實現效果：

主要用到的方法是 grabCut方法。

目前網上沒有找到關於OpenCVForUnity的文件，磕磕絆絆的看著網上較多的opencv c++程式碼參考實現的，成功實現背景分離的時候感覺還是很美好的，記錄下。

首先是關於這方法的說明：

GrabCut函式說明

void grabCut(InputArray img, InputOutputArray mask, Rect rect, InputOutputArraybgdModel, InputOutputArrayfgdModel, intiterCount, intmode=GC_EVAL)

具體程式碼：

using UnityEngine;
using OpenCVForUnity;

public class TextureCutOut : MonoBehaviour {
    private Mat image ;
    private Mat mask, binMask, dstMat;
    private OpenCVForUnity.Rect rect;
    private Mat bgModel, fgModel;
    byte[] maskPixels;
    // Use this for initialization
    void Start () {
        Texture2D inputTexture = Resources.Load("inputTexture") as Texture2D;
        image = new Mat(inputTexture.height, inputTexture.width, CvType.CV_8UC3);
        Utils.texture2DToMat(inputTexture, image);
        mask = new Mat(inputTexture.height, inputTexture.width, CvType.CV_8UC1);
        mask.setTo(new Scalar(Imgproc.GC_BGD));

        rect = new OpenCVForUnity.Rect(45,25, 175, 210);

        //掩碼空間mask對應矩形空間內的掩碼設定為GC_PR_FGD（預設為前景）
        Mat matRectCol = mask.colRange(rect.x, rect.x + rect.width);
        Mat matRectRow = mask.rowRange(rect.y, rect.y + rect.height);
        matRectCol.setTo(new Scalar(Imgproc.GC_PR_FGD));
        matRectRow.setTo(new Scalar(Imgproc.GC_PR_FGD));

        //框出預設前景
        Imgproc.rectangle(image,new Point(rect.x, rect.y),new Point(rect.x + rect.width, rect.y + rect.height), new Scalar(0, 0, 255, 255), 2);
        //Texture2D outputTexture = new Texture2D(image.cols(), image.rows(), TextureFormat.RGBA32, false);
        //Utils.matToTexture2D(image, outputTexture);

        //重新讀取原始圖片 切割
        Utils.texture2DToMat(inputTexture, image);
        bgModel = new Mat();
        fgModel = new Mat();
        Imgproc.grabCut(image, mask, rect, bgModel, fgModel, 1);
        GetBinMask(ref mask, ref binMask);
        dstMat = new Mat(mask.size(), CvType.CV_8UC1);
        image.copyTo(dstMat, binMask);

        Texture2D outputTexture = new Texture2D(dstMat.cols(), dstMat.rows(), TextureFormat.RGBA32, false);
        Utils.matToTexture2D(dstMat, outputTexture);

        gameObject.GetComponent<Renderer>().material.mainTexture = outputTexture;
    }
	
	// Update is called once per frame
	void Update () {
	}

    void GetBinMask(ref Mat comMask,ref Mat binMask)
    {
        binMask = new Mat(comMask.size(), CvType.CV_8UC1);
        //commask中有0，1，2，3四種取值，這句就是取值為0，2的在binmask都為0，取值1，3的在binmask中為1
        //binMask = comMask & 1;
        maskPixels = new byte[comMask.cols() * comMask.rows()*comMask.channels()];
        comMask.get(0,0,maskPixels);
        for (int i = 0; i < maskPixels.Length; i++)
        {
            if (maskPixels[i] == 0 || maskPixels[i] == 2)
            {
                maskPixels[i] = 0;
            }
            if (maskPixels[i] == 1 || maskPixels[i] == 3)
            {
                maskPixels[i] = 1;
            }
        }
        binMask.put(0,0, maskPixels);
    }
}

20170912更新：

原先的切割後背景是黑色，根據專案需求要變成透明的，試驗了多次，OpenCVForUnity自帶的copyTo()方法沒有實現，就自己寫了一個帶遮罩的圖片疊加方法，效果如下：

程式碼：

using UnityEngine;
using OpenCVForUnity;

public class TextureCutOut : MonoBehaviour {
    private Mat image ;
    private Mat mask, binMask, dstMat,tranparentMat;
    private OpenCVForUnity.Rect rect;
    private Mat bgModel, fgModel;
    byte[] maskPixels;
    byte[] srcPixels;
    byte[] dstPixels;
    // Use this for initialization
    void Start () {
        Texture2D inputTexture = Resources.Load("1") as Texture2D;
        image = new Mat(inputTexture.height, inputTexture.width, CvType.CV_8UC3);
        Utils.texture2DToMat(inputTexture, image);
        mask = new Mat(inputTexture.height, inputTexture.width, CvType.CV_8UC1);
        mask.setTo(new Scalar(Imgproc.GC_BGD));

        rect = new OpenCVForUnity.Rect(30,50, 450, 900);

