High Contrast Shading
The human visual ability is designed by environmental pressures to acquire quickly an understanding of the creature's surroundings. This is automatic and, although hue is important, value is the fundamental distinction that this ability relys on.  Value is a term that has a specialized meaning in the visual arts.  It is the name for light and dark.  High value is light.  Low value is dark.  Grays are intermediate values.
 When presented with an image,  the mind quickly resolves the value pattern into an understandable scene.    Experienced artists often rely on the viewer's mind to do this and therefore leave unneccesary details out of the picture.
  Look carefully at this illustration of a hammer.  Notice that there are no contour lines delineating the handle or head.  You may think you see edges buried in the black and white areas, but it is an "optical" illusion.  The brain is "filling in information in order to make sense of the image.   This is true also for the image of the container at the top of the page.
 
 

Do this: (40 minutes)
   Use an unshaded (incandescent) light bulb to lluminate four opaque objects. Select simple objects, such as a ball, a can, a sea shell, a fruit or vegetable, and position each carefully with respect to the light so that the light/dark pattern reveals sufficient information about the object that the viewer can identify it.  Try to chose objects that have little or no confusing pattern of colors on their surface.
    Look at one of the objects.  Shut one of your eyes to "flatten the scene.  Close your other eye until you can barely see anything at all.   Notice that any colors have largely disappeared.  Notice that all you can see is a light and dark pattern.
With a soft lead pencil, duplicate that pattern.
DO NOT DRAW OUTLINES OF THE SHAPES WITH THE INTENTION OF FILLING THEM IN.
Start by creating a small "puddle" of dark with your pencil in a place that corresponds to the darkest part of the light/dark pattern.  Usually this is where the cast shadow "touches the object.  Enlarge this puddle both into the cast shadow and the darkened side of the object.  Continue enlarging it, shaping it as you do until it fills the dark shape that is the cast shadow and the object's dark side.  Look for any other dark shapes that are created by the interaction of the object with the light.  Create them the same way. Do not draw lines to "connect" edges.  Let the mind's eye complete the image.

Pay attention to the actual shape of the image.  Use your knowledge of perspective to see accurately.  Think about relationships of parts of the shapes: How they contribute to the viewer's understanding of the object.  For instance, the two dark shapes in this image relate to one another in a way that creates the illuminated lip of the top of the cylinder

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