&gt; Additionally, functionality like alpha channels and 16-Bit color depth can only be achieved via extensions<p>PPM explicitly supports 16 bit colour depth.<p><a href="http:&#x2F;&#x2F;netpbm.sourceforge.net&#x2F;doc&#x2F;ppm.html" rel="nofollow">http:&#x2F;&#x2F;netpbm.sourceforge.net&#x2F;doc&#x2F;ppm.html</a> says &quot;Each sample is represented in pure binary by either 1 or 2 bytes.&quot;<p>And PAM explicitly supports transparency (which I guess you could consider an &quot;extension&quot; but since it&#x27;s handled by libnetpbm, I&#x27;d quibble.)<p><a href="http:&#x2F;&#x2F;netpbm.sourceforge.net&#x2F;doc&#x2F;pam.html" rel="nofollow">http:&#x2F;&#x2F;netpbm.sourceforge.net&#x2F;doc&#x2F;pam.html</a> says &quot;Each of the visual image formats mentioned above has a variation that contains transparency information.&quot;<p>Also,<p>&gt; Due to it being a textual format it also lacks the desired compression characteristics.<p>...isn&#x27;t true since P6 is binary (excepting the header which is likely trivial on a normal sized image).