Light Logic

Light Logic is the term Betty Edwards uses in her book The New Drawing on the Right Side of the Brain?. (seen here) ("The New" is there to make sure everyone knows it's the third edition of the book.)

Light Logic, or how light interacts with objects, has four elements according to Ms. Edwards. However, other people, like my ART125 Tone, Color, Comp teacher teacher Rob Kmiec at DigiPen? say that there's nine elements to the classical order of light.

I (Stickman) believe that light logic, or "The Classical Order of Light" as Rob Kmiec calls it, is best simplified into five sections. Here they are, from lightest to darkest. The numbers correspond with the above picture.

1. Highlights?. This is the brightest spot. The brightest spot is where light hits perpendicular to the object. (Perpendicular is the opposite of parallel.)

2. Local Value?. This is the normal value? the object would have if it was under ambient light?.

3. Crest Shadows?. The shadows that the object casts on itself.

4. Reflected Light?. Light hits other things and bounces back. Reflected light, in my opinion and that of Rob Kmiec, is one of the key things to making something look three dimensional.

5. Cast Shadows?. The cast shadows are always the darkest part. These are the shadows that the object cuts out of the light that's shining, either on the ground or on other objects.

If you do highlights properly, you're going to end up adding value to almost the entire figure (or even image), except for the highlight spots. The easiest way to do that is to set a ground?. For example, take a stick of graphite and apply it all over the page, then smear it evenly. This creates a "base" or "ground?." Then you draw the crest shadows, and the cast shadows, and erase the highlights. The value of the ground will determine what you draw and what you erase. If a ground was dark enough you would have to erase everything except the cast shadows. At that point it would be better to buy black paper and draw with a white medium.

As such, it's probably easier just to leave the highlights out of it and stick to cast and crest shadows.

Then we meet the matter of making cast shadows darker than crest shadows. The easiest way to deal with this, I believe, is logically. Let's hop back and take a look at the threshold of an absolute scale in simultaneous contrast.

In an absolute scale, such as a black to white value scale?, differences of less than 20% of the scale are suppressed by the retina of the eye. Differences of greater than 20% are amplified. So to create the appearance of difference between the cast and crest shadows, we should have a difference of at least 20% of the value scale we're using for our drawing (generally our scale would be: as white as the paper to as dark as our graphite?/medium).

How talented are you at creating a value scale? If you make a value scale of five values (including black and white) they will be 20% differences. That's right on the contrast threshold. But what if you mess up and go under that 20%? You don't get the intended effect. But drop the value count to four and you get 25% differences on the scale. That's the white of the paper, the darkest graphite possible, and two grays in between. It's also much easier to play with 25% differences than 20% differences.

If you use those four values as your bases -- highlight, reflected light, crest shadow, cast shadow, then things pop out a lot better as to what they're supposed to be.

But like everything else, light logic is just a tool. You can pick and choose how much of what parts you want to use to get the desired effect. For the truely lazy, you can get away with local value, crest shadow, and reflected light. Those three values are the minimum you need to create a three dimensional illusion.