Animation as a whole slowly went through this phase from The Silent Age of Animation to The Golden Age of Animation. Early cartoons were very crudely made—they were very stiff, rigid and mechanical in appearance and movement, had no construction, no line of action, lots of symmetry (which made them look flat) and body parts piled onto each other, rather than being directly connected by form. This began changing when Walt Disney began forming and refining The Twelve Principles of Animation, as well as animators like Fred Moore altering Mickey Mouse's design to become more pearlike and organic, allowing it not only to be three-dimensional, but also to be more pliable and organic than the earlier, rigid designs from shorts like Plane Crazy and Steamboat Willie. Disney immediately adapted this to its other characters, and everyone else in the animation industry copied this immediately, sending classic rubberhose animation to its grave within a few years.
The animation and character designs in Adventure Time have gone through some pretty noticeable changes over time. Finn, as a result of going through puberty, is noticeably taller, Jake's design is not as circle-like as it used to be and is a bit more detailed/streamlined. Marceline is no longer a total noodle person, as she has gained some curves over the course of the show (compare "Evicted" from season 1 to "Red Starved" in season 5 and one can actually make out hips and breasts on her design in the latter). The animation itself has also gotten smoother over time, and background details are more rich in the show's later seasons.
The second season of American Dragon Jake Long animation changed to a more detailed and fluid style of animation compared to the first. The character designs, especially Jake's dragon form, changed considerably; in the first season he is muscular and in season two and onwards he is scrawny.
Aqua Teen Hunger Force was never very solid to begin with, but it somehow improved vastly. Initially, Meatwad's shape would vary constantly, before settling in on its more meaty roundness at some point in the middle of the first season. The biggest evolution was the vast improvement in lip syncing, where the first episodes typically had the characters' mouths move in the same one or two animations for all dialogue, they now move faster and more accurately to the words. More recent episodes also saw animation breakthroughs such as Carl running (complete with jiggling!) and loads more realistic animation and poses for real people and the Aqua Teens.
The serialised Teens are quite different in appearance (especially Frylock), and distribution of personality, to their original appearance on Space Ghost Coast to Coast.
The Backyardigans' five-kid-cast got improved with Season 2. You can barely notice any change on Uniqua, and it's more noticeable on Austin (who looks nothing like he did during the first season).
Batman: The Animated Series had very full and weighty character designs and animation. When it was uncanceled the designers wanted to modify a lot of the designs for several reasons: the weightier designs were more difficult to animate, improve design "mistakes" in their opinion, simpler designs made the emotional actions clearer. Many fans didn't like the relatively "blander" designs, but mostly the new Joker. (Even the creators acknowledge it didn't turn out the way they wanted — it looked good on paper but not animated, and lacked the deep redness of the lips that gives him such a manic grin). This said, some, like Scarecrow's new nightmarish appearance, Poison Ivy's green skin, and Penguin's less Burton-esque revamp came out okay. A good discussion of the changes can be found here.
For purposes of more illustration, take a look. The same villain (Baby-Doll) as seen in TAS◊ and in New Adventures◊.
With Batman Beyond and more so in Justice League, the Timmverse got its final revamp. While the BTAS era had every character given a weighty look and the STAS era gave everyone a sleek look, the 3rd revamp added more lines. The best example of the changes would be the Joker, though, as he was present in all three eras and was wearing the same clothes, for the most part. In the BTAS era he had a rounded face and detailed features. In the STAS era he was drawn very basically with few lines. Even his eyes were simplified and his mouth became simple curves. In the JL era he got more lines and a face more similar to the BTAS version, but with the sharper lines and details (and red irises in his eyes.)
Batman is a good guide as well. Here are three images, from BTAS◊, the New Batman Adventures◊ and the Justice League◊. Notice how the inner leg in the TNBA one is represented by a single curved line, while in the JL one there is a distinction between thigh and calf, and in the BTAS one both the thigh and calf have curved lines. And the gauntlets are different. In the TNBA one they are drawn in a single zigzag, and in the JL one each is its own defined curve.
