Palatino 766 pipes, pools, cliffs, streams... (by Taccola)

[Jorge_Stolfi]

Sometimes [taccola's] application of this cast shadow is not very elegant, and that's where it gets interesting. He will follow the outside border of the object that's casting the shadow, almost drawing like parallel hairs on it. This is a very specific technique (or misuse of one) that people would not use spontaneously.... See the pillar held by the ship: it is "hairy" all around.

The most common function of such hatching is not to depict shadows, but to indicate depth.  The hairs along a line indicate that it is the silhouette of an object, and show which side of it is the background.  That device does rely on our eyebrain's use of shadows to deduce the 3d dimension; but it is exploited locally, around each point of the silhouette.  Thus it does not have to be consistent all around, nor consistent with the presumed direction of the light in the scene.  

Even though we see colors with three color coordinates, at a very early stage -- starting just below the retina -- those thee coordinates are remapped to two "chroma" coordinates, that tell the hue and saturation of the color, and one "brightness" coordinate. Those values are converted to log-like scales, and the brightness is then subtracted from the chroma values; so that a red apple will seem to have the same color whether it is in full sunlight or in the shade.  Meanwhile the gradient (spatial derivatives) of the brightness are computed and sent to the brain.  The recognition of shapes, including letters, uses only this signal, ignoring chroma.

This mechanism has a number of interesting consequences.  Like, it explains why we have names for hue/saturation combinations like orange and pink, but not for shades of gray; and why one cannot tell whether two parts of an image are the same level of gray unless they are right next to each other -- in which case we can easily notice a difference of 1% in brightness.  And it explains why we can recognize people and spoons and armadillos in pen drawings, like cartoons, even if they are painted blue or unpainted.   And how an artist can create the illusion of a light-emitting lamp or moon simply by painting a darker halo around it.  And why one should not use yellow letters on white background, or blue letters on black background -- even though, mathematically, those color pairs should be as distinct as green on black.

All the best, --stolfi