图像二值化—改进版Wellner算法

        if (imageistruecolor($res)){
            imagetruecolortopalette($res, false, 256);//如果是真彩色图象，将真彩色图像转换为调色板图像
        }
        
        $imgHeight = imagesy($res);
        $imgWidth = imagesx($res);
        $grayTable = array();//用于存储图像每个像素点的灰度值。

        for($y=0; $y<$imgHeight; $y++){
            for($x=0; $x<$imgWidth; $x++){
                //获得当前像素点颜色索引值
                $colorIndex = imagecolorat($res, $x, $y);
                //通过颜色索引值获得颜色的RGB数组表示形式
                $rgb = imagecolorsforindex($res, $colorIndex);
                //计算该点的灰度值
                $gray = round(0.299 * $rgb['red'] + 0.587 * $rgb['green'] + 0.114 * $rgb['blue']);
                $grayTable[$x][$y] = $gray;  
            }
        } 
        $sumColorTable = array(); //颜色求和表
        for($y=0; $y<$imgHeight; ++$y){
            for($x=0; $x<$imgWidth; ++$x){
                if($x==0 && $y==0){
                    $sumColorTable[$x][$y] = $grayTable[$x][$y];
                }else if($y==0 && $x>0){ //第一行的像素灰度和
                    $sumColorTable[$x][$y] = $sumColorTable[$x-1][$y] + $grayTable[$x][$y];
                }else if($x==0 && $y>0){ //第一列的像素灰度和
                    $sumColorTable[$x][$y] = $sumColorTable[$x][$y-1] + $grayTable[$x][$y];
                }else{
                    $sumColorTable[$x][$y] = $sumColorTable[$x-1][$y] + $sumColorTable[$x][$y-1] - $sumColorTable[$x-1][$y-1] + $grayTable[$x][$y];
                }
            }
        }
        
        $_S = intval($imgWidth / 8);
        $_T = 15;
        $ss = intval($_S/2);
        $imgData = array();
        
        for($y=0; $y<$imgHeight; ++$y){
            for($x=0; $x<$imgWidth; ++$x){
                $x1 = $x-$ss;
                $y1 = $y-$ss;
                $x2 = $x+$ss;
                $y2 = $y+$ss;
                if($x1<0) $x1=0;
                if($y1<0) $y1=0;
                if($x2>($imgWidth-1)) $x2 = $imgWidth-1;
                if($y2>($imgHeight-1)) $y2 = $imgHeight-1;
                
                if($x1==0 && $y1==0){
                    $crossArea = 0;
                    $upArea = 0;
                    $leftArea = 0;
                }else if($y1==0 && $x1>0){
                    $crossArea = 0;
                    $upArea = 0;
                    $leftArea = $sumColorTable[$x1-1][$y1];
                }else if($x1==0 && $y1>0){
                    $crossArea = 0;
                    $upArea = $sumColorTable[$x1][$y1-1];
                    $leftArea = 0;  
                }else {
                    $crossArea = $sumColorTable[$x1-1][$y1-1];
                    $upArea = $sumColorTable[$x2][$y1-1];
                    $leftArea = $sumColorTable[$x1-1][$y2];
                }
                
                $D = $sumColorTable[$x2][$y2];
                $average = ($D-$upArea-$leftArea+$crossArea)/(($y2-$y1+1)*($x2-$x1+1));
                $grayColor = $grayTable[$x][$y];
                $colorIndex = imagecolorat($res, $x, $y);
                if($grayColor<($average*(100-$_T)/100)){
                    imagecolorset($res, $colorIndex, 0, 0, 0);
                    $imgData[$y][$x] = 1;
                }else{
                    $imgData[$y][$x] = 0;
                    imagecolorset($res, $colorIndex, 255, 255, 255);  
                }
            }
        }
        imagejpeg($res,time().'_demo.jpeg');//保存图片