Camera calibration: Converting k1, k2, k3, k4 to lens distortion a, b and c parameters.

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I have k1, k2, k3, k4 radial distortion coefficients for a fisheye lens. I want to convert them into a, b and c parameters to use with Hugin: http://hugin.sourceforge.net/docs/man...

This is all the data I have (including k1, k2, k3, k4 in the lens tag):

<camera width="2560" height="1920" layout="equatorial">
    <pose>
        <rotation m0="0.03483457" m1="-0.9967604" m2="0.0724933" m3="0.8140989" m4="-0.01377276" m5="-0.5805629" m6="0.5796806" m7="0.07924037" m8="0.8109818" yaw_deg="-35.55676" pitch_deg="-4.544904" roll_deg="90.79163"/>
        <translation x_cm="-0.9366078" y_cm="-0.1736482" z_cm="-0.3043223"/>
    </pose>
    <optics>
        <focal_length focal_pixels="1242.36"/>
        <principal_point center_offset_x="2.571655" center_offset_y="10.38043"/>
        <lens type="fisheye" k1="0.03462446" k2="-0.03762977" k3="0" k4="0"/>
        <fisheye_radius radius_pixels="1570"/>
    </optics>
</camera>

Doing the reverse would also be helpful, having a, b, c parameters get k1, k2, k3, k4 parameters.

Any chance someone can help me with this or point into the right direction?