの色雑音も抑圧できる。 In addition, when used for a color video camera, since it takes time to generate a smoothed color difference signal, the color difference signal and the smoothed color difference signal included in the input image are separate frames or field images, so that the motion in the image is not changed. There is a large difference between the chrominance signal value and the smoothed chrominance signal value in a certain part, and the color difference signal of the input image is mainly output, so there is no time delay of the display color, and the chrominance signal value and smoothing in the still part of the image Since the difference between the color difference signal values is small and the color difference signal smoothed in the image is mainly output, the color noise of the fixed pattern can be suppressed.
平滑化色差信号は、明るさ信号を一次微分した値の絶対値の稜線で領域境界を求め、領域境界で分断された2次元の平滑化処理を施すだけで得られるため計算量は少ない。また、平滑化色差信号は侠帯域信号なので、入力カラー画像を間引き画素数の少ない画像にしてから平滑化色差信号を算出しても問題なく、さらに計算量を少なくできる。 Since the smoothed color difference signal is obtained simply by obtaining a region boundary with a ridge line of the absolute value of the value obtained by first-order differentiation of the brightness signal and performing a two-dimensional smoothing process divided at the region boundary, the amount of calculation is small. Further, since the smoothed color difference signal is a narrow band signal, there is no problem even if the smoothed color difference signal is calculated after converting the input color image to an image with a small number of thinned pixels, and the amount of calculation can be further reduced.
本発明では入力されたカラー画像信号に含まれる明るさ信号を一次微分した値の絶対値をとり勾配信号を生成する。この明るさ信号としては、入力カラー画像が輝度信号と色差信号で構成されているときは輝度信号を使い、入力カラー画像がR(赤)G(緑)B(緑)の3原色信号のときは重み付け加算して合成した輝度信号やG信号を使っても良い。また、平滑化色差信号は侠帯域信号なので、入力カラー画像を間引いた画素数の少ない明るさ信号の画像を作成しても良い。一次微分した値の絶対値としては、上下方向・左右方向に近接する画素の差信号の二乗和の平方根が等方性があり望ましいが、領域境界を求めることが目的なので上下・左右方向の差信号の絶対値の和でも問題ない。この生成された勾配信号値が隣接する画素の半数以上の勾配信号値より大きい画素を選択することで、連続した明るさ信号が急変する画素が選択され、画像中の領域の境界が決定できる。 In the present invention, a gradient signal is generated by taking an absolute value of a value obtained by first-order differentiation of a brightness signal included in an input color image signal. As the brightness signal, when the input color image is composed of a luminance signal and a color difference signal, the luminance signal is used, and when the input color image is an R (red), G (green), and B (green) primary color signal. May be a luminance signal or G signal synthesized by weighted addition. Further, since the smoothed color difference signal is a narrowband signal, an image of a brightness signal with a small number of pixels obtained by thinning out the input color image may be created. As the absolute value of the first-order differentiated value, the square root of the sum of squares of the difference signal of pixels adjacent in the vertical and horizontal directions is isotropic and desirable. There is no problem with the sum of the absolute values of the signals. By selecting a pixel whose generated gradient signal value is larger than the gradient signal value of half or more of adjacent pixels, a pixel whose continuous brightness signal changes suddenly is selected, and the boundary of the region in the image can be determined.
次に、入力されたカラー画像信号に含まれる複数の色差信号をこの領域境界画素で分断された2次元平滑化処理を施す。入力カラー画像が輝度信号と色差信号で構成されているときはそのまま色差信号を、入力カラー画像がRGBの3原色信号のときはB−Y・R−Y信号やB−G・R−G信号等の色差信号を生成し、画像メモリーに取り込む。2次元平滑化処理は、左から右・上から下・右から左・下から上という順番で画像メモリーの走査を複数回繰り返しながら、領域境界画素とその次の画素では信号値を保持し、その他の画素ではその画素の信号値と1画素前の信号値とを合成した値に更新する。これにより、平滑化処理は領域境界画素で分断され、領域境界画素で囲まれた領域の広範囲の画素の色差信号値から合成された値が得られ、色雑音が除去される。 Next, a two-dimensional smoothing process in which a plurality of color difference signals included in the input color image signal are divided by the region boundary pixels is performed. When the input color image is composed of luminance signals and color difference signals, the color difference signal is used as it is. When the input color image is the RGB primary color signal, the BY / R / Y and B / G / RG signals are used. The color difference signal such as is generated and is taken into the image memory. The two-dimensional smoothing process repeats the image memory scan several times in the order of left to right, top to bottom, right to left and bottom to top, while holding the signal value at the region boundary pixel and the next pixel, For other pixels, the signal value of the pixel and the signal value of the previous pixel are updated to a synthesized value. Thereby, the smoothing process is divided at the region boundary pixels, and a value synthesized from the color difference signal values of a wide range of pixels in the region surrounded by the region boundary pixels is obtained, and the color noise is removed.
