GA 3311: Materials and Lighting In this class students will be introduced to materials, textures and lighting strategies to add detail and realism to objects without adding complexity to the model. Students will simulate real world surfaces containing reflection radiosity and other effects. |
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JUMP TO A SPECIFIC WEEK:
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Week 3: Light and Shadow (and that fuzzy line inbetween)
Types of Lights:
spot lights- A spot light has a cone of influence
in a specific direction. This is controlled by the Cone Angle attribute
which is measured in degrees from edge to edge. The Spot Light also has
Decay, Dropoff, and Penumbra. Great for lamps, grazing accent lights,
and dramatic effects, as well as for producing light fog.
directional lights-The Purpose of a directional
light is to simulate a distant light source, such as the sun, where light
rays are coherent and parallel. This type of light will produce a harsher,
more intense quality of light with harder edges and no subtle changes
in surface shading becasue of its parallel rays with no decay. Directional
Lights are not very expensive to render because the angle is consitent
for all rays and decay is not computed. It also cannot simulate light
fog.
point lights-The Point Light emits rays
from a single point outwards in all directions. It can accurately simulate
a lightbulb or candle as a result. Point lights DO have decay and will
typically produce more subtle, yet richer shading on surfaces. THIS
IS LIGHT EMITING FROM 1 DIMENSION IN 3D SPACE.
area lights-Unlike point, directional, and
spot lights, area lights are far more naturalistic in that all light sources
must exisit in multiple planes of 3d space. Thusly it is a less abstract
shading model, but also more expensive to render. An are light reflects
the size and orientation of the light. They are great for straight, long,
specular, highlights. For soft lighting across a scene. And for Realistic
shadows that vary from hard to soft. All area lights have built-in quadratic
decay (see below). THEY DO NOT WORK WELL WITH DMAP SHADOWS. THIS
IS LIGHT EMITING FROM 2 DIMENSIONS IN 3D SPACE.
volume lights-This is the most physically
accurate definition for a light, but it usually far to costly for us to
render with. The Volume light will illuminate objects within given volume. THIS
IS LIGHT EXSITING IN 3 DIMENSIONS OF 3D SPACE.
ambient lights-An ambient light is a non-directional
light that can be used in low intensities to simulate the diffused, scattered
or reflected light you see in real life. When doing more advanced rendering,
global illumination simulatios are used instead. The ambient light has
an ambient shade characteristic as well.
the default light-If
there is no light in a scen, maya will create a directional light when
the scene is rendered. This light is parented to the rendered camera and
illumates the scene regarless f where the camera is facing. After the
render is complete, Maya removes the default light from the scene
Light Properties:
Intensity-Intensitycan
be defined as the actual or compartive brightness of the light. It can
be modified with the slider, or texture mapped. We will look at texturing
intensity with ripple patterns to fake caustic lighting effects in underwater
simulations in the upcomng weeks.
Color ->View the Color Temperature chart
below, indicating relative color values for certain light sources:
The color the light can also be mapped.
Dropoff (spot
light specific)- is
similar to decay except for that its function is to cause the light to
diminish in intensity perpendicular to the light axis instead of along
the light axis.
Penumbra
(spot light specific)- The Penumbra is the
area at the edge of a spot light where the light diminishes in intensity.
The larger the radius of the penumbra, the softer the circle of light
projected by a spot light will be.
Ambient shade (ambient light specific)- This
attribute allows you to specify how much light comes frm the source. I
set to 0, it applies evenly across the whole scene, where as a value of
1 a the other extreme causes there to be direction associated with that
ambient light, based on it's position.
Decay rate (for point,spot and area)- Decay
refers to how light diminishes with distance. In maya, it is possible
to alter the rate of decay in four sepeate ways: No Decay, Linear, Quadratic,
and Cubic
No Decay |
Light will contine forever; will reach all objects in the scene. |
| Linear | Liht intensity decreases in direct proportion to distance. Longer to render, but diminishes less than in nature. |
| Quadratic | This is how light decays in real life. (I=1/d*d). Longer to render still. |
| Cubic | Light decays faster than real life. (I=1/d*d*d). Longest to render. |
Raytrace
Shadows are
the natural simulation of shadowing effects in 3d. Raytrace Shadows
are produced on a ground surface when light in the form of a ray, being
casted from a light source such as a point light or spot light or directional
light (etc...), is occluded out by an intercepting object. As the rays
are blocked, no light can reach the ground plane, and thusly a shadow
is produced. See Image 1:
By
Default, Raytrace Shadows produce very hard edges. This is unnatural to
what we see in real life (View the floor in Gothic Cathedral Image above).
