img = img1*mask + img2*(1-mask) How do that ? - jsxyhelu

img = img1mask + img2(1-mask) How do that ?

原文地址： http://answers.opencv.org/question/160599/img-img1mask-img21-mask-how-do-that/

如何提高一个简单操作的速度？最后的 multiply方法很有启示。

Hello,

I would like merge two color images with an image mask.

img1 and img2 are color image with 3 channelsmask is grey image with 1 channel

for merge the two image with the mask I do a loop for each pixel.

float c1,c2;
for( int j = 0; j < img1.rows ; j ++ ){
    for( int i = 0; i < img1.cols ; i ++ ){
        c1 = (greyGoodScale.at <uchar >(j, i)) / 255. 0;
        c2 = 1 -c1;
        img.at <Vec3b >(j, i)[ 0] = c2 *img1.at <Vec3b >(j, i)[ 0] + c1 *img2.at <Vec3b >(j, i)[ 0];
        img.at <Vec3b >(j, i)[ 1] = c2 *img1.at <Vec3b >(j, i)[ 1] + c1 *img2.at <Vec3b >(j, i)[ 1];
        img.at <Vec3b >(j, i)[ 2] = c2 *img1.at <Vec3b >(j, i)[ 2] + c1 *img2.at <Vec3b >(j, i)[ 2];
    }
}

OK, it's work but my image is 720x500 and i have 70ms of processing time is TOO LONG, I need to be real time.I can't do process on GPU.

Is a way to reduce processing time ?

thank.christopheopenCV 3.x

有用的回答：

const uchar *scale = &greyGoodScale.at <uchar >(j, 0);
uchar *imgdata = &img.at <uchar >(j, 0);
const uchar *imgdata1 = &img1.at <uchar >(j, 0);
const uchar *imgdata2 = &img2.at <uchar >(j, 0);
for( int i = 0; i < img1.cols ; i ++ ){
    c1 = scale[i] / 255. 0;
    c2 = 1 -c1;
    int pos = 3 *i;
    for ( int k = 0; k < 3; k ++){
        imgdata[pos + k] = c2 *imgdata1[pos + k] + c1 *imgdata2[pos + k];
    }
}

is one a the best solution with 16ms of processing time.

Now I would like divide the loop into 4 threads. My code is ok :

std : :thread t1(firstQuarter...
..
std : :thread t4(fourthQuarter...
t1.join;
...
t4.join;

but time is exactly same of one thread even time is upper.It's just classic access array memory and for the out image they don't need mutex because each thread have a quarter aof the image. How free array matrix for multithread for this easy case ?

  Mat img1 = imread( "f:/lib/opencv/samples/data/lena.jpg", IMREAD_COLOR);
    Mat img2 = imread( "f:/lib/opencv/samples/data/orange.jpg", IMREAD_COLOR);
    Mat img3,img4;
    Mat mask1(img1.rows, img1.cols, CV_8UC1, Scalar( 0));
    circle(mask1, Point( 200, 200), 100, Scalar( 92), - 1);
    Mat mask2 = 255 -mask1;
    Mat mask2c,mask1c;
    vector <Mat > pMask = { mask1,mask1,mask1 };
    merge(pMask, mask1c);
    pMask.clear();
    pMask.push_back(mask2);
    pMask.push_back(mask2);
    pMask.push_back(mask2);
    merge(pMask, mask2c);

    multiply(img1, mask1c, img3,  1. 0  /  255, CV_32F);
    multiply(img2, mask2c, img4, 1. 0 / 255, CV_32F);
    img4 =img3 +img4;
    img4.convertTo(img3,CV_8U);
    imshow( "test", img3);
    waitKey();