        //掩碼空間mask對應矩形空間內的掩碼設定為GC_PR_FGD（預設為前景）
        Mat matRectCol = mask.colRange(rect.x, rect.x + rect.width);
        Mat matRectRow = mask.rowRange( rect.y, rect.y + rect.height);
        matRectCol.setTo(new Scalar(Imgproc.GC_PR_FGD));
        matRectRow.setTo(new Scalar(Imgproc.GC_PR_FGD));

        //框出預設前景
        Imgproc.rectangle(image,new Point(rect.x, rect.y),new Point(rect.x + rect.width, rect.y + rect.height), new Scalar(0, 0, 255, 255), 2);
        //Texture2D outputTexture = new Texture2D(image.cols(), image.rows(), TextureFormat.RGBA32, false);
        //Utils.matToTexture2D(image, outputTexture);

        //重新讀取原始圖片 切割
        Utils.texture2DToMat(inputTexture, image);
        bgModel = new Mat();
        fgModel = new Mat();
        Imgproc.grabCut(image, mask, rect, bgModel, fgModel, 1);
        GetBinMask(ref mask, ref binMask);
        

        dstMat = new Mat(mask.size(), CvType.CV_8UC4);
        //自己寫的圖片疊加函式，實現背景部分透明
        MyCopt(ref image, ref dstMat, ref binMask);

        //自帶的帶掩碼複製方法，背景部分是黑色
        //image.copyTo(dstMat, binMask);
        //SetBlackPixelTranspalent(ref dstMat);

        Texture2D outputTexture = new Texture2D(dstMat.cols(), dstMat.rows(), TextureFormat.RGBA32, false);
        Utils.matToTexture2D(dstMat, outputTexture);

        gameObject.GetComponent<Renderer>().material.mainTexture = outputTexture;
    }
	
	// Update is called once per frame
	void Update () {
	}

    void GetBinMask(ref Mat comMask,ref Mat binMask)
    {
        binMask = new Mat(comMask.size(), CvType.CV_8UC1);
        //commask中有0，1，2，3四種取值，這句就是取值為0，2的在binmask都為0，取值1，3的在binmask中為1;
        //值為 0為背景，1為前景
        //binMask = comMask & 1;
        maskPixels = new byte[comMask.cols() * comMask.rows()*comMask.channels()];
        comMask.get(0,0,maskPixels);
        for (int i = 0; i < maskPixels.Length; i++)
        {
            if (maskPixels[i] == 0 || maskPixels[i] == 2)
            {
                maskPixels[i] = 0;
            }
            if (maskPixels[i] == 1 || maskPixels[i] == 3)
            {
                maskPixels[i] = 1;
            }
        }
        binMask.put(0,0, maskPixels);
    }

    //把圖片中黑色部分變透明
    void SetBlackPixelTranspalent(ref Mat mask)
    {
        Imgproc.cvtColor(mask, mask, Imgproc.COLOR_BGR2BGRA);
        srcPixels = new byte[mask.cols() * mask.rows() * mask.channels()];
        mask.get(0, 0, srcPixels);
        byte b;
        byte g;
        byte r;
        for (int i = 0; i < srcPixels.Length; i += 4)
        {
            b = srcPixels[i];
            g = srcPixels[i + 1];
            r = srcPixels[i + 2];
            if (b == 0 && g == 0 && r == 0)
            {
                srcPixels[i + 3] = 0;
            }
            else
                srcPixels[i + 3] = 255;
        }
        mask.put(0,0, srcPixels);
    }

    void MyCopt(ref Mat srcMat,ref Mat dstMat,ref Mat mask)
    {
        maskPixels = new byte[mask.cols() * mask.rows() * mask.channels()];
        srcPixels = new byte[srcMat.cols() * srcMat.rows() * srcMat.channels()];
        dstPixels = new byte[dstMat.cols() * dstMat.rows() * dstMat.channels()];
        int n1 = mask.channels();
        int n2 = srcMat.channels();
        int n3 = dstMat.channels();
        
        mask.get(0,0,maskPixels);
        srcMat.get(0,0,srcPixels);
        dstMat.get(0,0,dstPixels);
        for (int i = 0; i < maskPixels.Length; i++)
        {
            if (maskPixels[i] == 1)
            {
                dstPixels[i * 4 + 0] = srcPixels[i * 3 + 0];
                dstPixels[i * 4 + 1] = srcPixels[i * 3 + 1];
                dstPixels[i * 4 + 2] = srcPixels[i * 3 + 2];
                dstPixels[i * 4 + 3] = 255;
            }
        }
        dstMat.put(0,0,dstPixels);
    }
}