What is Ray Projection?
Ray projection is a fundamental concept in computer graphics related to how 3D objects are represented in a 2D space.
At its core, ray projection is a method for translating three-dimensional coordinates into two-dimensional ones, simulating how we see objects. While often associated with ray tracing (a rendering technique), the term "ray projection" as defined in specific contexts like the provided reference refers to a particular type of geometric projection.
According to the reference, ray projection refers to a type of oblique projection in computer graphics where the projection is parallel, but the projection plane is not necessarily perpendicular to the projection rays.
This definition places ray projection firmly within the category of parallel projections, distinguishing it from perspective projections where projection lines converge to a single point (like human vision).
Parallel vs. Perspective Projection
- Parallel Projection: Lines of projection are parallel. Objects maintain their relative size regardless of distance from the viewer.
- Perspective Projection: Lines of projection converge. Objects appear smaller the farther away they are.
Oblique vs. Orthographic Projection (within Parallel Projection)
Parallel projections are further divided based on the angle between the projection rays and the projection plane:
- Orthographic Projection: The projection rays are perpendicular (at a 90-degree angle) to the projection plane. This includes common views like top, front, side, and isometric projections used in technical drawings.
- Oblique Projection (Ray Projection): The projection rays are not perpendicular to the projection plane. This is the key characteristic of ray projection as described.
Key Characteristics of Ray Projection (Oblique Parallel Projection)
Based on the definition, ray projection exhibits these features:
- Parallel Rays: All projection lines are parallel to each other.
- Non-Perpendicular Plane: The plane onto which the objects are projected is at an angle other than 90 degrees to the projection rays.
- Preserves Parallelism: Parallel lines on the 3D object remain parallel in the 2D projection.
- Can Distort Shapes: While parallel lines are preserved, angles and shapes that are not parallel to the projection plane can appear distorted compared to their true form.
Practical Insight
Imagine looking at a cube. In an orthographic view looking straight at a face, it's a perfect square. If you rotate it slightly for an isometric view (a type of orthographic), parallel edges remain parallel, and certain angles are standardized.
In an oblique projection (ray projection), you might still see one face as a true shape (like a square), but the edges receding away from that face are drawn parallel to each other, but at an arbitrary angle (not 90 degrees) to the plane, and their length can be shown true or foreshortened. This provides a sense of depth while keeping parallel lines parallel, which isn't possible in perspective projection.
Applications in Computer Graphics
While perhaps not as universally common as perspective or orthographic projections, oblique projections like ray projection can be used in computer graphics for:
- Specific artistic or technical visualizations where maintaining true shape on one plane while showing depth via parallel lines is desired.
- Certain types of game perspectives or visual styles.
- As a step or component in more complex rendering pipelines.
Ray projection, as defined, offers a specific balance between maintaining parallel relationships (like orthographic) and providing a sense of depth (like perspective, but without convergence), achieved by projecting parallel lines onto a tilted plane.
