Lect 4 Representing the Real World Flashcards
- Describe Local vs. Global Illumination models.
- A local illumination model only considers direct light i.e. light coming directly from light sources. However, much of the light we see is indirect light.
- In local illumination models, we can fudge this using “ambient light”. Global illumination models take into account direct and indirect illumination – we don’t need to use an ambient value.
- Due to this we get effects like colour bleeding and caustics that are not possible in **local illumination. **
- List details of the 4 basic graphics definitions
- RGB,
- PIXEL,
- RASTER and
- CLIPPING
• RGB
– colour specification in which intensity values for Red, Green, and Blue components of a colour are used
• Pixel
– Individual unit of display, single “dot” of colour. Matrix of pixels on the screen.
• Raster
– Block of graphics memory which contains information to be displayed (often as a continuous block of RGB triplets, each representing a pixel)
• Clipping
– Determining which virtual objects (or parts of objects) should or should not be shown.
- What are the basic camera/eye specifications?
– Eyepoint
• The logical point (in reference to the origin) at which the “camera” or “eye” is located
– View Vector
• Direction in which the camera is pointing
– Up Vector
• Perpendicular to View Vector, orients the camera
- What is the viewing plane?
• Viewing plane
– Usually the near clipping plane, this is the “window” you have into the graphical world
- Define three Rays involved in **Ray Tracing. **
• Eye Ray (primary ray)
– Ray from Eyepoint (position of viewer) through a pixel on the viewing plane, used to determine visible points
• Secondary Ray
– Ray generated from object surface at ray intersection point to gather information about environment effects on the surface
• Shadow Ray
– Secondary ray toward a light source to determine if an object occludes light source. (blocks it creating shadows)
- Describe “The Rendering Equation”
• The rendering equation is a way of describing the way light moves through an environment.
• The rendering equation describes the total amount of light emitted from a point along a particular viewing direction, given a function for incoming light and a BRDF (Bi-directional Reflection Distribution Function).
• Global illumination algorithms like ray tracing and radiosity aim to solve approximations of the rendering equation.
7.
Describe BRDF
(Bi-directional Reflection Distribution Function)
- The BRDF simply describes what we all observe every day: that objects look differently when viewed from different angles, and when illuminated from different directions.
- A BRDF describes how much light is reflected when light makes contact with a certain material.
- The Bidirectional Reflectance Distribution Function (BRDF) describes the appearance of a material by its interaction with light at a surface point.
- How does an opaque material deal with light?
- For opaque materials, the majority of incident light is transformed into reflected light and absorbed light.
- As a result, when an observer views an illuminated surface, what is seen is reflected light, i.e. the light that is reflected towards the observer from all visible surface regions.
- To what degree is light reflected with BDRF?
- In general, the degree to which light is reflected (or transmitted) depends on the viewer and light position relative to the surface normal and tangent.
- Consequently, a BRDF is a function of incoming (light) direction and outgoing (view) direction relative to a local orientation at the light interaction point.
- Why is BRDF important?
To model a photo realistic environment we need accurate models. To simulate certain lighting conditions the BRDF is needed.
It can be complex to measure the light interaction point but there are online databases with models available to help. The MERL BRDF database contains reflectance functions of 100 different materials (http://www.merl.com/brdf/).
- Explain why BRDF is a function of wavelength.
When light interacts with a surface, different wavelengths (colours) of light may be absorbed, reflected, and transmitted to varying degrees depending upon the physical properties of the material itself.
This means that a BRDF is also a function of wavelength.
- How is the value of a BRDF determined?
• There are different ways to determine the value of a BRDF.
– One way is to evaluate mathematical functions derived from analytical
models.
– The other is by resampling BRDF data acquired by empirical measurements of real-world surfaces.
- What provides theoretically perfect results?
The Rendering Equation
• In computer graphics, the rendering equation (Kajiya, 1986) describes the flow of light energy throughout a scene. Based on the physics of light, it provides theoretically perfect results, in contrast to the various rendering techniques, which approximate this ideal.
- Give details of how The Rendering Equation works.
• At a particular position and direction, the outgoing light (Lr) is the sum of the emitted light (Le) and the reflected light.
The reflected light itself is the sum of the incoming light (Li) from all directions, multiplied by the surface reflection and incoming angle.
- Describe Ray Tracing.
- Ray-tracing is the most complete simulation of an illumination- reflection model in computer graphics.
- It can handle light that originates from the scene environment (indirect light).
- It can be very realistic but is computationally expensive with extremely high processing overheads – scenes can take hours, minutes or days to render.
- The advantage is that it combines hidden surface removal with shading due to direct illumination, shading due to global illumination, and shadow computation within a single model.
- How does Ray-tracing Work??
- A ray of light is traced backwards through the scene, starting from what the eye or camera sees.
- When it intersects with objects in the scene its reflection, refraction, or absorption is calculated.
- What coordinate system are the virtual Screen and Virtual Viewpoint in when implementing Ray Tracing?
In Ray Tracing, a virtual screen and a virtual viewpoint are defined in the same coordinate system as the objects to be rendered.
- Describe a line “Ray” in Ray Tracing.
