Thread: Fundamentals of light - Demo

1. Fundamentals of light - Demo

Hi Ca !

Light is the most basic thing of all. It makes us all able to see and allows us to create.
For us as artists and draughtsmen understanding the basic fundamentals of how light interacts
with form is crucial. Although there are already many sources that explain these principles, like
Tony Ryder's book, I did not know of any here on the site. So I thought I would do a little ball
exercise to refresh the things I learned and at the same time share it and hope that people will find this small contribution usefull.
I painted this in Photoshop without an actual ball to reference from, but I hope it will still be clear:
Attachment 272590

1. Direct light. In an indoor situation with natural light ,light hits your
subject in straight rays. But the light that hits your subject always has a
certain angle and direction. The pitch of the terminator will tell you what the
direction of the light is and the amount of available light on your subject will
tell you about it's angle. Proximity and orientation are 2 tools by which we
determine how light or how dark something will be in the light mass.
Proximity : is the distance from the subject to the light source. Light
diminishes in strength as it travels a certain distance. the further it travels the weaker it becomes.
So something further away from the light source will be less brightly lit then something closer by.
Orientation : is the pitch or angle of the plane relative to the light
source. On a round form such as the sphere, the plane changes constantly
and gradually, so when it turns away from the light source it will go darker, if it turns towards it it will get brighter = form light.

2. The light mass is the part of your subject that is lit before it falls into shadow.
Most of the light you see on matte surfaces is form light. on shiny
surfaces you will see less form light and stronger highlights.

3. The shadow mass is the part of your subject where no direct light
falls onto. The part that is in shadow.

4. Terminator, not to be confused with Arnold. Is the part on your
subject that receives the least amount of light. Think of the terminator or
casting edge as points of tangents instead of a line. The rays that hit the
subject are tangent to the rounded surface of the sphere. It's the part where
form turns from the light into the shadow. From dusk into nighttime.

5, 6. For our purposes it's useful to split up form light in lighter light
and dark light. On organic forms you will see that they're both
interlocking and wedge shaped. On our ball we see more lighter light spreading
across the top of the form and consequently more dark light spreading across
the bottom of the form. ( I could have painted this part better though )

7. Highlights are places where the surface of the object reflects the
light in only 1 direction. The angle in which it bounces towards you is equal and
opposite the angle it comes in.
Therefore, the placement of the highlights depends on where you are relative
to the object. If you move, the highlights will travel with you. test it out if you like !
So when modelling form, never round up to a highlight ! They can sit anywhere in the form light.

8. The cast shadow is produced by the blocking of the light by the
subject. It casts a shadow onto another plane. They always fall on planes
that are angled towards the light source. If that plane changes, the cast

9. Reflective light is the amount of light that gets bounced back into
the shadow from a plane that's in the light. It follows the same rules as the
direct light, only it's much weaker. And the direction might sometimes be hard
to determine because often many planes reflect light back into the shadow.
The tonal range in the shadow is under normal conditions much closer but the
progression of values acts in the same way as in the light side. Gradually
getting lighter as it rounds from the terminator towards the secondary light
source ( the reflected light ). And shaped like the LL and DL but in this instance

10. Dark accents ( not very noticeble in my painting ) are places of concentrated shadow.

I will edit this thread if I think of something important to add or if people have other suggestions.

Tom
Last edited by Art_Addict; January 4th, 2008 at 07:04 PM.

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I especially like the way you haven't placed the highlight at the point directly facing the light source, which a lot of people do.

Be very careful to make all of these elements consistent with a single light source position. Your highlight position and "lighter light" shape are consistent with a light source coming from the direction you indicate but close to side-on to the sphere, whereas the cast shadow, by running somewhat away from us, implies a light source further back over our right shoulder. The path of your terminator needs to be a bit more distinct for me to say exactly what direction it indicates.

"Reflected light" is often shown as you have indicated here, but would have a rather different shape in this setup, and would include a specular as well as a diffuse component. I've gone into this in some detail here (hidden away on my Dimensions of Colour site):

http://www.huevaluechroma.com/021.php
http://www.huevaluechroma.com/022.php
http://www.huevaluechroma.com/023.php

Here are a couple of relevant illustrations from the site:

5. Briggsy ,

Thanks a lot for the pointers. Congratulations on the site btw, looks fantastic!
I should have drawn the ball from life as to avoid the mistakes, stupid.
In time I will change the picture making it more true to life.

Cheers
Tom

6. Hey David, how about doing a similar set-up with a matte ball, for comparison?

7. Originally Posted by Elwell
Hey David, how about doing a similar set-up with a matte ball, for comparison?
That would be very nice.

8. Well, I bet you Art_Addict could paint that now every bit as well as I could, and it is his thread. Feel like giving it a shot my friend?

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