Questions on Form and Light

[yangdaniel027]

Hey guys, I'm realizing that I never really went into much depth when studying light/form and now my lack of understanding is coming back to bite me. Anyways I'm really struggling to apply the form principle when it comes to more complex objects such as faces, and in general finding where the separation of lights and darks occur.

I'm attaching two images here, one for reference and the other marked with areas where I'm confused as to whats going on. I don't have the greatest handwriting, and so I'll also just write out my questions for clarity.

Are the lines I marked out down the nose, below his eyes, and cheeks where the terminator is? Would the corresponding section afterwards would be the form shadow? Would the completely black areas be cast shadows or occlusion shadows? Is his forehead in light or shadow since it seems to be a gradation? How would I describe the left side of his face since it's in shadow but a small amount of light is still hitting it? Would the left side be considered in light or shadow?

I apologize if I've complicated this but it's been frustrating me for the past few days; to the point I'm literally asking myself, man the hell is going on here? In any case, anything that would help me understand this better would be appreciated :)

[OtherMuzz]

First off, this is a terrible image to study, it has way too much going on, is an extreme crop, and has multiple materials and textures in it. There is a reason why we study casts and statues instead of photos like this.



This photo is far more understandable to a student.

What you need to wrap your head around is the idea that a cast/core shadow exists any-time the form is 90deg or more from the light source, and everything else is lit. Within the lit area it is way more complicated, but the best way to explain it is that every angle will have a different value associated with it.

If it is perpendicular with the ray, it is fully lit, if it is at an angle, then it is partially lit. Lamberts cosine law is the mathematical equivalent of what you are trying to learn. The easiest way to express that is cos(angle) where the output is a percentage between 0-1.

I've found the best way to hammer these things into your brain is subdividing activities, like making a box out of value, and then subdividing it, using a fully opaque brush. Like this.

[KurtJeremy]

Othermuzz is spot on with simplifying the Lighting process.
as for your question, I made this overpainting, Theres a proko video about Light and shadow on YT thats pretty much where I got this from.

The difference with Occlusion and cast shadows when observing from life are open to interpretation. but they mean quite different things, with Occlusion shadows being more of a CG term rather than painting.






The forehead will be considered lit. you just need to find the line of the shadow( terminator) ,again , referencing from your picture it can be rather hard to decipher which one is which.

[AlfonsoX]

This is one of the best reads about light.
http://www.itchy-animation.co.uk/light.htm

Maybe try not to think about the shadow but what is lighting that particular surface.

Light will almost never come from a single point but from a full sphere around us.
On a given point on a given surface an hemisphere is launching photons to that single point. 
or
Imagine you have a very litle camera wide angle fish lens 180, then place it on where you want to shade facing out. You will see a full scene throwing light at that little camera.

Yes the sun is a dot but you can never isolate its light. You will always have light bouncing everywhere, on the floor, on the wall, on the clouds, on the same object surface.

For example "occlusion" its just a little cave that its having that hemisphere being reduced so its get darker.
Technicaly the hemisphere is still there but the surface that wrapds that hemisphere its just darker.

Casted shadow just means the main strongest light is being cut but it will still be lighted by a bunch of things around.

The names cast, form, oclusion, terminator. In general refer to a single point of light wich is something impossible on itself.
They are there just so we can talk about and explain things but are abstractions.

Im sorry if a make anyone more confused. Its just my way of seeing it.

At least read that link if you havent already. Its the best.
http://www.itchy-animation.co.uk/light.htm

[OtherMuzz]

Alfonso, sorry to do this, but i think your answer is just a distraction from the information YangDaniel needs.

We are talking specifically about turning form with light, which is something you can only answer by focusing specifically on that issue. My problem with things like the link you provided, is they don't go into the form value relationship at all, and instead it just becomes a laundry list of observations about the world, not explanations. Hell that first ball illustration is inaccurate to the point of it being useless.

Almost 100% of the time peoples issues with describing form with light isn't if they are considering ambient light and ambient or not, it comes from simple light plane form turning.

I think you should go back and read my post talking about Lamberts Cosine Law (which was a link btw).

[yangdaniel027]

Thanks guys for your responses, all quite helpful actually. Thinking about light in terms of planes makes sense.

[OtherMuzz]

This tutorial was released the other day actually, and is really solid. http://www.fundamentalsofpainting.com/li...g-problem/