Imagination is 360 degrees… from life, the light & shadow is fixed.
Try drawing WITHOUT lines. Any lines. At least 200-500 sketches. Pure light and shadow. If you start to freak out, mentally? Do some in pointalism, like old newsprint. Light and shadow alone create tremendously strong lines, all on their lonesome.
Then add lines back in… and prepare to be blown away.
When I’m drawing a very structured object, such as, a human head or complex animal structure, such as a horse, even when drawn entirely from my imagination, I will often begin with a few light guide lines. This helps me to establish the object’s general perspective, size and placement on the paper. Then I’ll often tentatively begin to lightly lay-in the shadow shapes - sometimes using the side of a charcoal stick, if not, a series of hatch-lines. But these lines I use for structured objects. For freer works I would prefer to start with tone.
The key difficulty I’ve had when drawing shadows and light from my imagination has been in finding the terminator edge, which is basically the division between the lit-side and shadow-side of the form. Without this division the form will appear flat, ‘child like’ and lacking in 3-dimensional effect.
The problem for me is that, the imagined imagery within my mind always seems to be lacking any recognizable division between this lit-side and shadow-side of the form. My mind will simply ignore both the light and shadow patterns, as well, as the light source itself.
Perhaps, my difficulty in finding the terminator edge has something to do with the principles of Gestalt. My mind might be ignoring this splitting of the object’s light and shadow pattern while, at the same time, trying to fill-in the darkened obscure areas with more recognizable imaginary. In other words, my subconscious might be omitting this division of light and shadow in an effort to more clearly visualize this imaginary object in its entirety.
I’ve noticed that I have very little difficulty finding the terminators when working from a life-model - as you’ve mentioned earlier, these light and shadow patterns are ‘fixed’ and already determined for me.
In my past, some people had mistakenly assumed that, I could create an imaginary image in my mind and then, simply copy it to my paper. In no way could I ever do this, as these images are continually changing. If anything is stable and consistent it is only what’s already be applied to my paper.
I will often begin an imaginary artwork with no idea of what the objects will become - and here, I agree, the best approach would be to begin with variations of tone rather than line. With toned shapes and smudges on the paper, I will usually begin to see it as resembling something. Then I'll begin to clarify what I'm imagining is already there.
Using toned paper with charcoal or pencil for the shadow areas, and a white chalk or white pencil fir the highlights is a good start. The paper itself then served as the mid-tone or half-tone. Conte crayons are a nice alternative.
During my early 20’s, drawing intuitively like this would greatly provoke my anxieties. I would actually become physical ill and have to quit. However, during that same time, I would be able to draw from photographs without the anxiety.
Line is interesting in that, it’s only an invention that doesn’t exist in reality yet, is extremely useful in visual thinking. The skill is in making all of these visual elements work together.
I sometimes enjoy using my imagination beyond my artwork. For example, earlier this summer, I was cutting down a small tree in my backyard which, when only partially removed resembled a figurine of a tall, thin man wearing a yellow straw hat with dark clothing with his arms extended outward, as if, he were feeding the birds. I enjoying imagining this scenario for several days before I had to finish my tree cutting removal. Little imaginary pleasures, like this, seem to help me with my depression.