</TR>
</TABLE>
<p>

Read image mask.  The mask is associated with the image read in by the
`<font color="#82140F"><code>read image</code></font>' command in the following way.  When computing image
histograms, Gri ignores any pixels in the image for which the
corresponding pixel in the mask is set to `<font color="#82140F"><code>1</code></font>'.
<p>
<ul>
<p>
<li> `<font color="#82140F"><code>read image mask rasterfile</code></font>'
The image size is specified in the rasterfile file itself, so 
it is not s