最後に、入力画像の色差信号値と対応する生成された平滑化色差信号値をそれぞれ画素毎に比較し出力色差信号を合成する。これは、平滑化後の色差信号値から入力画像の色差信号値を減算し色差誤差信号を生成し、色差誤差信号の振幅が所定レベルより小さい場合にはそのまま、所定レベルより大きい場合には振幅を徐々に下げる非線形処理を施してから、入力画像の色差信号値に加算し合成色差信号出力を得る。これにより、色差誤差信号の振幅が小さい時には平滑化色差信号が出力され、色差誤差信号の振幅が大きくなるにつれ入力画像の色差信号に 近づく信号が出力される。従って、平滑化による色差信号の差が雑音変動とみなせるレベルの場合は色雑音が低減でき、雑音等の影響で領域境界画素の分断で平滑化色差信号ににじみがある場合でも入力画像の色差信号が出力されるため色にじみにはならず、またカラービデオ信号の動きのある部分で色差信号が大きく変化する場合には入力画像の色差信号が出力されるため色残像は生じない。結果として、色にじみも色残像もなく色雑音が低減された高画質なカラー画像信号を得ることができる。 Finally, the color difference signal value of the input image and the generated smoothed color difference signal value corresponding to each other are compared for each pixel to synthesize the output color difference signal. This is because the color difference signal value of the input image is subtracted from the smoothed color difference signal value to generate a color difference error signal. If the amplitude of the color difference error signal is smaller than the predetermined level, the amplitude is not changed. Is applied to the color difference signal value of the input image to obtain a combined color difference signal output. As a result, a smoothed color difference signal is output when the amplitude of the color difference error signal is small, and a signal that approaches the color difference signal of the input image is output as the amplitude of the color difference error signal increases. Therefore, color noise can be reduced when the difference in the color difference signal due to smoothing is at a level that can be regarded as noise fluctuation, and even if the smoothed color difference signal is blurred due to the division of the region boundary pixels due to noise or the like, the color difference signal of the input image Therefore, when the color difference signal changes greatly in a portion where the color video signal is moving, the color difference signal of the input image is output and no color afterimage is generated. As a result, it is possible to obtain a high-quality color image signal with no color blur and no color afterimage and reduced color noise.
以下この発明の実施例を図面を用いて説明する。図1は、本発明の第1の実施例に係る色雑音低減手法の機能ブロック図、図2は傾き検出の機能ブロック図、図3は稜線検出の機能ブロックである。入力カラー画像は輝度信号Yと2つの色差信号Cr,Cbで構成されている。輝度信号Yは傾き検出機能1に入力され、勾配信号DETを生成する。図2の傾き検出機能では、メモリーや遅延手段等を用い注目画素の近傍11の信号値を集め、減算機能12,13で上下に隣接する画素U,Dの差と左右に隣接する画素L,Rの差をそれぞれ出力し、絶対値機能14,15と加算機能16で合成し勾配信号DETが出力される。稜線検出機能2は、勾配信号から領域境界画素を次のように検出する。メモリーや遅延手段等を用い注目画素とその近傍画素21の勾配信号値を集め、比較機能22−1〜4で注目画素と上下左右に隣接する画素U,D,L,Rをそれぞれ比較し、雑音レベルを考慮し注目画素のほうが勾配信号値が明らかに高いと判断した場合に1を、それ以外の場合0を出力する。比較機能22−1〜4の出力は、加算機能23で加算され、判定機能24で半数(2)以上の場合に1を、それ以外の場合0を領域境界信号BRDRとして出力する。 Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a functional block diagram of a color noise reduction method according to the first embodiment of the present invention, FIG. 2 is a functional block diagram of inclination detection, and FIG. 3 is a functional block of edge detection. The input color image is composed of a luminance signal Y and two color difference signals Cr and Cb. The luminance signal Y is input to the inclination detection function 1 to generate a gradient signal DET. In the inclination detection function shown in FIG. 2, signal values in the vicinity 11 of the target pixel are collected using a memory, a delay unit, and the like, and the difference between the upper and lower adjacent pixels U and D and the left and right adjacent pixels L and The difference between R is output and combined by the absolute value functions 14 and 15 and the addition function 16 to output a gradient signal DET. The ridge line detection function 2 detects a region boundary pixel from the gradient signal as follows. Gather the gradient signal values of the pixel of interest and its neighboring pixels 21 using memory, delay means, etc., and compare the pixels of interest U, D, L, and R adjacent in the vertical and horizontal directions with the comparison functions 22-1-4, In consideration of the noise level, 1 is output when it is determined that the gradient signal value of the target pixel is clearly higher, and 0 is output otherwise. The outputs of the comparison functions 22-1 to 4 are added by the addition function 23, and 1 is output as a region boundary signal BRDR when the determination function 24 is more than half (2), and 0 is output otherwise.
