Turn a color to black if its r+b > 150
<p>I've looked around on SO but I couldn't find any similar question.</p>

<p>After opening an image in PHP I'm willing to check every pixel, and, if their red + green values is lower than 150 I would turn that pixel to white, instead if the sum is higher than 150 I would turn that pixel to black.</p>

<p>Is this possible?</p>

<p>I've tried with <code>imagefilter()</code> but I can't make it work as expected. Maybe there are more appropriate functions to use.</p>
