NEW YORK ( TheStreet) -- Since I last looked at 3-D printing companies three months ago, the industry has made a round-trip.The two largest such companies, 3D Systems ( DDD) and Stratasys ( SSYS), are now trading close to their December levels. What's going on? Bears and shorts began attacking the shares in earnest last month, writes 24/7 Wall Street, with claims the machines have yet to reach production lines and consumer markets remain untapped. There were other problems unique to 3D Systems. It "missed" on its latest quarter, Reuters writes, although sales were up 45%. (Sales were supposed to be up 48%, Forbes said.) Co-founder Charles Hull sold more than half his personal position, according to Investors.com. But a 45% rise is hard to call a miss, and Hull is in his mid-70s. Stratasys also fell, partly in sympathy and partly because its financial reports lack transparency, wrote Investors.com -- the current company was formed late last year in a merger with Objet, an Israeli firm. Here is what's really happening. The 3-D printing, or "additive manufacturing," revolution is brand new. When Mitch Free writes at Forbes that it "doesn't live up to the hype yet," the key word is "yet." Charles Hull got his first patent on the process in 1986. Machines today use only a limited number of materials. They're fairly slow. They're only practical in niches -- Stratasys now has a unit for orthodontic clinics, Engineering.com writes. That doesn't mean the revolution isn't happening, or that it won't happen. The two leading companies aren't guaranteed their place in the final standings, but they're the best horses to bet on. Being public and large, they can make acquisitions -- such as 3D Systems' recent purchase of GeoMagic, a software company. And 3-D printing itself may just be part of a larger revolution, which Neil Gershenfeld of MIT called "bringing programmability to the physical world" in a recent Wired profile. Gershenfeld's Center for Bits and Atoms is looking to digitize not just design, but materials and process, building something like Legos, 3-D parts that can be assembled accurately in standard ways. But is it likely that both 3D Systems and Stratasys are going to miss the boat on all this, or that they're going to be made obsolete by some innovation out of left field? In new fields you're always betting on management, on people like 3D CEO Avi Reichental, profiled here at Singularityu.org.