このように生成された領域境界信号BRDRと色差信号Cr,Cbは、2次元平滑化機能3,4にそれぞれ入力される。図4は2次元平滑化機能の機能ブロック図である。入力された色差信号と領域境界信号は、走査アドレス発生機能27により制御される色差メモリー25と境界メモリー26にそれぞれ格納される。色差メモリー25と境界メモリー26として3画面分を持たせ、図5に斜線領域で示すように、書き込みアドレス・平滑化処理アドレス・読み出しアドレスを走査アドレス発生機能27で1画面ごとにアドレス領域を切り替えながら適切に制御することで、連続画像に対する2次元平滑化信号を連続的に得ることができる。遅延機能28は、領域境界信号に1画素分の遅延を与え、OR機能32で論理和がとられ、領域境界画素とその次の画素でスイッチ31をb側に切り替える。スイッチ31がb側に切り替えられると、色差メモリー25に蓄えられた画素信号がそのまま遅延機能29と色差メモリー25に入力される。したがって、領域境界画素とその次の画素には平滑化処理がかからない。それ以外の画素ではスイッチ31が0側になっているため、色差メモリー25と遅延機能29の信号値が加算機能30によって混合され、遅延機能29と色差メモリー25に入力される。このようにして、1つの走査方向に対して領域境界画素で分断された平滑化処理が実現できる。 The region boundary signal BRDR and the color difference signals Cr and Cb thus generated are input to the two-dimensional smoothing functions 3 and 4, respectively. FIG. 4 is a functional block diagram of the two-dimensional smoothing function. The input color difference signal and area boundary signal are stored in the color difference memory 25 and the boundary memory 26 controlled by the scanning address generation function 27, respectively. The color difference memory 25 and the boundary memory 26 have three screens, and the write address, smoothing process address, and read address are switched for each screen by the scanning address generation function 27 as shown by the hatched area in FIG. However, by appropriately controlling, a two-dimensional smoothed signal for a continuous image can be obtained continuously. The delay function 28 gives a delay of one pixel to the region boundary signal, the logical sum is taken by the OR function 32, and the switch 31 is switched to the b side at the region boundary pixel and the next pixel. When the switch 31 is switched to the b side, the pixel signal stored in the color difference memory 25 is input to the delay function 29 and the color difference memory 25 as it is. Therefore, the region boundary pixel and the next pixel are not subjected to the smoothing process. In other pixels, the switch 31 is on the 0 side, so that the signal values of the color difference memory 25 and the delay function 29 are mixed by the addition function 30 and input to the delay function 29 and the color difference memory 25. In this way, smoothing processing divided at the region boundary pixels in one scanning direction can be realized.
図6は、色差メモリー25と境界メモリー26の走査方向を図示している。図6(a),(b),(c),(d)は、左から右,上から下,左から右,下から上方向への走査をそれぞれ示している。点線は帰線を示している。周辺画素に領域境界信号をセットすることで、両端の画素の色差信号を混入を防止できる。図7は、2次元平滑化処理の実現手順を流れ図を用いて示している。図8は画像の平坦部でのこの平滑化処理により、ほぼ中央のハッチのかかった画素位置に信号値4096があった場合に信号がどう広がるかを示している。平滑化方向を変えながら繰り返し平滑化し、単純な処理で500画素以上の広範囲の画素値を使って平滑値を合成できる。各画素の平滑値への寄与率は1%以下であり、ランダムに発生する雑音は1/10以下に低減される。図9は、領域境界がある場合に平滑化処理により、ハッチのかかった画素位置に信号値4096があった場合に信号がどう広がるかを示している。黒塗りの部分は、領域境界画素を示している。各画素の平滑値への寄与率は、領域境界内では平坦領域の場合より増加し、領域境界外では領域境界画素の分断点から信号のしみ出しはあるものの急速に低下する。したがって、雑音等の影響により領域境界画素が分断されたとしても、平滑化色差信号への影響は軽微である。このようにして、色差メモリー25内に生成された平滑化色差信号は、走査アドレス発生機能27により制御され、入力カラー画像の色差信号とタイミングが合わされて出力される。 FIG. 6 illustrates the scanning directions of the color difference memory 25 and the boundary memory 26. FIGS. 6A, 6B, 6C, and 6D illustrate scanning from left to right, top to bottom, left to right, and bottom to top, respectively. A dotted line indicates a return line. By setting the region boundary signal to the peripheral pixels, it is possible to prevent the color difference signals of the pixels at both ends from being mixed. FIG. 7 shows a procedure for realizing the two-dimensional smoothing process using a flowchart. FIG. 8 shows how the signal spreads when the signal value 4096 is present at the pixel position at the center of the hatching by this smoothing process in the flat portion of the image. Smoothing is repeatedly performed while changing the smoothing direction, and a smooth value can be synthesized using a wide range of pixel values of 500 pixels or more by simple processing. The contribution ratio of each pixel to the smooth value is 1% or less, and randomly generated noise is reduced to 1/10 or less. FIG. 9 shows how the signal spreads when there is a signal value 4096 at the hatched pixel position by the smoothing process when there is a region boundary. A black portion indicates a region boundary pixel. The contribution ratio of each pixel to the smooth value increases more in the region boundary than in the flat region, and rapidly decreases outside the region boundary, although a signal oozes out from the breakpoint of the region boundary pixel. Therefore, even if the region boundary pixel is divided due to the influence of noise or the like, the influence on the smoothed color difference signal is slight. In this way, the smoothed color difference signal generated in the color difference memory 25 is controlled by the scanning address generation function 27 and is output in time with the color difference signal of the input color image.