Beavis And Butthead had developed its own refined low-key style of animation by the last few seasons. The very early episodes comparatively look like they were drawn by blind toddlers.
The revival looks to be even more drastic, especially when comparing their characters' designs to the rest of the universes.
The Ben 10 meta-series had changed in looks between the original series and its sequels to look less cartoonish. For example, Ultimate Alien, which brought back the 10-year-old Ben in one episode, had slight changes in detail to him and the aliens he used.
Zigzagged, now they have shifted 3 times. Omniverse's artstyle is back to being cartoonish, but utilizes much more simple character designs and colors, creating a much more colorful and fluid show.
The Boondocks' art style changes every season, the character designs that have changed the most are Sarah Dubois, Ed Wuncler, and his grandson Ed Wuncler III. CN hired a new animation studio for the latest seasons.
The original YouTube pilot for Breadwinners didn't have very detailed backgrounds, and had a generally generic, sketchy feel. When the series received its own webpage on Nickelodeon's official website, there was also a remake of the original short. The remake featured more detailed backgrounds and more unique retro-video game style textures. The voice acting was re-recorded and it's better, to boot.
Casper: the friendly little ghost's first appearance showed him more chubby and duck-pin shaped; his adventures soon shifted him into the slimmer, round-headed ghost we know today.
Chaotic: The first season is animated in flash; this was an awkward style to some people. Then the second season was more like Afro Samurai with the colors of Avatar: The Last Airbender.
Chowder had completely different designs in the first episode, "The Froggy Apple Crumble Thumpkin". Mung had bigger ears and a smaller nose and eyes, Shnitzel has smaller eyes, and Chowder had a smaller face. In newer episodes, Mung has a bigger head and Chowder is thinner and has bigger eyes.
In The Critic, all the characters were given updates to their character designs. Jay Sherman and his sister Margo had the most changes.
Most of the characters (usually the main teenagers) started off looking like stick figures and in certain episodes, loose and gangly. Later, they gain thicker muscles and retain a more solid structure, sometimes to the point of stiffness.
In season 3, the fight scenes were better and improved. The first 2 seasons often showed basic punches and Ecto Rays with an irritating overuse of onomatopoeia comic book-ish freeze frame. season 3 made it better by reducing the last and bumping the others; the results are often surprisingly fast and fluid.
The later seasons experienced a drastic change in character and background designs, to the point where it lost its cherished Thick-Line Animation.
Dee-Dee had thicker eyebrowes and seemed to lack a chin in the first few episodes.
In the Oh Yeah! Cartoons shorts of The Fairly OddParents, the characters' appearances were slightly different from those in the show itself. For example, many of the characters had stretched heads and bodies, Wanda's skin tone was a little more pink, and Timmy's eyes were always crossed. In an issue of Nickelodeon Magazine, Butch Hartman, the creator, mentioned that Timmy looks much cuter than when he first appeared.
Another show with quite obvious change is Family Guy:
The attention to detail in the backgrounds and objects improved over time. In one of the commentaries, the writers noticed how the pancakes in one scene are so crudely drawn that it looks more like a beehive or a stack of huge Honeycombs cereal. The food in the more recent episodes look more like real food now.
Parodied when Brian and Stewie go five years into the future and everything is 3D animation. They also go back to 1999, the same year as the pilot episode and are shocked to see how different things look back then. They even poke fun at the Off Model issues the series suffered in its early years, such as one of Peter's eyes is drawn over his nose.
Background characters are also drawn with great detail, including the dancers for the intro, where there are characters from the series appearing as the dancers instead of generic characters.
Hey Arnold!: Season 1 looked much cruder compared to the other seasons (especially see Helga's face compared to the later seasons). Season 4 and 5 looked dramatically different from earlier seasons due to the switch to computer coloring.
Home Movies stopped using Squigglevision after its first season, and switched to a much more fluid Flash animation style from there on out.
Honey Halfwitch, an otherwise short-lived series from Paramount Pictures Cartoon Studio, went through an extreme character redesign near the end, where she went from looking like a Harvey Comic character (she was created by Howard Post, who drew for Harvey) to looking like a girl Charlie Brown. What happened was that Paramount fired Post and replaced him with veteran animator Shamus Culhane, who ordered to have the character redesigned.
Jimmy Two-Shoes: Starting in the second season, the animation got a bit more detailed and much smoother.
In season one of KaBlam!, Henry and June were very crudely drawn. Starting from season two, their appearances got cleaned up, and they finally got eye colors.
While Kevin Spencer is not known for its amazing animation (and occasionally lampshaded, being that they're a low-budget show), the first season was so ghastly that nothing was ever on model at any point. By the second season, the animation improved. The series got better in terms of animation, which can best be seen with Anastasia's hair becoming more realistic.
Season 2 of Kidd Video had a drastic change not only in the character design, but backgrounds as well, giving the show a more Disney-esque look than the more traditional designs of the first season.
On the show Kim Possible, Monique went from having black dot eyes, to white eyes with pupils by the second season.
Also, the characters had more movement in the first two seasons than in later ones. The animation also got smoother as time went on.
Shows like King of the Hill have a large change in art quality from early episodes to later episodes. The pilot is crudely drawn and looks very dated. There is some improvement as season one goes on, but the change overall in how the series looks throughout the show improves later on.
Buck Strickland's design has changed the most as in his first appearance he is very short and fat and has a different shaped head, by mid season 2 he is taller a bit thinner but with a pot belly and a thinner different shaped head.
Mainframe Entertainment improved their hardware several times, and the results were noticeable between seasons of their various shows.
In ReBoot, one hardware upgrade in mid-season two allowed for much more variety of motion than they had before. Season 3 showed another upgrade with much more texture like eyelashes, partially because it wasn't produced until a year and a half after season 2 ended. Season 4 was the uncancelled season three years later and the characters had a much greater sense of weight.
Beast Wars had a major jump at the beginning of season two, with the introduction of the "transmetals" the robotic appearances looked better then ever. Beast Machines featured some amazing and lush explosion effects as well as a decent range of emotion from at least one character that did not have a face.
Just in Beast Wars, the CGI evolved a great deal through its three season run, as the bare and flat plains with occasional rock formations gave way to lush jungles, deep forests and underwater environments. Even non-robotic characters were way ahead of previously featured organics in terms of visual appeal: Just compare the great looking Cyber Raptors from season 3 to Dinobot's original beast-mode that had visible "seams" where his body parts met, with a terribly segmented tail, and constantly deformed in awkward ways when he moved. Or the saber-toothed cat from season two to the tiger model from the first season, which also tended to look like separate body parts stuck together. None, however, evolved as much as the butterfly — although the outdated model did appear at later points, even when the better version had already been featured.
Metalocalypse was fairly consistent in style in the first two seasons with lines getting smoother, motion less clunky, and better coloring over time. Season 3 was a huge shift from the first two. Thanks to an increased budget, the characters now have pretty full range of motion compared to the first two seasons, where most of the action involved the band members standing around and talking.
Many classic cartoons have gone through design changes; Mickey Mouse is probably the most famous, but his was rather subtle evolution, being tweaked here and there as animation techniques were being refined, such as giving his eyes movable pupils. A much more obvious design change can be seen in Donald Duck who looked like this◊ when he debuted... a big difference compared to this◊
My Little Pony: Friendship Is Magic has cleaned up over the episodes, in subtle ways. One noticeable change is that unicorn horns now are a part of the head instead of being separate, and each individual unicorn's magic is colour-coded instead of being a generic faded colour. Another is that Applejack's freckles don't vanish when she runs anymore.
In all of Princess Luna's appearances after the pilot, including the Pensieve Flashback in the Season 4 premiere, her model is radically different from that seen in her debut after being depowered from Nightmare Moon form.
The whole setting is also much more clean in later episodes. The crowd scenes in early episodes, for example, feature groups of near-identical ponies who just stand there motionlessly, aside from occasionally blinking their eyes. In later seasons, the animators have given background ponies new animations and movements so they appear more lively instead of being just filler.
The animation has gotten a lot more dynamic over each season, taking advantages of angles and momentum to give the show a more realistic look overall. Compare the first episode to the latest; basic walk cycles are hardly used anymore.
As of Season 4, the animators have begun going absolutely wild with facial expressions. You'd be hard pressed to find a single episode that doesn't haveat leasta couple.
The early Peanuts specials have Sally wear a blue bow on the curly part of her hair. Later specials got rid of her bow, probably because it was too tedious to animate.
In the sneak preview for Recess, the characters (aside from Ms. Finster and Mikey) look very different than they do in the series proper.
During season 3, some episodes were done in digital coloring as opposed to season one and two's coloring which was only hand-painted cels. The digital and hand-painted would often go back and forth with mixed results until the final digitally colored episode, "Bonky Fever", in 2000.
Actually, the episodes (not including movies) with digital coloring inverted this. Animation errors were much more prone. "Buried Treasure" and "Here Comes Mr. Perfect" considerably had the worst animation, with the former being animated too much like cutscenes from a 1990s point-and-click CD-ROM, and the latter having choppy animation.
T.J. had a slight shifting design throughout the series. In season one, he was a bit chubbier, shorter, and had highlights in his eyes. Starting in season two, the highlights were gone, but were sometimes shown in more Off Model moments and on DVD and video covers. In season four, his eyes got bigger, and in season five, he began getting taller and skinnier.
There were also subtle character design changes as well; sometimes Ren and Stimpy would look more like their normal, original incarnations, but now they look more off-model than before. Ren's ears got shorter, Stimpy's three head hairs longer. They looked a lot more fluid, but were coloured differently, with brighter, more violent colours.
Rugrats began very crudely drawn, with rather grotesque, angular, exaggerated shapes and muted colors. By the end of the series, it featured smoother lines and more realistic coloring.
The Simpsons has lampshaded its art evolution since the Tracey Ullman Show shorts a few times. In the episode "Lisa's Sax", the early shorts are described as "crudely-drawn filler material". These shorts were so poor because Matt Groening quickly sketched his character designs to serve as inspiration for the animators, but they copied the designs exactly.
The standards of art and design in The Simpsons were mostly tied to the budget. The initial animation team consisted of four men: Matt Groening, who did (very crude) sketches and storyboards, and just three animators at Klasky-Csupo. As the series progressed, and ratings remained high, the budget, the team, and its capabilities increased. When the Tracy Ullman Show finished, Fox wanted to spin off The Simpsons as a half-hour program series, and asked Groening to make a pilot. Not wanting to go to the effort of making a pilot, which might be shelved without ever being aired, Groening refused. If the Fox executives wouldn't approve a full season based on past performance, he said, he would rather leave them and go back to his popular Life in Hell strip. In the end, they compromised, agreeing on a half-length season of 13 episodes. When this season had aired with ratings success, they got a full budget for a full second season and were able to improve the animation qualify and spend time firming up the designs.
South Park: It began with "only show people from cardinal directions", and everyone shuffled around, never moving their legs. Although the art style has remained identical, now they can show a fairly wide range of movement. What's even more surprising is the fact that they can still make them in a couple days, due to switching from trying to "animate" static paper cut-outs to actual animations on Maya, one of the most powerful 3D animation programs in existence.
The show itself has acknowledged this a couple of times.
"Free Hat": Not in the plot itself, but features a fake commercial for a "digitally remastered" version of "Cartman Gets an Anal Probe" (the first episode), also featuring a Conspicuous CG spaceship and aliens, plot changes, and Imperial Walkers in the background (the whole thing is a parody of the Star Wars remakes). Notably, the animation style is a huge improvement over the original version...by being about on par with the style of "Free Hat" itself.
"A Very Crappy Christmas": One of the subplots features the kids making a cartoon about Christmas. They base the characters on themselves and do all the animation in construction paper. The joke is that the entire process is similar to how Trey and Matt first created the South Park characters to begin with, and the few scenes shown in the episode are lifted directly from their animated short "The Spirit of Christmas - Jesus vs. Frosty". Because the short is completely unaltered it ends up looking like a crude cartoon compared to the episode it appears in.
