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## PowerPoint Slideshow about ' Computer Graphics' - william-mason

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Presentation Transcript

Overview

- Tuesday:
- Significance of Computer Graphics
- Brief history of Computer Graphics
- Overview of 3D Graphics Concepts
- Modeling objects in a 3D environment

- Thursday:
- Rendering images
- Lighting
- Animation

Definition of “Graphics”

- A branch of computer science that applies computer technology to produce and/or manipulate visual representations
- 2D vs. 3D graphics:
- 2D ~ photographs, paintings, signage
- 3D ~ ‘simulation,’ or conversion of a 3-dimensional scene on a 2-dimensional platform (screen)
- Step 1: design the scene (creative, artistic)
- Step 2: convert it to a 2-dimensional ‘photograph’ (technical)
- This is where we work to make images more realistic!

What’s so Great about Graphics?

- Humans are very visual creatures
- We use graphics/vision to:
- Learn new things
- Make decisions
- Present/Comprehend information
- Be entertained

- Most people, when thinking of Graphics, think of:
- Games
- Movies

Graphics in Games

- One Focus: ‘realism’ and detail

Graphics in Games

- Another focus: creativity and fun

Graphics in Games

- Yet another focus: Cinematics and Story

Graphics in Movies

- Virtual + Live Action

Graphics in Movies

- Fully Animated

Graphics for Learning, Decision Making

- Do you learn more from something like this:
1 2 4 11 14 22

3 5 6 12 1 8

4 3 2 9 8 44

5 2 4 19 5 2

2 3 9 1 15 9

8 7 1 3 18 22

0 9 1 5 8 7

2 1 9 7 4 0

Graphics for Learning, Decision Making

- Or something like this?

Graphics for Presenting/Understanding Information

- Using Shading to improve medical analysis:

Some Historical Perspective

- Televisions: invented around 1923
- First computers: 1940 – 1943
- Computers with Monitors: 1956
- We knew how to produce images on a screen before computers were invented!
- Why were people so interested in connecting the two?
- Foresight to anticipate applications previously mentioned
- TV’s only copy/reproduce images from a camera, cannot create them
- Interactivity!

3D Graphics Creation

- Modeling
- What will be in the picture, what will it look like?

- Rendering
- If the model was real, and we took a picture of it, what would the picture look like?

- Displaying
- Saving the rendering as a bit-map image
- On file: movies and pictures
- On Screen: interactive applications (games, drafting tools, simulators, etc.)
- Most games try to Model, Render and Display 30—60 times every second!

The 3D graphics paradigm

Modeling Objects

- Shape: Represented by a polygonal mesh obtained from
- Traditional mathematical equations
- Berzier curves and surfaces
- Procedural models
- Other methods being researched

- Surface: Can be represented by a texture map

A polygonal mesh for a sphere

About Polygons

- They are easy to model!
- 2-dimensional
- Simple definition (connect the dots!)

- They are easy to combine to make more complex shapes
- 2D: Like combining a square and a triangle to get a house
- 3D: Like a house of cards or a balsa-wood model

- They don’t take up much memory either

A Bit More on Modeling

- Many of us are familiar with 2D systems:
- To draw a square, we can give coordinates for each corner:
- Square = (3,3), (6,3), (6,0), (3,0)

3D Coordinate System

- A 3D system is harder to visualize, even though we live in one!
- X,Y coordinates are the ‘floor’
- Add a Z coordinate that represents ‘height’
- Shapes drawn the same way, specifying vertices + lengths, but obviously will be more complex
- And now we can talk about cubes , spheres, etc!

z

y

x

2D Polygon Image

- Find the shapes!

3D Polygon Image

- Like a house of cards—Find the shapes!

A Bezier curve

Growing a polygonal mesh for a mountain range

http://www3.wooster.edu/cs/studentCoursework/graphics/proj3_2003.php

Where We’re Headed Next Time:

- We’ve only seen a single (and arguably easiest) step in creating a 3D image on a computer screen
- Where we’ll go next
- Lighting (shadows, bright spots, reflection, refraction)
- Displaying (rasterizing)
- 3D accelerator cards

- Funny thing, though:
- 90% of all graphical design work is finished after the modeling phase!
- Programmers use graphics engines to take care of lighting, rasterizing, card compatability
- But to be the best, you need to understand how it all works!

