This is with regards to both short pips and inverted rubber. How close to a "smooth, pip-free" sheet of rubber could the topsheet become?

Do you know why no pips is not allowed?

 it is, its called reverse, or do you mean just a sheet of rubber, no pips any side ? actually, if you do mean that, you have a point.

Yup!  I'd argue that by definition, all inverted rubbers have pips: otherwise there would be nothing to "invert"..  :D

I doubt it would be very hard to explain the benefits of pips - both for pips out and for inverted rubbers.  But ITTF sees it fit to ban pip-less rubbers.  Which makes me wonder what benefits there might be to pip-less rubbers..  (To be honest, I wouldn't be all that surprised if that was just another example of ITTF zeal: banning something that doesn't need to be banned.  But who knows?  Did players ever manage to gain some unfair advantage by using pip-less rubbers?)

yes, great post from you, I am interested with the technicalities of this though. I do know that in the 30's 40's, many players loved they set up to be so perished that the pips had worn away. Modernly, I just am not sure that there is a reason that pips have to be involved at all. Only because every rubber has to be approved by the ITTF

I think if pips were removed from smooth rubber altogether, sponges would become EVEN MORE important than they already are.

Other factors with the rubber topsheet would come to forefront rather than pip height/width, pip alignment, and pip density. Topsheet hardness, thickness and tackiness would be critical. I think softer, more supple topsheets would be the norm to allow the sponge to come through the most.

We need a physics nerd to go in-depth with this stuff!

And yet another conspiracy theory arises.  NO, the ITTF never did "ban" pimple less rubbers.  They didn't exist - to any significant degree - for them to decide against them.  As I understand it, pimpled rubber - a la hardbat - was the standard prior to the introduction of sponge.  The regulations were then written to govern how these could be used, either separately or in combination.

I never took claims beginning with "I wouldn't be surprised if" to be theories.  But point taken.  It was a gratuitous jab in ITTF's direction, and doesn't help the discussion.. 

Very interesting.  So, from your posts together with Zeio's posts, the move seems to have been meant (1) to make room for the most effective and most common approaches, and (2) to make oversight a manageable task by setting boundaries on the range of coverings that could be introduced moving forward.  That's quite sensible.

Liu Guoliang!  Very interesting.  I really don't know enough about rubber design to have an opinion.  But I was thinking it would be interesting to ask someone knowledgeable whether pip-less topsheets might have be effective for low spin offensive coverings.  (Basically for the same kind of game that offensive players use short pips for.)  I mean I'd be curious to learn whether they might allow, e.g. for more predictable behavior pips.  

What did Liu Guoliang use prior to the 1mm minimum?