入力画像の色差信号値と対応する生成された平滑化色差信号値は、減算機能5,6により減算され、色差誤差信号を生成する。非線形処理機能7,8により、色差誤差信号の振幅が所定レベルより小さい場合にはそのまま、所定レベルより大きい場合には振幅を徐々に下げる非線形処理が施され、加算機能9,10により入力画像の色差信号値に加算され合成色差信号出力を得る。これにより、色差誤差信号の振幅が小さい時には平滑化色差信号が出力され、色差誤差信号の振幅が大きくなるにつれ入力画像の色差信号に近付く信号が出力される。したがって、平滑化による色差信号の差が雑音変動とみなせるレベルの場合は色雑音が低減できる。雑音等の影響で領域境界画素の分断で平滑化色差信号ににじみがある場合でも入力画像の色差信号が出力されるため色にじみにはならず、またカラービデオ信号の動きのある部分で色差信号が大きく変化する場合には入力画像の色差信号が出力されるため色残像は生じない。結果として、色にじみも色残像もなく色雑音が低減された高画質なカラー画像信号を得ることができる。 The generated smoothed color difference signal value corresponding to the color difference signal value of the input image is subtracted by the subtraction functions 5 and 6 to generate a color difference error signal. The non-linear processing functions 7 and 8 perform non-linear processing for gradually decreasing the amplitude when the amplitude of the color difference error signal is smaller than a predetermined level, and when the amplitude is larger than the predetermined level, and the addition functions 9 and 10 It is added to the color difference signal value to obtain a combined color difference signal output. As a result, a smoothed color difference signal is output when the amplitude of the color difference error signal is small, and a signal that approaches the color difference signal of the input image is output as the amplitude of the color difference error signal increases. Therefore, color noise can be reduced when the difference between the color difference signals due to smoothing is at a level that can be regarded as noise fluctuation. Even if the smoothed chrominance signal is blurred due to the division of the area boundary pixels due to noise, etc., the chrominance signal of the input image is output, so it does not bleed, and the chrominance signal in the part where the color video signal moves When the value changes greatly, the color difference signal of the input image is output, so that no color afterimage occurs. As a result, it is possible to obtain a high-quality color image signal with no color blur and no color afterimage and reduced color noise.
図10は、本発明の第2の実施例に係る色雑音低減装置のブロック図であり、図1と同一機能の部分に対しては同一の符号を付している。入力信号は光の3原色信号のG B Rであり、間引き回路33により、入力画像信号を縦n画素×横m画素の大きさの複数のブロックに分けたときの平均値信号をそれぞれ出力する。人の目の解像度に大きな影響のあるG信号とR信号は、それぞれ傾き検出回路1に入力され、加算器35で加算され勾配信号を出力する。減算器34は、間引きされたB信号とR信号からそれぞれ間引きされたG信号を引き、色差信号B−GとR−Gを出力する。稜線検出回路22で検出した領域境界に基づいて、この色差信号B−G、R−Gは、それぞれ2次元平滑化回路3,4で平滑化色差信号に変換される。 FIG. 10 is a block diagram of a color noise reduction apparatus according to the second embodiment of the present invention, and parts having the same functions as those in FIG. The input signal is G BR of the three primary color signals of light, and the thinning circuit 33 outputs an average value signal when the input image signal is divided into a plurality of blocks each having a size of vertical n pixels × horizontal m pixels. The G signal and the R signal that have a great influence on the resolution of the human eye are respectively input to the inclination detection circuit 1 and added by the adder 35 to output a gradient signal. The subtracter 34 subtracts the G signal thinned out from the thinned B signal and R signal, and outputs color difference signals BG and RG. Based on the region boundary detected by the ridge line detection circuit 22, the color difference signals BG and RG are converted into smoothed color difference signals by the two-dimensional smoothing circuits 3 and 4, respectively.
