Art Frank Stella At the Museum of Modern Art until May 31

THE QUESTION of what to paint continually haunts a contemporary artist. The monumental canvases of recent years have been stripped to rectangles of few colors with a minimum of activity inside the frame. The constant purifying of the medium leads to the dilemma of what can come next. Frank Stella, a particularly inventive artist, has opened painting in a new direction by changing the traditional shape of the canvas. In the process of experiment, his paintings have led him beyond the rectangular frame to shapes of triangle, rectangle, and circle.

A retrospective exhibition of his work, currently at the Museum of Modern Art in New York, shows how he has changed from black rectangles patterned with a design of pinstripes done in 1958 to bold inventions of interlocked geometrical forms. No single canvas in his group of paintings can make the viewer nervous before its cold stare. Each of his paintings makes sense as part of the evolution of his art and appears less bewildering than when it is displayed with the work of other contemporary artists.

The label of Minimal Art has latched onto Stella's work. He makes enormous paintings-with surfaces as smooth as a car. Flat color without a shadow is neatly placed within ruled or compassed lines so that no trace of the artist's hand remains. He could command a computer to execute it. But a startling combination of geometrical forms makes his pictures look different from the other Minimal Artists.

The earliest shaped-canvases in this exhibition reveal that Stella first cut out rectangular pieces from the edges of the painting. Rows of white pinstripes repeat the pattern of the sides. He coats the picture with a uniform quiet color-maroon, grey, or often black. And by evenly covering the whole surface, the thin stripes emphasize the outer shape. One of these canvases runs its zigzag olive green surface 23 feet along the wall. Here Stella plays with shadowed color that makes the stripes seem to move in three dimensional space as well as on the flat surface.

With a burst into more daring shapes. Stella gave up pinstripes and fired the canvas with new color. At first glance, "Chocorua," named for a New Hampshire mountain, looks like a triangle imposed upon a rectangle, piercing the edge. But on closer scrutiny, this drawing is complex. Wide bands of color outline the shapes, locking the geometrical pieces into a unit.

In a small room of drawings, you see how he plans the canvases, outlining them with a ruler often on graph paper. Using a felt pen he has made some interesting color models, but they lack the power which enormous size blows into them. As you pass a corner along the exhibition, a circle ten feet in diameter faces you from the end of a brief hall. Light colored bands drive in and among each other, breaking and reconnecting around the circle. This pinwheel vision dominates the show.

Recently Stella has changed his imagery to large leaf-like forms made from two intersecting circles. Rainbowed with pastel, these canvases have the delicate look of butterfly wings.

The geometric imagery of Frank Stella seems unrelated to anything beyond the realm of painting or problems of color and form. Abstract art does not directly comment on the world that faces it. But as you stand in the midst of the Frank Stella exhibition you can imagine that this exhibition, like other contemporary shows, sits at the focus of the city. The Modern Museum seems an open space in between the tangled grid of New York, where all the confusion has been distilled.