Illuminating Rendered Objects

• The goal of lighting models is to render in a way that is as close to realistic or as close to a desired style as possible.

Parameterizing Light §

• Rather than capture all wavelengths, we use a sample of wavelengths at different intensities. The standard case is to use RGB. The intensity of the light is from .
• Angle of Incidence - refers to the angle by which light strikes a surface and thus how much light is received.
• Angle of View - refers to the angle by which light comes from a surface and onto the eye.
• Attenuation - refers to adjusting light such that its intensity is smaller when viewed from farther away.
  • This can be done via linear attenuation or quadratic attenuation so that the intensity is inversely proportional to distance and square distance respectively.
  • Alternatively, we can clamp the attenuation such that if the light is above a certain distance from the source, it has intensity.
• Specular Exponent - controls the degree of specular lighting to be used in lighting.
• Maximum Intensity - the maximum intensity of the light (within HDR space). Any light above this intensity gets clamped.

Light Sources §

• Local Illumination pertains to lighting that only comes from light sources.

• Global Illumination pertains to lighting that comes from both light sources and the reflection of light off of surfaces.

• Directional Light Sources direct light to a particular direction.

  • This is useful for when we do not want the angle of incidence to change significantly when we move the light source close.
  • These model light sources that are extremely far away from an object to the point that light only strikes in one direction.

• Ambient Light - lighting that emanates off of every object on the scene, coming from all directions equally. In effect, every object receives a little bit of light intensity.

  • The intensity of ambient light can be changed (for example to simulate day and night).
  • This is meant to simulate global illumination.

• Point Light - light that has a position and from which light shines with in all directions.

• Spotlight - light source that emits from a position in a generally conical or cylindrical shape at a given direction.

Lighting Techniques §

• Diffuse Lighting pertains to when the light reflects from the surface at many angles rather than as a mirror.

  • Lambertian Reflections pertain to the ideal case where light is reflected evenly on all directions.

• Specular Lighting pertains to when the light strongly reflects off the surface in a perpendicular manner.

  • This gives objects shininess.
  • Specular Highlights are highlights due to specular lighting caused by direct illumination from a light source.
  • Specular Highlights do not linearly interpolate well.

• Microfacets - refer to imperfections on the surface which introduce roughness.

  • This is controllable by adjusting the distribution of surface normals.
  • The Phong Model is a simple model which states that light reflected in the direction of the viewer varies based on the difference between the view direction and direction of perfect reflection.
    • One limitation of this is depicting rough surfaces that still have specular highlights since it does not allow specular contributions from areas outside a certain region
    • Another limitation is it only allows for view angles less than have a specular contribution.
  • The Blinn-Phong Model is an extension of the Phong Model.
    • It does this by checking if the half angle is aligned with the surface normal.
  • The Gaussian Specular Model is an extension of the Blinn model which actually simulates the microfacets. It assumes that at a given point, the intensity follows a normal distribution parameterized by the half-angle offset from the surface normal.
    • This is parameterized by a Gaussian Smoothness parameter that controls the apparent smoothness (from )
    • It offers more distinct highlights for shiny surfaces.

• High Dynamic Range Rendering simulates how the human iris adjusts based on the luminance of the environment (i.e., at day, the iris lets in less light to not damage the retina).

  • This manifests as filtering out some of the light.
  • Additionally, we note that light does not have global maximum brightness, but this necessitates tone mapping from HDR to Low Dynamic Range.

• Gamma Correction involves controlling the overall brightness of the image to account for non-linear color spaces in the display device.

  • It involves a parameter . The correction simply involves doing the following for the linear color

  • Gamma encoding, where makes the dark regions lighter.

  • Gamma decoding, where makes the shadows darker.

  • Without gamma correction, light sources tend to look extremely bright even when the rest of the scene is dark.

  • It also allows for details within the darkness to show up more easily.

  • Do not use gamma correction for color spaces that are already non-linear such as sRGB.

Artifacts §

• Light Bands occur in specular diffusion, manifesting as a sharp cutoff between the specular highlight and dim areas. This is due to the light facing the object a certain way.
• Light clipping occurs when the intensity of the light exceeds the allowable range of , causing the object to lose all detail and instead be rendered as absolute white.

Shading Techniques §

• Gouraud Shading involves lighting each vertex and then interpolating the light across the triangle surface.
  • This is an ancient and inaccurate technique, however, as it is limited to diffuse lighting and the performance constraints are no longer a concern.
• Fragment Lighting involves interpolating both normals and the light direction on the fragment
  • This removes artifacts due to using vertex only shading, which are more apparent when the triangle is close to the light.

Materials §

• Materials are effectively containers which store parameters for how a surface should be illuminated.
• The diffuse color pertains to the color to be given off due to diffuse lighting.
• The specular color pertains to the color to be given off due to specular lighting.

Impostors §

• Lighting can be used to make an object appear different from what is expected from its geometry. This is through impostors.
• This allows using objects as placeholders to invoke the fragment shader to make it look like something else (e.g., a box mesh that is rendered as a perfect sphere)
• This can be made accurate with ray tracing so that the impostor behaves correctly when viewed from the camera

Computational Aspects §

• We make use of normal vectors. Each vertex in a mesh will be assigned a normal vector.
  • Regions in a fragment are then shaded based on the normals of its vertices.

Links §

• Basic Rendering and the Rendering Pipeline - for more information on how to render and position objects.

• Learning Modern 3D Graphics Programming Ch. 3