図11は、本発明の第2の実施例に係る平滑化回路のブロック図、図12は本発明の第2の実施例に係る2次元平滑化処理の実現手順を示す流れ図である。図4と同一機能の部分に対しては、同一の符号を付している。図4との違いは、平滑化回路がカスケードになっている点である。平滑化回路37は、図4の平滑化機能と同じであり、主走査方向に沿って境界画素で分断された平滑化処理を実行する。一方、遅延素子39は主走査方向と直行する副走査方向に隣接する画素の信号を出力する遅延素子であり、平滑化回路38は副走査方向に沿って境界画素で分断された平滑化処理を実行する。例えば、図6(a)の走査方向でメモリーアクセスを行うことで、左から右への平滑化処理をした信号に、さらに上から下への平滑化処理をした信号がメモリーに生成される。また、図6(c)の走査方向でメモリーアクセスを行うことで、右から左への平滑化処理をした信号に、さらに下から上への平滑化処理をした信号がメモリーに生成される。したがって、図12の流れ図で、図7の流れ図と同じ平滑化処理が実現でき、メモリーアクセスの回数を半減できる。 FIG. 11 is a block diagram of a smoothing circuit according to the second embodiment of the present invention, and FIG. 12 is a flowchart showing a procedure for realizing a two-dimensional smoothing process according to the second embodiment of the present invention. Parts having the same functions as those in FIG. 4 are denoted by the same reference numerals. The difference from FIG. 4 is that the smoothing circuits are cascaded. The smoothing circuit 37 is the same as the smoothing function of FIG. 4 and executes a smoothing process that is divided at boundary pixels along the main scanning direction. On the other hand, the delay element 39 is a delay element that outputs a signal of a pixel adjacent in the sub-scanning direction orthogonal to the main scanning direction, and the smoothing circuit 38 performs a smoothing process divided by boundary pixels along the sub-scanning direction. Execute. For example, by performing memory access in the scanning direction of FIG. 6A, a signal that has been smoothed from left to right and a signal that has been smoothed from top to bottom are generated in the memory. Further, by performing memory access in the scanning direction of FIG. 6C, a signal that has been smoothed from right to left and a signal that has been smoothed from bottom to top are generated in the memory. Therefore, in the flowchart of FIG. 12, the same smoothing process as that of the flowchart of FIG. 7 can be realized, and the number of memory accesses can be halved.
このように生成された平滑化色差信号は減算器5,6に入力され、平滑化色差信号から入力画像信号に対する色差信号B−G、R−Gをそれぞれ減算し、色差誤差信号を生成する。一般にカラー画像信号中の雑音レベルは暗部で大きく所定レベル以上ではほぼ一定になるため、雑音レベル推定器36は、入力画像のG信号レベルから雑音レベルを推定し、非線形処理回路7,8の非線形特性を制御する。非線形処理回路7の出力は、加算器9でB信号へ1/2を加算し、減算器37でG信号から1/2を減算することで、B信号とG信号への影響の少ない色差信号B−G信号成分の雑音抑圧ができる。非線形処理回路8の出力も、加算器10と減算器38に入力され、色差信号R−G成分の雑音が低減されたR信号とG信号が出力できる。 The smoothed color difference signals generated in this way are input to the subtracters 5 and 6, and the color difference signals BG and RG for the input image signal are subtracted from the smoothed color difference signals to generate a color difference error signal. In general, since the noise level in the color image signal is large in a dark portion and becomes substantially constant above a predetermined level, the noise level estimator 36 estimates the noise level from the G signal level of the input image, and the nonlinear processing circuits 7 and 8 are nonlinear. Control properties. The output of the non-linear processing circuit 7 is a chrominance signal with little influence on the B signal and the G signal by adding 1/2 to the B signal by the adder 9 and subtracting 1/2 from the G signal by the subtractor 37. Noise suppression of B-G signal components can be performed. The output of the non-linear processing circuit 8 is also input to the adder 10 and the subtractor 38, so that an R signal and a G signal in which noise of the color difference signal RG component is reduced can be output.
間引き回路33を用いることにより、縦横の画素数がそれぞれ1/n倍・1/m倍になり、勾配信号DETや領域境界信号BRDRを生成するのに必要な遅延素子やメモリーのサイズが1/m倍に、色差メモリーや境界メモリーのサイズが1/(n×m)倍に小さくなる。また、2次元平滑化処理の計算量も1/(n×m)倍に小さくなり、結果として垂直ブランキング期間で平滑化処理が納まれば、図13のように1画面分のメモリーで2次元平滑化処理ができる、つまり、間引きブロックの開始時点で平滑化された色差信号値を読み出して色差信号合成に使い、間引きブロックの終了時点で色差信号のブロック内平均値を書き込み、ブランキング期間で2次元平滑化処理を実行し平滑化された色差信号値を書き込むことで、メモリー容量が小さくかつ時間遅れの少ない色雑音低減が実現できる。 By using the thinning circuit 33, the number of vertical and horizontal pixels becomes 1 / n times and 1 / m times, respectively, and the size of the delay element and the memory required to generate the gradient signal DET and the region boundary signal BRDR is reduced to 1 /. The size of color difference memory and boundary memory is reduced to 1 / (n × m) times. Further, the calculation amount of the two-dimensional smoothing process is also reduced to 1 / (n × m) times. As a result, if the smoothing process is accommodated in the vertical blanking period, the memory for one screen as shown in FIG. Dimensional smoothing is possible, that is, the color difference signal value smoothed at the start of the thinning block is read out and used for color difference signal synthesis, the average value in the block of the color difference signal is written at the end of the thinning block, and the blanking period By executing the two-dimensional smoothing process and writing the smoothed color difference signal value, it is possible to reduce color noise with a small memory capacity and a small time delay.
