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# GRPHICS04 - Rendering (1)

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This is a course on the theoretical underpinnings of 3D Graphics in computing, suitable for students with a suitable grounding in technical computing.

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### GRPHICS04 - Rendering (1)

1. 1. RENDERING Michael Heron
2. 2. INTRODUCTION  As part of the process of creating a 3D scene, we are building up a model of objects. The geometry of those objects The relationship of those objects to each other The relationship of the objects to the camera.  However, in order to turn this scene into a useable image, we must go through a separate process. Rendering
3. 3. RENDERING  The aim of rendering is to view objects as solids. Not polymeshes, wireframes, or any other of the representations we have discussed.  Requires several steps Identify which surfaces are visible Project surfaces onto the 2D viewing plane Shade surfaces appropriately Rasterise  Convert from mathematical representation into bitmaps.
4. 4. RENDERING – TWO FLAVOURS  Depending on the intended application, rendering can have one of two flavours.  Real-time  Games, simulations  Non Real-Time  Movies, static scenes  In both cases, the end goal is the same.  Turn the mathematical representations of a 3D scene into something pretty.
5. 5. REAL-TIME RENDERING  For real-time scenarios, trade-offs are required. Scenes must be rendered quickly enough for the human eye to be fooled  Around 30 frames per second Trade off is between photorealism and speed of processing.  Can use tricks to ‘fool’ viewers.  As time goes by, this trade-off becomes less important. Computers get faster, our eyes don’t.
6. 6. NON REAL-TIME RENDERING  For non-interactive media, time is not important.  We can spent many more seconds rendering an image because we have no throughput to reach.  Each frame may take minutes/hours to render.  Played back at 25-30 frames per second.  Like old fashioned cartoon animations.
7. 7. RENDERING - LIGHT  Simply assigning a colour to a surface gives an unrealistic appearance. The interaction of light with objects is one of the primary mechanisms for achieving ‘realism’  Light interaction is complex Illumination Reflection Shading Shadows Texture of surfaces Transparency/Translucency
8. 8. RENDERING - LIGHT  The appearance of an object depends on:  The type of light source  Distance to the light source  Orientation of the object relative to the light source  Surface properties of the object  Determines the nature of light interaction  Repeat for all light sources.  Lots of processing.
9. 9. RENDERING - LIGHT  In non real-time situations, we can precisely model the physics of light interaction.  Such as with ray-tracing  Or radiosity  For real-time situations, an approximation will suffice.  As long as it looks good.
10. 10. LIGHT SOURCES  There are several kinds of light sources that are used in 3D modelling. Point light sources Ambient light sources Directional light sources Spotlight sources  They are all used to give differing ‘textures’ of light and shadow. Choosing the right kind and number of light sources an important part of good scene output.
11. 11. LIGHT SOURCES - AMBIENT  Ambient light is the general background illumination.  All objects reflect light to some degree  Light can reach parts of a room/scene that are in no direct line of light.  Illumination is constant on all surfaces  Regardless of position or orientation  Light is uniform across a scene.
12. 12. LIGHT SOURCES - DIRECTIONAL  Directional light sources emit light strongly in a particular direction. Like the sun  Objects in the line of light more brightly illuminated than other objects.  Light is uniformly provided in a fixed direction.  Light source is assumed to be very far away. No need to deal with light distance calculations.  Objects at the back of a scene lit to the same degree as objects at the front Provided they are directly in the path of light.
13. 13. LIGHT SOURCES - POINT  Point light sources have  Location  Colour  They emit light equally in all directions.  Like a lightbulb.  Light strength attenuates  Gets weaker with distance  Gradients of illumination across a surface  Can be moved around with a scene.
14. 14. LIGHT SOURCES - SPOTLIGHT  Very similar to a point light source  Difference is that spotlight source does not radiate in all directions.  Direction of light can be varied within a scene.  This distinguishes it from directional light.  Light attenuates with distance.  This also distinguishes it from directional light.
15. 15. SURFACES  The surface of an object represents the next implication for light interaction.  Surfaces can:  Absorb light  Reflect light  Refract light  Emit light  They can glow in the dark
16. 16. ABSORPTION  The colour of the surface determines the level of absorption  Black absorbs all visible spectrums  White reflects all visible spectrums.  Coloured surfaces vary depending on colour.  Reflect some wavelengths  Absorbs others  Wavelength interaction with colours a matter of physics.
17. 17. REFLECTION  Reflection can be either  Specular  Reflects like a mirror  Light gets reflected at angles  Angle of reflection  Ever tried shooting a laser pointer at a mirror?  It’s like that  Diffuse  Reflects at many angles at once  Diffuse reflection is the primary way in which we can see objects.
18. 18. REFLECTION http://www.curriki.org/xwiki/bin/view/Coll_Athabasca/Unit3- Lesson2TheMovementofLight
19. 19. REFLECTION  Reflection is dependant upon  The arrangements of facets on a surface  The roughness of the surface  The orientation of the surface  The absorbption of the surface.  Light can also be refracted  As light moves from one medium to another, the path of the light bends.  Mechanics of this are not trivial
20. 20. REFRACTION http://www.curriki.org/xwiki/bin/view/Coll_Athabasca/Unit3- Lesson2TheMovementofLight
21. 21. TRANSMISSION  Sometimes, objects transmit light through themselves.  Such objects can be transparent (like glass) or translucent  Transparent objects refract light passing through.  White light through a prism  Translucent objects diffuse the light that passes through  Frosted glass
22. 22. LIGHT RENDERING  Two approaches used to render light  Local reflection models  Everything floats in ‘dark space’  Only one reflection of light considered  Global reflection models  Light reflects off of objects which reflect onto other objects  Indirect light  Very costly to render  Not yet perfected
23. 23. SUMMARY  One aspect of rendering is the interaction of light upon surfaces. This can be  Real-time  Non Real-Time  Issue complicated by the physics of light Reflection, Refraction Different kinds of lights  Lighting is an important part of rendering believable 3D scenes.