It’s a good thing that people get passionate about stylized looks visually – by viewing animations or imagining how an illustration would look should it come to life. Its a good thing, because it would be difficult to arouse interest using the word used to describe this undertaking: non-photorealistic rendering or “NPR”.
A graphics researcher is likely to interpret “NPR” by the mechanisms with which traditional techniques of image creation such as oil painting, watercolors, charcoal, etc. can be replicated. This is already a wide and varied field, concerned with simulation of pigment dispersion, artistic input or analogy-based replication of specific painting styles. The intention is both scientific (to propose and confirm underlying principles) and practical (to the degree in which these principals can be tested in a proof-of-concept).
zelda / William Kentridge / Pixar "Cars"
An artist is likely to understand “NPR” as a sketchy line rendered from a 3D object, or – more often than not – cel-shading with clean color gradients as in the image of Zelda above. The more technical artist will argue that the slick look that Pixar achieves in its films is not “NPR” – but rather a stylized realism, and argue that the surfaces and contours rely on physically correct models of light in determining their properties. The less technical (or ‘traditional’) animator may even be thinking of stop-motion or claymation when we mention 3D, and may well have National Public Radio in mind when we speak of NPR.
A producer – who has every reason to be attentive to a film’s look – is likely to draw a complete blank at the term – and otherwise have a dangerously limited vocabulary to discuss what a ‘look’ is or isn’t. Worst case, he or she will rely on empty verbiage like “the style we want is photorealism” or “we want it to look like Pixar”. So… what is non-photorealistic rendering?
Cel Shading – a case history
Cel-shading is a good example of the issues involved in defining and developing NPR looks and tools. A cel is a transparent sheet of acetate. Once upon a time, artists would ink in or xerograph frames of animation on cels, then paint in color fields on the reverse side. These would be then laid over background plates using register marks and photographed. Subsequent cels would be replaced for the subsequent frames. While hand-drawn animation, tweening and clean-up is still done on paper, the coloring process once done on cels has long since been replaced by digital processes.
And its easy to see why: thousands of individual images are required for an animated film – even a ten-minute film on twos (12 fps) consists of 7,200 individual frames. The cel technique lived and died beneath the forces of efficiency. The experienced animator is difficult to replace, and so has some weight in deciding whether paper or digital tools are used. Also, the increase in efficiency by digital drawing is mostly due to digitalization processes, so if you already have a scanning pipeline set-up that’s not such a big argument. But the coloring assistant is quickly trained and the production speed is increased dramatically with digital tools. So the cel technique has disppeared.
Cel animation was a technique – not a style.
Typical of NPR, we now refer to a technique as a style and do our best to replicate something we haven’t fully analyzed. The cel technique supported a wide variety of looks, from Bambi and Bugs Bunny to Akira. As Yuri Norstein showed, it could be combined with cut-out animation and stop-motion techniques to create lush, personal visions. The results of the techique were as varied as the artistic visions employing it, yet marked by the materials, tools and intentions that defined it.
We might characterize cel animation as:
- animated figures made up of clear shapes, each made up of one key color
- an outline of (more or less) clean ink lines
- still background plates (perhaps panned or zoomed)
- additional effects via layers of cels
The backgrounds generally have a higher level of detail and are more painterly in quality, as the artist can take more time with them – they appear throughout a larger number of frames. The shapes are sometimes given more form with additional tonal values alluding to shadow and highlight, which reflect the position of a light source and the represented surface information.
In the image above (from Ghost in the Shell), for example, the shapes read as certian materials with very little textural information: stretched, jagged forms read as hair (long, bunched fibers and anisotropic highlights) and a pink shape with a tight, round specular reads as lips. The shapes of shadow and highlight have little to do with the form of an actual contoured head… they deliver just enough clues to be intuitively read by the viewer.
Most NPR looks have this in common: material attributes are not portrayed directly, but rather communicated with the use of the stylistic means available. Hair is not portrayed fiber for fiber, but in the careful exageration of specific qualities and the reduction / exclusion of others.
Then there are the temporal qualities:
- animated figures have flexible frame rates according to the required motion (rapid movements can be on ones, normal motion on twos, and long holds can be held for a longer number of frames)
- looped elements can be used to keep still character from “freezing” (ie. lines boil, or hair sways in wind)
- motion blur is sometimes faked with brushed lines, but more generally incorporated into the character’s shapes via stretching and deformation… smearing effects such as in charcoal animation aren’t supported by the dried pigments.
- layers might be animated to create parallax effects (moving clouds, etc.)
(Piglet / Boris Dezhkin)
And production concerns:
- the artistic process is highly segmented, with both highly skilled animators and layout artists and less skilled assistants assuming more tedious processes
- more detailed characters are generally associated with reduced temporal animation
- shadows from the lights are cast during photography, particularly if more than one cel are used
- in hybrid animation methods, the cels themselves might be seen…
A closer look at the inspiration reveals further characteristics that we may or may not wish to emulate. So the first step is to determine what goals the look needs to fulfill: production targets, compatibility between visual elements, etc. Gradually, new possibilities stretch this “style” beyond its inspiration as the remnants of the original technique give way to expanded artistic goals.
On to “cel shading” in 3D…
Here are two impressive examples of ‘cel shaded’ 3D art. (IronMan fan art / adam baroody ) These images make evident how much stylistic potential NPR looks can have. They also reveal how many issues can be taken for granted. Here, the contour defining shadows is much busier, as it correctly (and thus incorrectly) follows the volume of the represented form. High-interest areas such as the eyes are treated with much the same rules as broader, lower-interest areas such as the body chest. The viewer is presented with an image without hierarchy, as the artistic processes of exclusion and reduction in service to composition have been neglected. Yet the result is promising, and the production efficiency undeniable.
Adam Baroody’s test above is interesting in that he has taken imagery generated from 3D to use in 2D filter processes (Photoshop). In so doing, he was able to create more convincing line qualities which were unattainable directly from 3D geometry. I highly recommend visiting his tutorial. What I find fascinating in NPR look development is how consistently “correct” can look wrong.
Next: the rights of wronged perspective.