図14に、本発明の第3の実施例に係る静止画用カラー撮像装置のブロック図を示す。制御部40は、駆動回路41を介して固体撮像素子42を駆動する。固体撮像素子42には、例えば図15に示す原色形ベイヤー配列の色フィルタが付けられており、画素位置でエンコードされた3原色信号R,G,Bが出力される。A/D変換器43は、固体撮像素子42の出力信号をディジタル値に変換してプロセッサ44に出力する。A/D変換利得を制御部40から制御することで撮像装置の感度を設定できる。プロセッサ44は、隣接画素情報が取り出し易いようA/D変換された信号を固体撮像素子42の画素配置と同一の順序に、制御部40からの情報に基づいてメモリー45に再配置して格納する。プロセッサ44は、ガンマ補正・信号補間・輪郭補正等の処理を施してカラー画像信号を生成し出力する。 FIG. 14 shows a block diagram of a still image color imaging apparatus according to the third embodiment of the present invention. The control unit 40 drives the solid-state image sensor 42 via the drive circuit 41. The solid-state image sensor 42 is provided with, for example, a primary color Bayer array color filter shown in FIG. 15, and three primary color signals R, G, and B encoded at pixel positions are output. The A / D converter 43 converts the output signal of the solid-state image sensor 42 into a digital value and outputs it to the processor 44. The sensitivity of the imaging device can be set by controlling the A / D conversion gain from the control unit 40. The processor 44 rearranges and stores the A / D converted signals in the same order as the pixel arrangement of the solid-state image sensor 42 in the memory 45 based on information from the control unit 40 so that adjacent pixel information can be easily extracted. . The processor 44 performs processing such as gamma correction, signal interpolation, and contour correction to generate and output a color image signal.
プロセッサ44が出力するカラー画像信号の信号レベルと雑音レベルの関係を、図14を用いて説明する。図16(a)は、固体撮像素子42の信号量とそれに含まれる雑音量の関係を示している。背景雑音は信号量が変化しても一定の雑音成分であり、光ショット雑音は検出される信号電荷数がポアソン分布に従うため信号量の平方根に比例した雑音量となる雑音成分であり、シーン雑音は各画素の感度ばらつきなどにより発生し信号量に比例した雑音量となる雑音成分である。近年の固体撮像素子では、電荷検出感度が向上し、かつ画素が高精度に作り込まれているため、シーン雑音は無視できるレベルになっている。 The relationship between the signal level of the color image signal output from the processor 44 and the noise level will be described with reference to FIG. FIG. 16A shows the relationship between the signal amount of the solid-state image sensor 42 and the amount of noise included therein. The background noise is a constant noise component even if the signal amount changes, and the optical shot noise is a noise component whose noise amount is proportional to the square root of the signal amount because the number of detected signal charges follows the Poisson distribution, and scene noise. Is a noise component which is generated due to sensitivity variation of each pixel and becomes a noise amount proportional to the signal amount. In recent solid-state imaging devices, since the charge detection sensitivity is improved and the pixels are built with high accuracy, the scene noise is at a negligible level.
背景雑音の主要構成要素は、暗電流(光電変換素子や電荷転送部でのリーク電流で遮光時でも流れる電流)のショット雑音、暗電流のばらつきによる固定パターン雑音、そして電荷電圧変換アンプやAD変換器などのアンプ雑音である。図16(b)は、暗時信号量と背景雑音量の関係を示し、長時間露光で暗時信号量が増加すると背景雑音量が露光時間にほぼ比例して増加する。図16(c)は、プロセッサ44の入力信号に対するガンマ補正特性とその微分利得を示している。入力信号に重畳される雑音成分は、この微分利得倍になり出力される。 The main components of the background noise are shot noise of dark current (current that flows even during light shielding due to leakage current in photoelectric conversion elements and charge transfer units), fixed pattern noise due to variations in dark current, and charge-voltage conversion amplifier and AD conversion It is amplifier noise of the instrument. FIG. 16B shows the relationship between the dark signal amount and the background noise amount. When the dark signal amount increases with long exposure, the background noise amount increases in proportion to the exposure time. FIG. 16C shows the gamma correction characteristic for the input signal of the processor 44 and its differential gain. The noise component superimposed on the input signal is multiplied by this differential gain and output.
図16(d)は、撮像条件によりプロセッサ44の出力信号レベルとそれに重畳される雑音レベルの関係がどう変わるかを示している。長時間露光にすると、暗時信号量が増加しその結果として背景雑音が増える。一方、光ショット雑音は入力信号が同一なので、信号レベルが小さいときに出力雑音レベルが増加するものの、信号レベルが大きくなるにつれ標準撮像状態と同じ雑音レベルとなる。感度UP時には、制御部40がA/D変換利得を大きく設定するため、背景雑音と光ショット雑音ともに増加し、信号レベル全域にわたって雑音レベルが増加する。 FIG. 16D shows how the relationship between the output signal level of the processor 44 and the noise level superimposed thereon changes depending on the imaging conditions. Long exposure increases the amount of dark signal and consequently increases background noise. On the other hand, since the optical shot noise has the same input signal, the output noise level increases when the signal level is small, but becomes the same noise level as the standard imaging state as the signal level increases. When the sensitivity is increased, since the control unit 40 sets the A / D conversion gain to be large, both the background noise and the light shot noise increase, and the noise level increases over the entire signal level.
