Monday, February 15, 2010

Light and Form, Part 1

Light striking a geometric solid such as a sphere or a cube creates an orderly and predictable series of tones. Learning to identify these tones and to place them in their proper relationship is one of the keys to achieving a look of solidity.

The form principle is the analysis of nature in terms of geometrical solids, which can be rendered according to laws of tonal contrast.

Modeling Factors
The photograph above shows a sphere in direct sunlight. It has a distinct set of tonal steps from light to shadow, known as modeling factors.

In direct sunlight, there’s a strong division of light and shade. The light side includes the light and dark halftones, the center light, and the highlight. The center light is the point at which the light rays strike the form most vertically. The highlight is the point where, in a shiny surface, we see a reflection of the light source.

Note that the center light and highlight are not at the same location. The Terminator The terminator is the area where the form transitions from light into shadow. It occurs where the light rays from the source are tangent to the edge of the form. If the source is soft and indirect, the transition from light to shadow at the terminator will be more gradual. The form shadow begins just beyond the terminator.

To test which areas are in light and which are in shadow, you can cast a shadow with a pencil on the object. The cast shadow will show up only on the lighted side, not on the shadow side.


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Light and Form, Part 2
Light and Form, Part 3
You also might be interested in these posts:
Occlusion shadows
Reflected light.
More about all this in my book: Color and Light: A Guide for the Realist Painter

18 comments:

William R. Moore said...

James, what are your thoughts on the often stated: "Warm light has cool shadows and cool light has warm shadows" ? The demo picture ( on my screen) appears to have cool light and warm shadows. (Like light from the sky... cool light/ warm shadows) The description says direct sunlight... warm light/cool shadows.

DavidStill said...

I thought the highlight was the point that is most towards the light source? But on this picture the light seems to come directly from the right, and not from the right and a bit toward us. How can you determine where the highlight will be if it is not the point most towards the light source?

Kent M said...

DavidStill: reread the original post - you are referring to the 'center light' as the highlight.
The 'highlight' referred to is sometimes also called a 'specular highlight' and is basically a reflected image (a poor image if the surface is not glossy) of the light source. Like any reflection, the image will appear on the object where the angle the light hits the surface equals the angle of the viewer in relation to the surface.

Frank said...

If I may be so bold, I think this diagram is overly complicated.

In essence, there's just shadow, light, and reflected light. There's really no such thing as a shadow's "core," since that only exists because of the presence of reflected light. I'm simplifying it down quite a bit, but thinking of it in this way greatly facilitated my understanding. What do you think, James?

Daroo said...

Frank -- I agree except I find the core concept useful. Reflected "light" should be grouped with the shadow family set of values (with a clear separation from the values in the light -- especially in this high con example). Because it contains the term "light" -- I would tend to push it to be equal with the light side.

Also once you identify the core, finding the halftones and keeping them grouped with the light values becomes easier.

Tyler J said...

I think Kent nailed the reason for the highlight confusion. To add to his description, the issue here is that the surface is rounded and so the image of the sun is distorted.

Think of a mirror ball and how the reflected image wraps around the surface. There is something counter intuitive about to me, but I am still working on the whole "seeing" part of art.

I agree with Frank and Daroo that simple is usually better, especially when thinking of the fundamentals. However, there are various levels of understanding by the readers of this blog.

Reflected light is a great example. Most artist initially do not include reflected light in their work. Hence most efforts have that cartoon or comic abstraction quality (which also has its merit).

But once reflected light is accounted for, it brings the level of believability and/or realism up a notch. And when the reflected light accounts for the color shift based on what surface is bouncing from, then that adds another level of sophistication.

To back up a bit, I wonder if the more abstract, one-tone shadow is how our brain processes light information in a quick, once-pass way? Sort of a survival method of getting the most visual information in the fastest way possible. If so, it would make sense that everyone could understand this method of handling shadows.

Anyway, good post and I look forward to more of the light-shadow sneak peaks =)

The fearless threader said...

Thanks for this, I have started on the fisrt steps of my journey in paint and I'm struggling with shading and shadow in paint. This has helped me enormously. I look forward to tomorrow's post.

