'Returns the offset of the baseline from the bottom of the
image in pixels.
*texstr*
A valid mathtext string, e.g., r\'IQ: $\sigma_i=15$\'
*dpi*
The dots-per-inch to render the text
*fontsize*
The font size in points'

def get_depth(self, texstr, dpi=120, fontsize=14):

    assert (self._output == u'bitmap')
    prop = FontProperties(size=fontsize)
    (ftimage, depth) = self.parse(texstr, dpi=dpi, prop=prop)
    return depth