色雑音低減機能46には、メモリー45に記憶されたガンマ補正の施されたプロセッサ44の出力信号が繰り返し入力される。制御部40から撮像条件に合わせた雑音レベル推定機能36の特性が与えられる。色雑音低減機能46は、前述の手順で2次元平滑化した色差信号を生成したのち、プロセッサ44の出力信号と平滑化色差信号の差信号である色差誤差信号が生成される。雑音レベル推定機能36で撮像条件に整合した雑音レベルが算定される。色差誤差信号の振幅が雑音レベル以内の場合には、非線形処理機能7,8は色差誤差信号を通過させ、プロセッサ44の出力信号と合成して色差信号の雑音成分が除去される。また、有為な色差信号と判断される色差誤差信号の振幅が雑音レベルより大きい場合には、非線形処理機能7,8は色差誤差信号の振幅を下げ、主に入力された色差信号を合成した色差信号が出力される。結果として、色差信号雑音を最適に低減した画像信号が得られる。 The output signal of the processor 44 subjected to gamma correction stored in the memory 45 is repeatedly input to the color noise reduction function 46. The control unit 40 gives the characteristics of the noise level estimation function 36 according to the imaging conditions. The color noise reduction function 46 generates a color difference error signal that is a difference signal between the output signal of the processor 44 and the smoothed color difference signal after generating a color difference signal that has been two-dimensionally smoothed by the above-described procedure. The noise level estimation function 36 calculates a noise level that matches the imaging conditions. When the amplitude of the color difference error signal is within the noise level, the non-linear processing functions 7 and 8 pass the color difference error signal and combine it with the output signal of the processor 44 to remove the noise component of the color difference signal. Further, when the amplitude of the color difference error signal determined to be a significant color difference signal is larger than the noise level, the nonlinear processing functions 7 and 8 reduce the amplitude of the color difference error signal and mainly synthesize the input color difference signal. A color difference signal is output. As a result, an image signal in which color difference signal noise is optimally reduced can be obtained.
以上説明したように、本発明の色雑音低減手法によれば、画像エッジ部での色にじみなく、1枚の入力画像信号の色雑音や、固体撮像素子の暗電流むらのような固定パターンの色雑音でも低減することができる。また、本発明の色雑音低減手法の実装形態はハードウエア・ソフトウエアのいずれでも良く、色信号の平滑化処理の係数は例示した値以外のものであっても良い。 As described above, according to the color noise reduction method of the present invention, the fixed pattern such as the color noise of one input image signal or the dark current unevenness of the solid-state image sensor without blurring the color at the image edge portion. Even color noise can be reduced. The color noise reduction method according to the present invention may be implemented by either hardware or software, and the color signal smoothing coefficient may be other than the exemplified values.
本発明の色雑音低減手法に係る機能ブロック図。The functional block diagram which concerns on the color noise reduction method of this invention. 傾き検出の機能ブロック図。The functional block diagram of inclination detection. 稜線検出の機能ブロック図。The functional block diagram of a ridgeline detection. 平滑化機能の機能ブロック図。The functional block diagram of a smoothing function. 2次元平滑化処理のメモリーアクセスを説明するための図。The figure for demonstrating the memory access of a two-dimensional smoothing process. 走査方向を説明するための図。The figure for demonstrating the scanning direction. 2次元平滑化処理の実現手順を示す流れ図。The flowchart which shows the implementation | achievement procedure of a two-dimensional smoothing process. 画像の平坦部での平滑化処理の平滑値への寄与率を示す図。The figure which shows the contribution rate to the smooth value of the smoothing process in the flat part of an image. 領域境界がある場合の平滑化処理の平滑値への寄与率を示す図。The figure which shows the contribution rate to the smooth value of the smoothing process when there exists an area | region boundary. 本発明の第2の実施例に係るブロック図。The block diagram which concerns on the 2nd Example of this invention. 本発明の第2の実施例に係る平滑化回路のブロック図。The block diagram of the smoothing circuit which concerns on the 2nd Example of this invention. 本発明の第2の実施例に係る2次元平滑化処理の実現手順を示す流れ図。The flowchart which shows the implementation | achievement procedure of the two-dimensional smoothing process which concerns on 2nd Example of this invention. 本発明の第2の実施例に係る2次元平滑化処理のメモリーアクセスを説明するための図。The figure for demonstrating the memory access of the two-dimensional smoothing process which concerns on 2nd Example of this invention. 本発明の第3の実施例に係るカラー撮像装置のブロック図。FIG. 6 is a block diagram of a color imaging apparatus according to a third embodiment of the present invention. 本発明に関わる単板カラーカメラに使用される色フィルタの配列の一例を示す図The figure which shows an example of the arrangement | sequence of the color filter used for the single-panel color camera concerning this invention 信号レベルと雑音レベルの関係を説明するための図。The figure for demonstrating the relationship between a signal level and a noise level.
