Six years ago tomorrow, 6-year-old Etan Patz disappeared while walking to the school bus in Manhattan's SoHo district. Despite intensive and continuing efforts by his parents, New York and federal law enforcement authorities to find him, no trace of Etan has been uncovered.

Missing posters featuring photographs taken less than a month before his apparent abduction show and describe Etan as 40 inches tall, weighing 50 pounds. With the passing of time, the photos have become dated and are less likely to help uncover the youth, who would now be almost 13.

Until recently outdated photos were the only hope parents had that their missing children would be spotted. Today, on the eve of the third anniversary of Missing Children Day, two promising alternatives are being developed: sketched composites and computer-synthesized "photos."

In the first process, an illustrator uses photos, family likenesses and other data to create "aged" drawings -- what the child would look like, for instance, 6 years after the abduction. Two children were found recently when a pair of such drawings -- along with early photos -- were shown on a TV documentary. The children, abducted by their father 7 1/2 years earlier, were recognized by a school official and within days the girls were reunited with their mother.

"I was approached," says medical illustrator Scott Barrows, "by research people with the documentary. I worked with photos taken of the girls -- Kathleen and Deborah Caruso -- the week before they disappeared. I looked at a photo of their mother as a child, studied her face and earlier photos of the girls and took it from there."

Barrows and a colleague, Louis Sadler, both have experience in forensic anthropology: "When someone finds a skull, we try to reconstruct the face for identification, using sculpture techniques and tissue depth landmarks." Using similar techniques, as well as normal facial growth patterns and other data, Barrows and Sadler begin with photos of missing children and add progressively more mature overlays until they arrive at the right "age."

The Caruso case -- the first Barrows took on -- has led to others. He currently is working on 12 missing-children cases, and he and Sadler each are testing their procedure by doing "blind" studies. "We're each doing 15 different people, and several graduate students are working on the same 15 as I. We have a cookbook recipe-style technique, so there shouldn't be much variation in what we come up with."

Neither Barrows and his graduate students nor Sadler have seen actual current photos of their subjects. That comes later.

"We should be close," says Barrows, who is delighted with the way his Caruso sketches matched the girls themselves. "A few areas were off, but it was about 80 percent on."

Computer-synthesized "photos" are derived from a patented process developed by New York-based conceptual artist Nancy Burson, along with computer scientists Richard Carling and David Kramlich.

Using photos of the abduction victim, and others in the family that resemble the child, "we actually update the facial structure of the child," says Burson.

They scan photos of the child and the relatives into a computer; the computer translates the images into digital information and then creates a somewhat fuzzy composite image. Burson then manipulates the image into a final, computer-generated "photo" of the aged child.

Two such composites (Burson has done three to date) have resulted in sightings of the missing children: Kurt Newton, born in 1971, was abducted from a campground in his native Maine in 1975. "There was an incredible search and nothing turned up," says Burson. "They found his tricycle hidden in the woods. That's it."

Two years ago, Burson did a computer-composite of him, using photos of his father as a youngster, and of an older sister. Since then, Burson says, there have been several sightings. An earlier composite, done of 12-year-old Dorothy (DeeDee) Scofield, abducted August 1976 in Florida, also resulted in several sightings.

The FBI, on learning of the Scofield and Newton sightings, approached Burson and asked her to do a composite of Etan Patz. That photo has not been widely distributed, apparently because Etan's parents fear it might not resemble the nearly 13-year-old boy.

"Nobody really knows," says Burson, whether the composites would throw people off. But, she counters, "when you have a photo of a child who's 6 years old, it becomes a liability when you're looking for a child who's nearly 13."

Barrows says he's seen computer-generated aging processes applied to missing children's photos but still tends "to go with the sketch. It's not as finished looking and allows room for error on the viewer's part." He does recommend that the sketches or synthesized photos be paired with the most recent photographs available of the missing children.

Burson says she and her coworkers haven't yet done a blind study -- "No one has given us the funding to try it" -- adding that the process "still is in the prototype stage right now. We're doing this on a part-time basis. It takes a lot of time and the process is expensive right now."

Each of the first three composites cost around $2,500 and took her, Kramlich and Carling around 30 hours to complete. "The price will come down," says Burson, "when there is a more formal system to do this on. Also, we hope to speed the time up when the system is formalized.