Mario said...

The terminator is called by Loomis the "hump". This name suggests that it's actually the darkest part of the shadow (at least locally).

Richard J. Luschek II said...

One of the things in my training that I thought very helpful was the simplification of this separation of light and shadow. I agree with Frank, except to say that even reflected light is often overstated. We were taught that shadows are flat, a pretty Boston School thing. Gammell apparently used to say "Shadows are flat, flat as a hat, flatter than that". He was referring to black top hats in this little funny quote- I guess they were flat.
Point was that form happens in the light side. Shadows are for atmosphere, so they are understated and not looked into, not rendered with form and full variety of value. All the work is done in the light. So a Boston school diagram would have the shadow from the terminator- what we call the bed bug line, move through the cast shadow. That could all be shaded as one unit, maybe getting darker as needed, but essentially flat. We were taught to look at shadows, not into them- if that makes sense.
I would be interested in your thoughts on this.

Daroo said...

To clarify my earlier, unclear point:
"Because it (reflected light) contains the term "light" -- I would tend to push it to be equal with the light side -- which is wrong. This bad habit can be solved by squinting down and not staring into the shadows for long periods of time (they will appear to become lighter as your pupils dilate). Squinting and quick glances between shadow and light to compare their relative values will yield a more accurate result.

Will said...

That's neat that you should post about this because that's exactly what I'm studying this semester in my Analysis of Form class. Light & shadow is an amazing study, and using it well is the best way to draw & paint realistically.

Don Cox said...

The terminator could be the darkest part of the shadow if the shadow is lit by light reflected back from nearby surfaces.

But usually the very darkest part is the "crack" where the ball touches the ground.

As for "warm highlights, cool shadows" - it depends on the colour of the main light, and on the colour of the light that is lighting the shadow. In warm sunlight under a blue sky, there will be warm highlights and cool shadows. In a North light studio, the main light bcomes from the blue sky, so there you have cool highlights. The shadows are lit by warm light from the studio (brown or cream) walls.

On the moon, you have white highlights and black shadows.

jeff jordan said...

One of the few advantages I can think of in being nearsighted is that I just have to step back a foot and leave the glasses. Good for tonal shifts.

James Gurney said...

Thanks for all these great comments. I was a little nervous introducing such basic "textbook" stuff, but I had a feeling that each of us would have a slightly different take on everything.

I agree with many of you that shadows should generally be handled simply and that reflected light is often overstated. But in outdoor conditions especially, I like to think of the shadow side not just in terms of the absence of light, but rather as a region governed by other secondary sources (as we'll see in the next post).

The core, or hump, or accent, as it's variously called, is usually in my understanding a little past the terminator. But it's doesn't always appear on cue, depending on the conditions, and while its use in comics can be a useful shorthand, the artist realizes that it's a convention that does not always appear in reality.

I think you all explained the difference between the highlight and the center light well. They're almost never the same thing (unless the light is coming right over your shoulder). A good way to think of a specular highlight is that it's at the angle a pocket mirror would have to be to reflect the light source.

James Gurney said...

Richard--to answer your question about flat shadows, I think the philosophy you describe serves most cases.

But there were plenty of Boston painters like Paxton who featured modeling in shadows, especially in a toplit or backlit setting--say a woman in a white dress with the light coming toward us. There the subject really becomes the modeling of the shadows and the contrast of warm and cool effects, sometimes at equal value.

So my response to a rule like that would be...."it all depends."

DavidStill said...

Ah, thank you, I understand now. I think I'm more used to calling the center light highlight, and the highlight reflection. It all makes sense now.

DavidStill said...

also, on the question on rendering/flattening the shadows. My understanding is that as a rule of thumb, you can say that you should either model the lights, and flatten the shadows, or vice versa, but modeling both leads to over-modeling.

James Gurney said...

David, yes, I think that's a good rule of thumb to follow.

You can also do careful modeling within a very narrow tonal range in either the lights or the shadows, or you can turn a form with hue alone, but I'll get into that in another post.