Since physically, Light
can exhibit both Photonic (particle) and Wave like properties, we
have to take this into consideration when simulating raytrace effects.
By their very nature, anything exhibitng the properties of a wave, (and
all quanta materials will), can have those waves reflected, refracted,
and interferred with. These principles are exactly behind the theories
of Global Illumination, Radiosity, and Refractivity in Media Denser than
air, which we will look at throughout the course.
However, we need to concern ourselves with the ability of a wave to propogate past an interferring object. See Image 2 below. Just as a wave in the ocean will spread and disperse (see blue lines) instead of heading in a single direction (see red lines), raytrace shadows should exhibit blurring proportional to their distance after the light ray intercepts an object. This effect is known as Shadow Attenuation. and must be simluated in our raytrace settings to produce accurate shadows. (see image 3, and note the blurring of the shadow on the far left, furthest from the light, as the rays have more time to enter the shadow area). We can adjust the blurring of the shadow by changing the amount of rays that are cast from the point of interception, and by altering the angle at which they bend (see image 4).
Image
2: ...................................................................Image
3:
Image 4:
Depth Map shadows work in a method unlike raytrace shadows alltogehter. In a depth map shadowed scene, a map is actually projected from the light source. This map is used to govern the placement of shadows. If the map has two small a resolution, we will notice pixelation on the map itself as we see in the first image below.Both of these images used the same Depth map shadowing effect. On the right however, the dmap_res has been dramatically increased. Dmap Resoultion will improve the quality of your render where Dmap shadows are used, but upping it enough will slow down render time noticeably. Often, rendering with Dmap shadows at a very low res and and bias can produce undesired self shadowing. These low res shadows can appear to flicker when rendered with a moving camera.
Per-Vertex Shading Versus Per-Pixel Shading:Per-Pixel Shading is what makes all this possible. Originally, graphics cards and the software that were dependent on them calculated realtime lighting effects based on Per-Vertex Shading Algorithms. What this means is that light and dark as supported in real time is calculated by finding the value of the light ONLY AT THE VERTICIES. Any area on the surface is shaded solely by a ramp derived based on that visible pixel's distance between two verticies. With this as the case, we do not have the ability to add complex maps across the surface in real time that effect the play of light.With the graphics cards such as the Nvidia GeForce2 and ATI Radeon 8600 and all their successors, we have the ability to view our graphics with a Per-Pixel Shading Algorithm, in selected applications. These more complex graphics cards can simulate the effects of light at all visible pixels on the surface, not just at the verticies. This produces much smoother real-time lighting effects and allows us the ability to render out Normal maps to effect the light at each pixel.
Light Linking: In Maya we have the option to (unlike in real life) have lights only effect certain objects or an indivual object only be effected by certain lights. This is called Light-Centric Light Linking and Object-Centric Light Linking respectively. We can access this control window to choose which lights are effecting which objects under the Rendering Menu Set-->Lighting/Shading-->Light Linking-->Light Centric (or Object Centric).
Homework: Using the already Uv'd Alleyway from last week, alter the material attributes for each object, remove the ramp-grid texture from the color slot, and apply apropriate lighting to the scene to simulate both night time light, and daylight (make sure to delete the ambiet light already in place). Render out TWO images to be turned in on week 4. If in modeling new objects for your scene you have created a lamp, candles, or other light emitting objects, feel free to turn these into light sources as well.
Awarded Andrew's Gold Medal of Awesom-i-tude:
Image by Ravi Giriputra
Image by Nhathy Ngo
Image by Jason MacHardy
Image by Ravi Giriputra
Image by Oliver Barraza
Image by Jessica Bradley
Image by Thomas Kelley
Image by Luis Ramos
