Umbra
“In optics and perceptual modeling, the umbra is the region of a cast shadow where direct light from a given source is completely blocked by an intervening object. It represents a state of geometric occlusion—a physical condition in which no direct light reaches the area in question. This definition applies regardless of how the region appears perceptually: an umbra is determined by the path of light, not by its perceived value.
In the context of visual art and the Waichulis Curriculum, the umbra plays a central role in the accurate depiction of cast shadows—those shadows projected onto adjacent surfaces. While the umbra always signals a complete interruption of direct illumination, its perceived value can vary significantly due to environmental conditions such as: the reflectance of nearby surfaces (e.g., white floors, walls, or table surfaces), the presence of ambient or bounce light, as well as the material properties of the surface receiving the shadow
As a result, not all regions of the umbra will appear equally dark, and in some cases, the umbra may appear lighter than surrounding forms or shadow structures (such as the cast shadow accent or deep attached recesses) due to increased reflected illumination.
The cast shadow accent, which often appears at the point of contact between the object and the surface, represents the darkest perceptual region within the cast shadow. While it lies within the umbra, it is not synonymous with the umbra itself. Instead, it is a localized emphasis where geometric occlusion combines with minimal ambient fill, producing high contrast and perceptual salience. Thus, while the umbra defines the physical condition, the cast shadow accent highlights the most visually dominant portion of that condition.
It is important to distinguish the umbra from: the attached shadow (or form shadow), which occurs on the surface of the object itself, the terminator (or attached shadow accent), which marks the boundary on the form where direct light ends and the penumbra, which describes the partially occluded transitional zone at the edge of a shadow, where only some rays from an extended light source are blocked.
Within the framework of the Waichulis Curriculum, the umbra is a critical structural component of the cast shadow—specifically, it is the region of complete occlusion within the broader cast shadow shape. The cast shadow as a whole may include areas of full occlusion (umbra) and partial occlusion (penumbra), each influenced by environmental dynamics and the geometry of the form-light relationship. The cast shadow accent, in turn, refers to the darkest localized region within the umbra, typically where the shadow begins—such as the contact point between the object and the surface it rests on—and where reflected light is least likely to penetrate. Thus, while the cast shadow defines the full projected effect, the umbra defines the fully blocked portion, and the cast shadow accent identifies its most perceptually dominant feature.”
Umber
“A natural earth pigment composed primarily of iron oxide and manganese oxide, yielding a muted, brownish hue that may range from yellow-brown to dark olive, depending on specific mineral content and particle size. It has been used in painting since antiquity and remains a staple pigment in both traditional and contemporary palettes.
Umber exists in two principal forms:
Raw Umber: A cool, greenish-brown pigment with low chromatic intensity and strong value-modulating power. Its subdued tone makes it particularly useful in neutralizing mixtures and developing grayscale-based underpaintings (grisaille).
Burnt Umber: Produced by heating raw umber, which increases its red content and warmth while reducing its relative opacity. Burnt umber tends toward a reddish or maroon-inflected brown and is often used for warmer shadows, imprimatura layers, or warm grisaille approaches.
In the Waichulis Curriculum and controlled color training, umber pigments—especially raw umber—are often employed for their low chroma, neutralizing behavior, and stable drying properties when mixed into oil-based painting mediums. Their value control characteristics make them effective for early form studies, dead-color layers, and environments that require deliberate temperature modulation within constrained palettes.
Umber should not be confused with general browns or mixed neutrals. Unlike synthetic browns derived from high-chroma mixtures, natural umbers have predictable behavior under light, consistent film formation, and excellent permanence when properly bound. As with other earth pigments, they exhibit low oil absorption and a dense particle structure, allowing for firm paint handling and reliable surface adhesion.
Understanding umber’s role in historical and contemporary techniques—particularly in conjunction with black, white, and ochres—provides a strong foundation for value-based modeling, chiaroscuro development, and materially sound painting construction.”
Uncanny Valley
“A phenomenon where a near-human representation elicits feelings of unease or discomfort due to its close yet imperfect resemblance to a human being. The term ‘uncanny valley’ was introduced by Japanese roboticist Masahiro Mori in 1970, describing the dip in comfort level as robots approach human likeness but fall short of full realism.”
Underpainting
“An initial monochromatic or simplified layer of paint applied to a surface that serves as a foundational structure for a final painting. It is often used to establish composition, value relationships, and form, providing a guide or armature for subsequent layers of color. Underpainting can be executed in various techniques, such as grisaille (greyscale), verdaccio (greenish hues often found advantageous for the subsequent development of some flesh tones), or imprimatura (a toned ground that introduces a specific context for subsequent work). It is often a very useful stage in indirect painting techniques, where transparent and opaque layers gradually refine the image.
According to Ralph Mayer in The Artist’s Handbook of Materials and Techniques, underpainting methods have varied across time and materials. Early Italian Renaissance painters used egg tempera and verdaccio to model flesh tones, while Flemish painters of the 15th century employed meticulous grisaille underpaintings before applying luminous glazes. The choice of underpainting color significantly influences the final work—warm underpaintings, such as burnt sienna and white, were historically used for earthy landscapes, while cooler underpaintings, like blue or green earth, helped offset warm flesh tones. The strategic use of underpainting can play an important role in how light interacts with transparent layers, subtly modifying the appearance of hues and values. Beyond its role in guiding composition and establishing value relationships, underpainting can contribute to a painting’s luminosity and durability. It provides a stable, absorbent base that may provide some advantage in preventing unwanted changes in paint appearance, like sinking. By structuring the visual hierarchy early in the process, artists can create a painting that maintains coherence and vibrancy as additional layers are applied. Underpainting remains a common, fundamental aspect of both traditional and contemporary painting practices, supporting many processes used to pursue depth, realism, and specific color relationships in indirect painting techniques.”