A line “ray” is projected from the viewpoint through every pixel in the screen, and onto the object space.
The ray is tested for intersection against every object in the scene.
If no object is hit, you see the background.
Otherwise, you see the closest object to the viewpoint.
- If an object is hit with a ray what happens?
If an object is hit, a ray is projected from the intersection point towards the simulated light source.
- If a ray hits a second object what does this mean?
If a ray hits another object on its way, the intersection point is in shadow. See below.
- What is a “Secondary ray”?
If the object happens to be reflective, a “secondary ray” is projected from the intersection point (angle of incidence equals angle of reflection).
- What happens when an object is transparent?
If the object is transparent, the secondary ray is projected in the direction indicated by the **index of refraction. **
- What are Tertiary Rays?
If a secondary ray hits somthing reflective, tertiary rays are generated. And so on, and so on, and so on……
Models allow a defining number of permissable ray bounces.. 9 to 10 is usually sufficient..
- Describe how Camera Rays work.
- We start by ‘shooting’ rays from the camera out into the scene
- We can render the pixels in any order we choose (even in random order), but we will keep it simple and go from top to bottom, and left to right
- We loop over all of the pixels and generate an initial primary ray (also called a camera ray or eye ray)
- The ray origin is simply the camera’s position in world space
- How do we create shadows?
- ** Shadows** are an important lighting effect that can easily be computed with ray tracing
- If we wish to compute the illumination with shadows for a point, we shoot an additional ray from the point to every light source
- A light is only allowed to contribute to the final colour if the ray doesn’t hit anything in between the point and the light source
- Describe how reflections work in **Ray Tracing. **
- If the reflection ray hits a normal material, we just compute the illumination and use that for the final colour.
- If the reflection ray hits another mirror, we just recursively generate a new reflection ray and trace that.
- In this way, we can render complex mirrored surfaces that include reflections, reflections of reflections, reflections of reflections of reflections…
- To prevent the system from geting caught in an infinite loop, it is common to put an upper limit on the ‘depth’ of the recursion. 10 or lower works for most scenes, except possibly for ones with lots of mirrored surfaces.
- In any case, most pixels will only require a few bounces, as they are likely to hit a non-mirrored surface sooner or later.
- What is refraction/(Transmission Rays)
- Ray tracing can also be used to accurately render the light bending in transparent surfaces due to refraction.
- Often, this is called transmission instead of refraction. Transmission is a more general term that also includes translucency, but I think the real reason this word is preferred is because ‘reflecton’ and ‘refracton’ look too similar.
- When a ray hits a transparent surface (like glass, or water…), we generate a new refracted ray and trace that, in a similar way as we did for reflection.
- We will assume that the transmitted ray will obey Snell’s law (n1sinθ1=n2sinθ2), where n1 and n2 are the index of refraction for the two materials.
- What is meant by recursive Ray Tracing?
- The classic ray tracing algorithm includes features like shadows, reflection, refraction, and custom materials
- A single primary ray may end up spawning many secondary and shadow rays, depending on the number of lights and the arrangement and type of materials
- These rays can be thought of as forming a tree like structure, recursive.
- Describe **Ray Tracing Output. **
- The quality of a ray traced scene depends on the number of ‘bounces’. The more bounces, the better the quality.
- Images appear very clean and sharp with hard shadows – they can look so clean that they appear unrealistic – too perfect.
• Introducing randomness can counter this. Distributed ray tracing, a refinement of ray tracing that generates ‘softer’ images, is one way of doing this.
- Real-time ray tracing is possible but currently produces scenes with a lower number of bounces.
- Pixar used ray-tracing for their film Cars.
- Describe Radiosity output.
- Works for purely diffuse surfaces.
- Gives good results for effects such as colour bleeding.
- Interactivity is achieved since the scene calculations are pre-computed and only need to be done once.
- Good for scenes (especially indoor scenes) where most lighting is indirect and where ray tracing copes badly. However, not very good for scenes involving transparency and non-diffuse reflection.
- What are patches?
The surfaces of the scene to be rendered are each divided up into one or more smaller surfaces (patches).
The patches nearer the top of the pillar, which could not see the sun, are in shadow, and those that can are brightly lit. Those that could see the sun partly obscured by the edge of the window are only dimly lit.
And so Radiosity proceeds in much the same fashion. As you have seen, shadows naturally appear in parts of the scene that cannot see a source of light.
- Explain how radiocity takes multiple passes to render.
After each iteration we have intermediate radiosity values for the patch.
These intermediate values correspond to bounce levels.
That is, after one iteration, we know how the scene looks after one light bounce, after two passes, two bounces, and so forth.
Progressive radiosity is useful for getting an interactive preview of the scene. Also, the user can stop the iterations once the image looks good enough, rather than wait for the computation to numerically converge.
- Describe Image based lighting (IBL)
- Image-based lighting is the process of illuminating scenes and objects (real or synthetic) with images of light from the real world.
- Real-world illumination is captured as an omni- directional light probe image.
- This image is mapped onto a representation of the environment (e.g. as a sphere encompassing the modelled scene).
- The light in that environment is simulated using ray tracing with the light probe image as the light source