Drawing a 3D Object: Shape, Transformation, Rotation, Surfacing

- Recall: 3D objects are just 2D polygons ‘glued’ together
- Draw the polygonal structure at the Origin (0,0,0) using 2D Polygons
- A cube could be:
- bottom: (0,0,0),(1,0,0),(1,1,0),(0,1,0), top: (0,0,1),(1,0,1),(1,1,1),(0,1,1)
- Front: (0,1,0),(1,1,0),(0,1,1),(1,1,1), back: (0,0,0),(1,0,0),(0,0,1),(1,0,1)
- Lside: (0,0,0),(0,1,0),(0,0,1),(0,1,1) Rside: (1,0,0),(1,1,0),(1,1,1),(1,0,1)

- Transformation: Give the polygon a new position in the world
- Rotation: Make the shape point in a new direction
- Surfacing: ‘fill-in’ each polygon with a color/image

z

x

y

Surfacing

- Most systems have a ‘fill’ effect:
- Pick one uniform color for every pixel inside a polygon

- May also use special tools to explicitly color every pixel
- Costly to produce (hiring artists)
- Difficult to render (typically not an option in games), takes lots of memory

- Texture Mapping
- Provide an image that is applied to a polygon like wallpaper (common for brick walls, wood, grass, etc.)

Building a Scene

- A scene in a 3D graphics environment consists of:
- Polygon models of each object:
- Shape
- Position/Orientation
- Surface Coloring

- A virtual ‘camera’:
- Includes position/orientation information

- 1 or more virtual light sources
- All have a position in the world
- Angles depend on the type of source:
- Some have full 360o coverage
- Others have ‘shades’

- Polygon models of each object:

The 3D Scene: A Virtual Photo Studio

Rendering

- Take a picture of the ‘virtual’ scene with the ‘virtual’ camera, to get a REAL photograph!
- Lots of mathematics are used to:
- Determine which polygons are actually seen by the camera (and which are definitely not seen)
- What shape a polygon will really have in the final picture
- Depends on the angle from which it is viewed

- Which pixels in the image will represent the polygon
- The color/brightness of each pixel in that polygon (lighting model)

Lighting

- In the real world, when light hits a surface different portions of that light source will be:
- Absorbed
- Reflected
- Refracted

- It is the physical properties of an object that determine how light behaves on it, and, ultimately, what it will look like

Simulating Light

- Imagine a source of light as an infinite number of ‘rays’ that we’ll represent as straight lines
- Given a single point on an object and a single light source:
- There is exactly one ray that reaches that point

- To simulate absorption, reflection, refraction, we only have to perform calculations for that single point and ray
- Ultimately done 1000’s of times over for each and every pixel in the picture

Simulating Reflection

- Angle of incidence: angle between ray of light and the polygon
- Angle of reflection: angle between reflected ray and surface, always equal to incidence

Surface Properties

- Specular Surfaces
- Smooth, shiny
- Angle of incidence is perfect with respect to polygon’s position
- Characterized by bright white reflection from light source
- A purely specular surface is a mirror

- Diffuse Surfaces
- Surface covered in tiny, rough and random bumps
- Light is still reflected, but angle of incidence is based on which ‘bump’ it hits
- Characterized by a warm, uniform coloring across entire surface
- Many types of cloth are purely diffuse

- Most surfaces have a mixture of the two

Light: A Viewer’s Perspective

- Reflected light will be viewed only if it is reflected in the camera’s direction
- A specular surface creates Specular Light
- Follows strict rules of reflection
- Only visible if the light is ‘aimed’ at the camera

- A diffuse surface creates Diffuse Light
- ‘Random’ bumps ultimately guarantee that some of the light is reflected in every direction
- Much more likely to reach the camera lens

- Light that is reflected so many times that it doesn’t technically have a source is called Ambient Light
- Like a tiny bit of light hitting a surface from all directions
- A diffuse surface looks the same under a bright light, or ambient light
- This is how the ‘back’ of an object can be slightly illuminated from a single light source

Specular versus diffuse light

Refraction

- When a surface is semi- or totally transparent, light will pass through
- When it passes, refraction will bend that light in a different direction
- