09/23/2012 UPDATE: Since I wrote this post, it has come to my attention that some are coming away with the lesson that I am recommending "zero tolerance" for any tangent under any condition.
This is not true. I regret some of the wording that leads to this conclusion. I have no plan to re-edit the text below, but let this disclaimer serve as reminder that no "rules" are absolute.
As Martin Fagan mentions in the comments of this very post, key aspects of the design of THE SECRET OF KELLS movie were based entirely on tangents and they are very well worked out. In a particularly stylized design system, intelligent tangents are not only possible but necessary and desirable. The operative word is intelligent. Very linear, very geometric stylization can tolerate tangents galore.
The SCOOBY and FLINTSTONE examples here are used because the kind of drawing is not particularly stylized and the tangents are clumsily done. They're supposed to be volumetric (in a sort of default generic way) which in and of itself isn't bad but the awkward collisions of various shapes and lines are all the more conspicuous because of it.
Friend, colleague and teacher MARK KENNEDY has done a series of art instruction refresher posts lately under the heading "A kick in the head" parts one, two and three on his essential " TEMPLE OF THE 7 GOLDEN CAMELS" blog.
Mark's first post concerns, fittingly enough, silhouette: probably the single most important of all art rules. I highly recommend it. He covers the subject so thoroughly I don't have much to add other than the anecdotal remark that I don't ever recall hearing anyone stress it's importance until my first week as a professional, back in Eric Larson's animation training course at Disney. As far as I can recall, even Preston Blair fails to mention it in his first ANIMATION book (he gets around to it in the second) and I don't even remember it being brought up much in art school, though we did cover negative space quite a bit, which is a roundabout way of thinking of silhouette.
Silhouette is one of those rules that is so important, so fundamental, that it isn't just important to animation: it is essential to clarity in all graphic art, including painting, sculpture and photography. I confess like many "bullheads" when I first heard about it: I immediately questioned whether it was 100 per cent valid in every circumstance. After all, when we are animating, we will have to draw many angles of a figure and many phases of action. True enough, but for that reason, it is important to remember to keep silhouette in mind for all key poses, unless the angle and composition forbids it. If you need (or want) to make an exception to this rule, you better know what you are doing and follow through intelligently.
To put it more simply:
If you can put something in silhouette, you should.
It invariably makes it read better. If it is impossible, make sure that your overlapping forms have good silhouette within the contour of the whole figure.
Allow me now, with Mark's permission, to sidetrack onto the related subject of tangents.
If silhouette is desirable, tangents are not. Tangents occur when lines or shapes begin to collide hap-hazardly, usually right on the contours they are describing.
Here's the Oxford definition of a tangent:
"a straight line or plane that touches a curve or curved surface at a point, but if extended does not cross it at that point."
I also expand the definition to include shapes that collide awkwardly and uneccesarilly, almost invariably compromising silhouette.
To illustrate, here are some examples from a glossy catalog of a Hanna-Barbera museum show that toured in the 1990's. First up: a classic image of the SCOOBY DOO cast on the run.
Never mind that I hate this show and the characters, let's note that there is an incredibly noticeable tangent at the point "Shaggy's" nose encounters the back of "Velma's" head. Forgivable one might say, since this is derived from a cycle of animation containing five characters all running.
Well, yes and no. While it would not be a crime in a single transitional frame of animation, as a key, it is less forgivable, and as a piece of art clearly cleaned up and codified as an icon, it just doesn't make sense. Someone either didn't know enough or care enough to rectify it. These are the sort of errors that always push H-B into the second rate category, even when they are trying to be first class.
In any event, as a still image, and even as a key, it would be better if there were any perceptible "air" between his nose and her head, or, failing that, if these two forms overlapped each other definitively, even if only slightly.
The area where "Fred's" arm encounters "Daphne's" torso is also awkward and technically a tangent, though less obvious than the one just observed, but again, in these circumstances, hard to rationalize.
Ironically, this complicated little area between "Scooby's" 'up' legs and the figures behind him has been relatively intelligently worked out. Also, For clarity's sake, someone has either wisely or accidentally kept a few legs from breaking Scooby's silhouette any more than necessary, although it's a toss up who's leg that is back there. Someone has decided to paint it as the "Daphne's" leg, but it looks more like "Velma's".
Also, this area, where "Scooby's" tail drags along the same line as "Shaggy's" leg is nearly a tangent, but avoids tangenting by slight degrees, which is all it takes to rectify such errors.
The fact that the tangent rule has been so haphazardly applied is something both to ponder and also to be avoided. Similar to the rule of thumb for silhouette:
If you can eliminate a tangent, do it.If your tangents are intentional (or otherwise unavoidable), strive to make them as elegant as possible (or at least inconspicuous). (added 01/04/13)
Here's another example, from the same catalog: a group shot of the FLINTSTONES, characters I love, but from some crappy TV special done decades after the characters lost all spark. Probably the single worst tangent here is where "Wilma's mom's" hair intersects "Betty's" eye and collides with her pupil. Given that this is a slick cell setup, signed, published and probably sold at a premium price, this is just sloppy and stupid, beyond the weakness of even the artwork.
Likewise is the improbable way "Dino's" nose slips behind "Mom's" arm, despite the fact he is sitting clearly in front of her. Probably just an error of stacking or registration, but it is not the kind of error you want in an image that's supposed to be any kind of icon.
I'll have a little more to add about this image next time.