Roger Dickerson is an improbable success among American composers.

Though with him, as well as with most of his peers, success has to be measured in inches rather than miles. Composing is profitable only for the very few, and for the others who write music, their work must be supplemented by other pursuits.

And for black composers like Dickerson, opportunities for writing classical music are almost uniformly more limited than for their white counterparts.

Furthermore, Dickerson lives in New Orleans, a way from the hallowed halls of academe where compositional skills are normally pursud - and recognized.

But, given these constraints, the 43-year-old composer has done better than most at what matters most - getting performed.

In 1971 the New Orleans Symphony asked him to write a requiem for Louis Armstrong that has been played aorund the country. And in 1975 the same orchestra played his new version on that recurrent musical subject, the Orpheus legend. It was his "Orpheus and His Slide Trombone."

Both had roots that combined the materials of classical music with jazz, which Dickerson has played in New Orleans' French Quarter for much of his career.

Dickerson's latest work in this vein, and the story of how it came to be written, is the subject of a Public Broadcasting Service special at 8 tonight, (Channel 26 and other PBS stations).

The regional aspect is quite explicit in this composition, the "New Orleans Concerto for Piano and Orchestra." But do not expect travelogue kitsch, a sort of jazzy "Grand Canyon Suite."

In stead, the concerto is a fresh and engaging variation on the lean and kinetic three-movement, fast-slow-fast 20th century concerto format as it has evolved from Ravel to Prokofiev ot Bartok to Barber and others.

Dickerson works within this framework with originality, both dramatic and lyric. The New Orleans element is one mood, and most certainly not pictorial. No French Quarter Adagio or Mardi Gras Presto here.

A moody monologue for solo piano opens the work, full of jazz-related tremolos and runs. But its most powerful quality is its stark sense of what seems like alienation. This dramatic passage is to return as the climax of the last movement, not played alone, but combined with the accumulation of orchestral baggage that has developed during the work's 25 minutes.

It is a striking effect and gives Dickerson's concerto a satisfying sense of resolution that many more famous works along this model lack.

Another lovely effect is the introduction into the slow movement of a soprano vocalise (this is where the singer sings but does not mouth syllables) more as another wind instrument than as a solo. The result is to impose upon the rather jagged orchestral line a lyric rein that leads the solo instruments to picl up that mode and develop the movement toward greater serenity.

I have never heard of this being done in a concerto before. It is an engaging idea of a composer who seems less the revolutionary than the evocative shaper of moods.

The concerto is the product of a 1974 bicentennial commission - to thich 1,200 persons contributed - by the New Orleans chapter of LINKS, Inc., and was meant to "evoke the spirit of New Orleans." It does this through indirection and without parochialism.

The soloist is the eminent Leon Bates, accompanied by the New Orleans Philharmonic under Werner Torkanowsky, one of the few white conductors to taken an interest in the works of black composers.

The plain truth is that with Torkanowsky as a patron Dickerson probably made a wise decision in passing up the big league Amercian music centers and returning to his native city after years of study at Indiana University and in Europe.

his explanantion in a recent interview might have sounded a little simplisitic had the interviewer not already heard the concerto. To regard the city as cource of esthetic stimulus because it is "a cultural crossroads" may sound dubious, but Dickerson makes it work.

The show's documentary aspsects seem less satisfactory. It belabors the link between the community and the composition - not that there is no story here, but they give us a little too much of it.

One brief epsiode, however, should not be missed. It is an interview with Dickerson's mother, a woman of considerable television presence and charm, in which she tells about his teen-age jazz jobs in the French Quarter and declares, "If I had known what kindof places he was in I just don't know what I would have done."