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I have multiple camera in different points.
I have their position and rotation as $(x,y,z)$ , $(\alpha,\beta,\gamma)$ or $( roll, pitch, yaw)$ .
And I have output like this :
Feed from camera-1, I know length of yellow and green.
How can I calculate position of the object, in this case head in 3D space.
For each camera, you can compute the direction on which is detected the object. The position and orientation of the camera let you find the direction of the center of the image. Depending on camera aperture, each pixel of the image corresponds to a slightly different direction. I don't know which definition you use for your angle so I won't risk any expression of it but you should convert the angle to obtain a vector in cartesian coordinates.
So from a camera with position at $(x_c, y_c, z_c)$ and detecting the object in the pixel of direction defined by the unitary vector $(x_d, y_d, z_d)$, you know the object is along the direction:
($x_c + x_d \times d, y_c + y_d \times d, z_c + z_d \times d$) with $d$ being the distance camera-object.
Now if you have 2 cameras, you can look for the nearest points from each line to the other and take their center as position of the object (see skew lines).
