less than zero or greater than unity were not allowed.

<dt><b>-linear</b>

<dd>This option indicates that the inputs are not true Netpbm images
but rather a non-gamma-adjusted variation.  This is relevant only when
you mix pixels, using the <b>-opacity</b> option or an alpha mask
(the <b>-alpha</b> option).

<p>The alpha mask and <b>-opacity</b> values indicate a fraction of
the light intensity of a pixel.  But the PNM and PNM-equivalent PAM
image formats represent intensities with gamma-adjusted numbers t