6 and pngmono devices respectively
provide 8-bit color, 4-bit color and black-and-white for special needs.</p>

<p>The pngalpha device is 32-bit RGBA color with transparency
indicating pixel coverage.  The background is transparent unless
it has been explicitly filled.  PDF 1.4 transparent files do not
give a transparent background with this device.  Text and graphics
anti-aliasing are enabled by default.</p>

<h4>Options</h4>

<p>The pngalpha device has one option.
The other png devices have no special opt