Local and Global Illumination Models

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Questions and Answers

Which statement most accurately differentiates between local and global illumination models in computer vision?

Global illumination models are simpler to compute and are used for real-time applications, whereas local models are computationally intensive and used for offline rendering.
Local illumination models are superior for rendering diffuse surfaces, while global models are necessary for specular surfaces and reflections.
Local illumination models account for light sources originating only from directly visible sources, while global models consider light from all sources in the scene, including reflections and inter-reflections. (correct)
Global illumination models only consider ambient light and ignore direct light sources, while local models focus on direct light sources and disregard ambient light.

Ambient light is primarily incorporated into illumination models to simulate the effect of direct, focused light sources, thereby enhancing the sharpness of shadows in a rendered scene.

False (B)

Explain why relying solely on pixel brightness is insufficient for robust scene understanding in computer vision, considering the factors that influence pixel brightness.

Pixel brightness is a product of multiple confounding factors including light source characteristics, surface orientation, material properties (albedo), inter-reflections, and sensor gain. Without disentangling these factors, a single pixel's brightness is ambiguous and does not uniquely determine any underlying scene property.

The fraction of incident light reflected by a surface, which is a crucial factor in determining pixel brightness and is intrinsic to the surface material, is termed ________.

albedo Signup and view all the answers

Match the color correction methods with their underlying assumptions about the image's color distribution:

White World Assumption = The brightest pixel in the scene is assumed to be white, and other colors are normalized relative to this. Gray World Assumption = The average color of the scene should be gray, implying that the average of all color channels should be equal. White Balancing = A specific reference point or area in the image is identified as white or gray, and the rest of the image colors are adjusted to align with this reference. Signup and view all the answers

Consider a scenario where an object's pixel intensity changes significantly across its surface in an image. Which of the following is LEAST likely to be the primary cause of this intensity variation, assuming uniform lighting?

Consistent and uniform texture across the object's surface. (B) Signup and view all the answers

Achieving perfect color constancy in computer vision systems, comparable to human perception, is currently a solved problem with readily available algorithms that reliably interpret surface colors irrespective of lighting conditions.

False (B) Signup and view all the answers

Describe how differences in intensity within an image can provide cues about the shape of objects, citing at least three distinct visual cues derived from intensity variations.

Intensity differences reveal shape through cues like: 1) Changes in surface normal: Gradual intensity changes often indicate smoothly curving surfaces as the angle of the surface relative to the light source varies. 2) Texture: Repeating patterns of intensity variations can define surface texture and its orientation. 3) Indents and bumps/Grooves and creases: Sharp intensity changes and gradients can delineate edges, contours of bumps, indents, grooves, and creases, signaling changes in depth and surface geometry. Signup and view all the answers

A distant point source of light, such as the _____, is characterized by providing illumination from essentially a single direction across all points in a scene.

sun Signup and view all the answers

In the context of color correction using the Gray World Assumption, if an image has an average red channel value significantly higher than the average green and blue channel values, how would the Gray World Assumption attempt to adjust the color balance?

Decrease the value of the red channel and potentially increase the green and blue channels to bring all average channel values closer to a common gray value. (A) Signup and view all the answers

Flashcards

Distant Point Source

A light source that is far away, resulting in parallel light rays.

Area Source

Light source with a significant surface area that emits light in many directions.