The screen is flat, but the space is round, both infinite and enveloping. The human figures in it, camouflaged, should blend in with the background. But they stick out in every direction, moving every which way.

This thrilling fragment of choreographer Merce Cunningham’s 1958 dance “Summerspace” is sufficient justification to make a documentary about Cunningham in 3-D. The technology conveys aspects of his radical aesthetic that are otherwise difficult to suggest on film. And that’s only one reason that Alla Kovgan’s “Cunningham” is an excellent introduction to a great body of work that can be hard to get a handle on.

One of Cunningham’s many innovations was to dissolve the spatial organization and frontal focus of the proscenium stage, making all parts equal, with dancers facing and moving in any orientation and often several things happening at once. Kovgan’s film doesn’t reproduce this so much as find a vivid equivalent.

The camera, choosing what you see, diminishes the feeling of simultaneity. But by moving into and through the dances in 3-D, it offers an immersive sense of what Cunningham called “a space in which anything can happen.” It helps you see how the air around a dancer can seem as alive as the flesh.

If 3-D helps put Cunningham across, it isn’t required. Some of the most seductive footage here isn’t the new performances of old works (1942-1972) that Kovgan filmed in 3-D, like “Summerspace,” but lower-resolution, 2-D archival footage, much of it rare and irreplaceable. What makes it irreplaceable are the original performers, especially Cunningham himself, surely one of the greatest dancers of all time. His longtime muse Carolyn Brown speaks of his “quiet center” and “animal authority”; we see all that and more.

It’s another strength of the film that the voices we hear are hers and his and those of other company members, recorded long ago. No current-day experts are needed to establish the core ideas of the Cunningham aesthetic, from the use of chance in composition to the independence of dance and music.

The choice not to use new or outside voices keeps us in the period, and it helps preserve a Cunningham-like tact around personal facts. Although much of the film proceeds in chronological order, there’s almost no biographical back story; the long-shrouded relationship between Cunningham and composer John Cage is delicately (and movingly) presented in a single exchange of inexplicit letters.

Likewise, tensions between Cunningham and his dancers aren’t hidden, but they’re not over-explained, either. Emotion roils beneath a deceptively placid surface. A story accrues.

The same tact extends to questions of meaning. Cunningham’s practical-gnomic explanations illuminate without shutting down options.

Kovgan doesn’t follow Cunningham’s courage all the way, though. As if to compensate for the lack of 3-D in the archival material, she collages it, splitting the screen into many panels. It’s artful but effortful, drawing too much attention to itself.

That’s also true of much of the new footage. The elegant camera movements and dramatic settings chosen by Kovgan — dance in a formal garden that the camera glides away from, over a pond, like a dragonfly — are beautiful but they often distract from what they are intended to display.

The 90-minute film excerpts 14 works, which means that the snippets are quite short. This excerpting has a precedent in Cunningham practice. He liked to mix up pieces of many works in one-off performances called Events. Formally and philosophically, his focus was on each moment, with little linear development. The well-chosen selections in “Cunningham” reproduce the variety of a Cunningham Event, and give the Cunningham experience of luminous instants.

But what the film doesn’t give is an accurate sense of Cunningham time. In a Cunningham dance, the mind can wander, experience different rates of change, be baffled, engrossed, astonished, bored. The price of Kovgan’s efficiency is impatience, always cutting away and moving on.

“Cunningham” registers the resistance that its subject encountered: the puzzlement, the thrown fruit. But as a film, it doesn’t take comparable risks, so it precludes the possibility of certain rewards, ones you have to see the full dances to get. That’s what makes it a good introduction.