message: Add a figure and some formulas to illustrate the PnP pose estimation principle.
diff: @@ -567,7 +567,97 @@ focal length.
  function requires exactly four object and image points.
  The function estimates the object pose given a set of object points, their corresponding image
  -projections, as well as the camera matrix and the distortion coefficients.
  +projections, as well as the camera matrix and the distortion coefficients, see the figure below
  +(more precisely, the X-axis of the camera frame is pointing to the right, the Y-axis downward
  +and the Z-axis forward).
  +
  +![](pnp.jpg)
  +
  +Points expressed in the world frame \f$ \bf{X}_w \f$ are projected into the image plane \f$ \left[ u, v \right] \f$
  +using the perspective projection model \f$ \Pi \f$ and the camera intrinsic parameters matrix \f$ \bf{A} \f$:
  +
  +\f[
  + \begin{align*}
  + \begin{bmatrix}
  + u \\
  + v \\
  + 1
  + \end{bmatrix} &=
  + \bf{A} \hspace{0.1em} \Pi \hspace{0.2em} ^{c}\bf{M}_w
  + \begin{bmatrix}
  + X_{w} \\
  + Y_{w} \\
  + Z_{w} \\
  + 1
  + \end{bmatrix} \\
  + \begin{bmatrix}
  + u \\
  + v \\
  + 1
  + \end{bmatrix} &=
  + \begin{bmatrix}
  + f_x & 0 & c_x \\
  + 0 & f_y & c_y \\
  + 0 & 0 & 1
  + \end{bmatrix}
  + \begin{bmatrix}
  + 1 & 0 & 0 & 0 \\
  + 0 & 1 & 0 & 0 \\
  + 0 & 0 & 1 & 0
  + \end{bmatrix}
  + \begin{bmatrix}
  + r_{11} & r_{12} & r_{13} & t_x \\
  + r_{21} & r_{22} & r_{23} & t_y \\
  + r_{31} & r_{32} & r_{33} & t_z \\
  + 0 & 0 & 0 & 1
  + \end{bmatrix}
  + \begin{bmatrix}
  + X_{w} \\
  + Y_{w} \\
  + Z_{w} \\
  + 1
  + \end{bmatrix}
  + \end{align*}
  +\f]
  +
  +The estimated pose is thus the rotation (`rvec`) and the translation (`tvec`) vectors that allow to transform
  +a 3D point expressed in the world frame into the camera frame:
  +
  +\f[
  + \begin{align*}
  + \begin{bmatrix}
  + X_c \\
  + Y_c \\
  + Z_c \\
  + 1
  + \end{bmatrix} &=
  + \hspace{0.2em} ^{c}\bf{M}_w
  + \begin{bmatrix}
  + X_{w} \\
  + Y_{w} \\
  + Z_{w} \\
  + 1
  + \end{bmatrix} \\
  + \begin{bmatrix}
  + X_c \\
  + Y_c \\
  + Z_c \\
  + 1
  + \end{bmatrix} &=
  + \begin{bmatrix}
  + r_{11} & r_{12} & r_{13} & t_x \\
  + r_{21} & r_{22} & r_{23} & t_y \\
  + r_{31} & r_{32} & r_{33} & t_z \\
  + 0 & 0 & 0 & 1
  + \end{bmatrix}
  + \begin{bmatrix}
  + X_{w} \\
  + Y_{w} \\
  + Z_{w} \\
  + 1
  + \end{bmatrix}
  + \end{align*}
  +\f]
  @note
  - An example of how to use solvePnP for planar augmented reality can be found at
  