And compare the way animals looked in season 1 to how they look in season 11, quite a difference. The animals of the earlier seasons looked very cartoony and in the later seasons they look very realistic and lifelike.
The evolution in character designs is especially noticeable when you compare any new adult characters to old ones like Ned, Officer Barbrady, or Jimbo.
The depiction of celebrities has evolved from crude representations to more realistic caricatures. Compare the appearance of Geraldo Rivera in the first season to his appearance in the tenth season here.
The backgrounds are also a lot more detailed, with the buildings actually having perspective.
The first few seasons involved computer animation being used to replicate the same cardboard animation used for the pilot and earlier "Spirit of Christmas" shorts. This was because using actual stop-motion cardboard techniques took far too much time to make a syndicated series out of. Eventually, the novelty of attempting the old minimalist approach wore out, and the show took its original artistic style and just ran with it, forgoing restraints. Seasons 4 and 5 showed signs of going this route (lampshaded in the "4th Grade" intro sequence), with Season 7 being a notable turning point, beginning with a "repeat" of the pilot episode's opening scene using the show's current-at-the-time rendering techniques.
Sponge Bob Square Pants. Season one looks very different from the rest of the seasons. It is the only season that used hand-painted animation cels (beginning with season 2, the series switched to digital ink-and-paint), and the animation is noticeably more crude and inconsistent. Also, Patrick's appearance is slightly different.
The Spot the Dog animated series has went through this several of times. Between the first season of the 1985 series to the last season from 2000, Tom the Crocodile loses those menacing incisors and Helen the Hippo loses those eye sacs. And everyone changes to a lighter shade of color they were originally in. Although the changes itself could be reflecting the art evolution of the books, though.
Star Wars: The Clone Wars. The animation quality has steadily ramped up through the last 3 seasons, both in overall visual quality and in the movements of the characters (which have become less puppet-like as the seasons progressed), which was one of the main criticisms of the show at the outset.
Superjail! received a more fluid and expressive animation style with its second season, although fan reaction was mixed (to say the least), as some felt making the animation too smooth ruined the rougher look present with Augenblick's episodes. The lines also became bolder and color palettes seemed brightened, while the broader focus on story meant that the crew focused less on the wild scene transitions and used more traditional fades and cuts within some episodes. To relieve some of the criticism, the crew worked to reinstate more camera angles and transitions within season 3's episodes.
Teen Titans originally looked quite angular and cartoon-y, with simple backgrounds and rather basic coloring. It kept the same basic style, but subtly evolved to have smoother lines, much more detail, and more natural coloring.
Also, in the early episodes, Beast Boy disappeared just offscreen to transform into whichever animal he was going to use. In later episodes, Beast Boy's transformations were on-screen. It starts some point within the first season, very awesomely in "Apprentice" which for a freezeframe you got Beast Boy as half-ram and half-snake.
Occurred constantly during the last 3rd of the second Teenage Mutant Ninja Turtles series. After 5 seasons of consistent designs, the series' art style was dramatically changed for its season 6 retool, which resulted in simpler, more angular designs, and a less subtle color palette. Then, after the series was retooled again for it season 7, most of the regular characters were redesigned to look more like their movie counterparts—most notably, the turtles' eyes now had visible irises, even when they wore their masks—and the color palette became somewhat more dark. Afterwards, for the Grand Finale movie Turtles Forever, the art style was tweaked yet again, with the turtles returning to their season 6 models (albeit lightly modified) and with several other characters—particularly Splinter and Hun—getting slightly more complex shading.
The 80's cartoon also went through several changes in art throughout the series.
Tom and Jerry were originally drawn fairly realistically, especially Tom, who had realistic cat's claws and moved and acted much like a real cat. When Hannah-Barbera figured out that slapstick involving a realistic cat doesn't work so well, Tom became more anthropomorphic.
The Venture Bros. subtly but noticeably steps up the animation at the beginning of each new season.
