Cartoon fan David Mackenzie has been on the Internet griping about a cartoon called “Gorilla My Dreams,” recently released as part of a DVD collection of 60 restored Looney Tunes classics from Warner Bros.

Mr. Mackenzie, an 18-year-old film student in Glasgow, Scotland, says there’s something missing from the seven-minute cartoon, about a motherly primate who takes Bugs Bunny on a romp through the jungle: Pause the DVD, and as the two get ready to swing through the air, a piece of vine seems to dissolve.

The glitch is easy to miss. But hardcore animation fans say the case of the vanishing vine is only the latest example of technology gone awry.

“Casual fans will think it’s just people nitpicking, but it’s really not,” Mr. Mackenzie says. “If Gene Kelly’s arm disappeared while he was dancing in ‘Singin’ in the Rain,’ everybody would notice.”

As studios release more classic movies and television shows on DVD, they are increasingly using digital restoration to smooth over scratches and dirt specks on old film. But the process can also remove some of the lines that make up the animation — for example, blurring Tom’s face in a Tom and Jerry cartoon, or erasing lines in Woody Woodpecker’s fast-moving beak.

The technology at issue — called “digital noise reduction,” or DNR — works by removing lines that appear in one frame of a film but not the next, reasoning that the line doesn’t belong. In live-action films, that usually works well. But in cartoons, the process gets sketchier. A fast-moving cartoon is made up of a series of drawings with sharp ink lines. To the casual viewer, the drawings appear to move fluidly from one moment to the next. But in fact, they often change radically from frame to frame. And when DNR is applied, a deliberately drawn line can be mistaken for a stray and removed.

“It’s really irritating to watch,” says Mr. Mackenzie, who grew up on the Ren & Stimpy cartoons of the 1990s. He keeps a log of what he sees as the most egregious errors on his Web site: One shot from the Looney Tunes cartoon “Have You Got Any Castles?” shows an image of a dancing old man disappearing into a grayish haze.

Until recently, film studios kept old animation tucked away in storage. But as DVDs become more popular, studios have done the math and found that classic cartoons are relatively cheap to restore and sell well. “DVD is the medium for collectors. They gobble this stuff up,” says Robert Mayo, senior vice president of video at Classic Media Inc., of New York, which has released two DVD sets of Rocky & Bullwinkle & Friends. “It has given all the studios, us included, a reason to go back and remaster all these things.”

Craig Hoffman, a spokesman for Time Warner Inc.’s Warner Bros., which released the Looney Tunes DVDs last fall, declines to comment on the complaints about the restored cartoons. “There’s a wide audience: children, collectors, people who grew up loving them,” he adds.

One commonly used DNR product is made by Sweden’s Digital Vision AB, which sells equipment ranging in price from $35,000 to $150,000. Hugh Heinsohn, president of the company’s U.S. unit, compares his product with a hammer: It’s a powerful tool that must be used with care. It can muddy the image depending on the skill of the technician using it.

Digital noise reduction isn’t unique to DVD: Earlier, it was used in transferring old cartoons to VHS and laserdisc. The problem was harder to spot in VHS. But studios now are releasing more classics on DVD, and fans are becoming more aware of the issue. Many say it points to a broader problem in Hollywood: Years after the golden age of animation in the 1950s, studios don’t pay much attention to classic cartoons.

“Animation is considered a poor stepchild to the whole motion-picture industry,” says Jerry Beck, who has written several books on classic animation and serves on the board of the International Animated Film Society. “It’s just not up there with movies starring Brad Pitt.”

Animator Milton Gray, who worked on “The Simpsons” in the 1990s, describes high-quality animation as “poetic” and “dance-like.” Every frame matters, he says. Like many enthusiasts, he studies the classics frame by frame — which makes it especially frustrating when pieces are missing, he says.

Studios say the elimination of scratches and dirt makes up for a blurred line here or there. And glitches can be largely avoided if digital noise reduction is done by a skilled technician. “Sometimes it’s being used with no ill effects,” says Jeff Stabenau, who oversaw the Rocky & Bullwinkle restoration at an outside DVD production company. “If it is used carefully, it can enhance the animation.”

Some animation fans say they see some adverse effects of DNR in some of the restored Rocky & Bullwinkle cartoons. Mr. Stabenau says he hasn’t heard any complaints. “I think the images look great,” he says. “Based on the age and condition of the material, it’s restored as faithfully as possible.”

Walt Disney Co. has largely avoided criticism of its cartoon restorations. For most of its projects, Disney doesn’t use digital noise reduction, relying instead on artists to inspect each frame of film and remove defects either manually or with proprietary software. “If you just take a film and throw it through a noise-reduction system, you’re never going to get the same standard of quality,” says Jeff Miller, president for world-wide post-production and operations.

DNR’s negative side effects are more exception than rule. But for purists, a few seconds of messed-up animation can spoil an otherwise perfect collection.

Thad Komorowski, a Niagara Falls, N.Y., 16-year-old, who helps run a Web site about classic animation, plays scenes over and over to study facial expressions and gestures of characters in early Warner Bros. cartoons — exactly the kind of fast-moving animation style that can be mangled by digital noise-reduction. “My friends don’t really see it as a big deal,” Mr. Komorowski says, “but it makes me really mad.”

Write to Vauhini Vara at