# Create a stereo projection matrix using rvec and tvec?

I am setting up Projection Matrices in a stereo camera rig, using the intrinsic matrix. Like so:

```
// Camera 1 Projection Matrix K[I|0]
cv::Mat P1(3, 4, cv::DataType<float>::type);
K.copyTo(P1.rowRange(0, 3).colRange(0, 3)); //K is the camera matrix
//stereo
float tx = 0.12; //Stereo baseline
float ty = 0;
float tz = 0;
//rotation ( images are rectified, so this is zero)
float rots[9] = { 1,0,0,0,1,0,0,0,1 };
cv::Mat R = cv::Mat(3, 3, CV_32F, rots);
//translation. (stereo camera, rectified images, 12 cm baseline)
float trans[3] = { tx,ty,tz };
cv::Mat t = cv::Mat(3, 1, CV_32F, trans);
// Camera 2 Projection Matrix K[R|t]
cv::Mat P2(3, 4, CV_32F);
R.copyTo(P2.rowRange(0, 3).colRange(0, 3));
t.copyTo(P2.rowRange(0, 3).col(3));
P2 = K*P2;
```

This give me:

```
matrix P1
[333.02081, 0, 318.51651, 0;
0, 333.02081, 171.93558, 0;
0, 0, 1, 0]
matrix P2
[333.02081, 0, 318.51651, 39.962498;
0, 333.02081, 171.93558, 0;
0, 0, 1, 0]
```

which seems to work well.

Now, I need to update these matrices using the current camera pose (`rvec`

and `tvec`

).

I am doing this with:

```
//camera pose
cv::Mat R(3, 3, cv::DataType<float>::type);
cv::Rodrigues(rvec, R); // R is 3x3
R = R.t(); // rotation of inverse
cv::Mat tvecInv = -R * tvec; // translation of inverse
//translation. (stereo camera, rectified images, 12 cm baseline)
float trans[3] = { tx,ty,tz };
cv::Mat t2 = cv::Mat(3, 1, CV_32F, trans);
//add the baseline to cam2
t2.at<float>(0) = tvecInv.at<float>(0) + tx;
t2.at<float>(1) = tvecInv.at<float>(1);
t2.at<float>(2) = tvecInv.at<float>(2);
// Camera 1 Projection Matrix K[I|0]
cv::Mat P1(3, 4, CV_32F, cv::Scalar(0));
R.copyTo(P1.rowRange(0, 3).colRange(0, 3));
tvecInv.copyTo(P1.rowRange(0, 3).col(3));
P1 = K*P1;
// Camera 2 Projection Matrix K[R|t]
cv::Mat P2(3, 4, CV_32F);
R.copyTo(P2.rowRange(0, 3).colRange(0, 3));
t2.copyTo(P2.rowRange(0, 3).col(3));
P2 = K*P2;
```

Which gives me:

```
update P1
[-362.74387, -131.39442, 252.00798, -272.50296;
-9.197648, -374.57056, 8.7753143, -74.796822;
-0.098709576, -0.4356361, 0.89469415, -0.89352131]
update P2
[-362.74387, -131.39442, 252.00798, 37.901237;
-9.197648, -374.57056, 8.7753143, 421.92899;
-0.098709576, -0.4356361, 0.89469415, -0.0064714332]
```

This is very wrong. What is the correct way to update stereo projection matrices using `rvec`

and `tvec`

values?

thank you.