Woody Woodpecker went through some of this. The early entries in the series were very crudely drawn, and Woody had a ghoulish avian design. By the 10th short, when Shamus Culhane took over direction, he simplified the backgrounds, and gave the series a more or less refined art style (as refined as that low budget Walter Lantz studio could get, anyways) that would continue changing from there on out.
From the jump from webseries to television cartoon, Making Fiends got noticeably cleaned up. The backgrounds are a lot more detailed, the characters are no longer scribbly, and background characters are given brighter colors. For a comparison here's one of the protagonists house◊ in the web cartoon and TV cartoon.
Despite being made by the same producers behind The Simpsons, whose animation had been perfected in the course of over a decade, Futurama started out looking alarmingly similar to the early days of its sister show. Fry's distorted, squished face when he says, "Michelle! Baby!" in the pilot episode is a good example. Luckily, the staff were able to get this under control in just a few episodes. Later seasons would up the artistic ante by having more detailed environments and more complex scenes, especially in the post-Fox era when the show switched to High Definition and took on more serioussituations.
The animation on Regular Show grew smoother over time. The evolution is most evident on "First Day", which is an extension of the original Cartoonstitute pilot — new scenes were drawn in to extend it from seven minutes to 11, and these contrast strongly with the original.
Pepper Ann switched to digital coloring from cels in its second season.
Doug had more rubbery and crude animation during its first season (and the pilot was done in Squigglevision). Season 2 smoothed it out a little more. When the show switched to Disney, the colors were more vibrant.
Many of the character designs were refined in Season 2 of The Amazing World of Gumball. This is most noticeable in the designs of the main characters' eyes, but other significant redesigns include Penny's body shape being more curved like an actual peanut (and how her mother and sister appeared in the first season) while Miss Simian's limbs and body are far less emaciated.
Transformers Prime started off with some wonderfully detailed character models, especially the robots, but was a bit more lacking in the environments and having anything close to filling out a human-centric scene (the same three background characters are seen at the local high school every time). Lastly there was a lot of recycling of locations, almost every fight took place in a desert canyon of some sort (or arctic canyon). Season two immediately started opening things up with a trip to Cybertron and from there continued to offer a new variety of locations and better detailing to others (like some foliage surrounding desert canyons).
The Terrytoons character Gandy Goose first appeared black-and-white with a very 1930s-ish rubber-hose design (see "G-Man Jitters"). After Executive Meddling forced the studio to beef up their quality and switch to Technicolor, Gandy was given a redesign to how we know him today.
Mighty Mouse looked very generic under his original name Super Mouse, and his costume was red and blue, very similar to Superman. "The Wreck Of The Hesperus" began a slow refinement to the more recognizable Mighty Mouse, with his costume now yellow body suit with red shorts (the banned cartoon "Eliza On The Ice" had him in a red body suit and yellow shorts).
The Fleischer Studios shorts from 1929 to mid-1930 looked very crude from the usual style. There were no gray shades, the characters looked stiff and the animation was somewhat limited. It wasn't until animators like Grim Natwick and Shamus Culhane arrived that the animation vastly improved, gray shades were added, the backgrounds had more dimension and the animation moved in the studio's trademark fluid and surreal fashion.
The Powerpuff Girls first appeared in their pilot shorts as very rubbery and stretchy (their Whoopass Girls shirt had them a little more squat). This was refined when the series started in 1998, giving their eyes more circular focus and making them look a little shorter in contrast. When the movie was greenlit, the girls were given shorter bodies and larger eyes; the Professor's face was more thin than it was, and Mojo Jojo was given a longer chin and pointed ears (as where his ears were more circular before).
The Looney Tunes Show: In the second season, the characters' outlines were colored and now look a little more like how they appeared in the classic shorts.
The Hero Factory mini-movies changed their animation studio for the episode Invasion from Below from Tinseltown Toons to Ghost VFX. Although the design of the Heroes has changed throughout the specials (seeing as each one had to advertise new toys), the style stayed consistent. But for this episode, the CGI's look became less rugged and more clean, with the Heroes receiving creepily smooth faces and human-like eyes with dark pupils in place of their glowing, robotic pupils.