opencv：How to access a Mat with multiple channels ?

Mat.at(row, col) , 雖然每次都得從頭算出位址, 是效率不高的存取方式,
但頗好用. 只是, 非單一 channel 的 matrix 要如何存取呢?

下文 testM 是有三個 channel 的 8-bits unsign char matrix.
可依舊以 (row, col) 存取, 但最小單位是 Vec3b


        Mat testM(3,4,CV_8UC3);

for (int i = 0; i < testM.rows; i++)

{

for (int j = 0; j < testM.cols; j++)

{

Vec3b v = testM.at(i,j);

v[0] = i;

v[1] = i+10;

v[2] = i+20;

testM.at(i,j) = v;

}

}

當然也可直接存取 data, serial data 如下

row0= {ch0 ch1 ch2}, {ch0 ch1 ch2}, {ch0 ch1 c2}, {ch0 ch1 ch2}
row1= {ch0 ch1 ch2}, {ch0 ch1 ch2}, {ch0 ch1 c2}, {ch0 ch1 ch2}  
row2= {ch0 ch1 ch2}, {ch0 ch1 ch2}, {ch0 ch1 c2}, {ch0 ch1 ch2}      

for (int i = 0; i < testM.rows; i++)

{

uchar* pdata = testM.data + i*testM.step;

int k = 0;

for (int j = 0; j < testM.cols; j++, k+=3)

{

printf("r=%d,c=%d: %d,%d,%d\n", i, j, pdata[k],pdata[k+1],pdata[k+2]);

}

}

輸出:

r=0,c=0: 0,10,20

r=0,c=1: 0,10,20

r=0,c=2: 0,10,20

r=0,c=3: 0,10,20

r=1,c=0: 1,11,21

r=1,c=1: 1,11,21

r=1,c=2: 1,11,21

r=1,c=3: 1,11,21

r=2,c=0: 2,12,22

r=2,c=1: 2,12,22

r=2,c=2: 2,12,22

r=2,c=3: 2,12,22