符号の説明Explanation of symbols
1 傾き検出機能
2 稜線検出機能
3,4 2次元平滑化機能
5,6,12,13,34,37,38 減算機能
7,8 非線形処理機能
9,10,16,23,30,35 加算機能
11 注目画素の近傍
14,15 絶対値機能
21 注目画素とその近傍画素
22 比較機能
24 判定機能
25,26,45 メモリー
27 走査アドレス発生機能
28,29 遅延機能
31 スイッチ
32 OR機能
33 間引き機能
36 雑音レベル推定機能
37 水平平滑化回路
38 垂直平滑化回路
39 遅延素子
40 制御部
41 駆動回路
42 固体撮像素子
43 A/D変換器
44 プロセッサ
46 色雑音低減機能
DESCRIPTION OF SYMBOLS 1 Inclination detection function 2 Ridge line detection function 3, 4 Two-dimensional smoothing function 5, 6, 12, 13, 34, 37, 38 Subtraction function 7, 8 Non-linear processing function 9, 10, 16, 23, 30, 35 Addition function Reference pixel 11 Reference pixel neighborhood 14, 15 Absolute value function 21 Target pixel and its neighboring pixels 22 Comparison function 24 Judgment function 25, 26, 45 Memory 27 Scan address generation function 28, 29 Delay function 31 Switch 32 OR function 33 Decimation function 36 Noise Level estimation function 37 Horizontal smoothing circuit 38 Vertical smoothing circuit 39 Delay element 40 Control unit 41 Drive circuit 42 Solid-state image sensor 43 A / D converter 44 Processor 46 Color noise reduction function

Claims (2)

  1. 入力されたカラー画像信号に含まれる明るさ信号を一次微分した値の絶対値をとり勾配信号を生成し、前記生成された勾配信号値が隣接する画素の半数以上の勾配信号値より大きい画素を領域境界画素として抽出し、色差信号画像と領域境界画像を記憶する手段を有し、前記記憶された色差信号画像と領域境界画像を走査し、領域境界画素とその次の画素位置では色差信号値を保持し、その他の画素では直前の色差信号値と注目画素の色差信号値の加重平均を色差信号値として更新する平滑化処理を、走査方向が左方向・下方向・右方向・上方向の平滑化処理を複数回実行させることで、2次元の色差平滑化信号を生成し、前記平滑化色差信号と前記入力画像の色差信号の差信号に応じて、前記平滑化色差信号から前記入力画像の色差信号を引いた色差誤差信号を生成し、前記色差誤差信号の振幅が小さい時にはそのまま、色差誤差信号の振幅が所定のレベル以上では振幅を小さくする非線形処理を施した信号を前記入力画像の色差信号に加算して色差信号を合成することを特徴とする色雑音低減手法。The gradient signal is generated by taking the absolute value of the value obtained by first-order differentiation of the brightness signal included in the input color image signal, and the generated gradient signal value is larger than the gradient signal value of more than half of the adjacent pixels. Means for extracting as a region boundary pixel and storing the color difference signal image and the region boundary image; scanning the stored color difference signal image and the region boundary image; and the color difference signal value at the region boundary pixel and the next pixel position In other pixels, smoothing processing is performed in which the weighted average of the previous color difference signal value and the color difference signal value of the target pixel is updated as the color difference signal value, and the scanning direction is left, down, right, or up. By executing the smoothing process a plurality of times, a two-dimensional color difference smoothed signal is generated, and the input image is obtained from the smoothed color difference signal according to a difference signal between the smoothed color difference signal and the color difference signal of the input image. Color difference signal A color difference error signal is generated, and when the amplitude of the color difference error signal is small, a signal subjected to non-linear processing for reducing the amplitude when the amplitude of the color difference error signal exceeds a predetermined level is added to the color difference signal of the input image. A color noise reduction method characterized by combining color difference signals.
  2. 光学系を介した光学像を光電変換して撮像信号を出力する撮像素子と、前記撮像素子の出力信号からカラー画像信号を生成する信号処理手段と、請求項1の色雑音低減手法で色雑音を低減する手段とを少なくとも有したカラー撮像装置において、前記色差誤差信号の非線形特性の所定レベルを撮像条件に応じて変えることを特徴とするカラー撮像装置。An image pickup device that photoelectrically converts an optical image via an optical system and outputs an image pickup signal, a signal processing unit that generates a color image signal from the output signal of the image pickup device, and a color noise reduction method according to claim 1 A color imaging apparatus having at least means for reducing the color difference, wherein the predetermined level of the non-linear characteristic of the color difference error signal is changed according to an imaging condition.
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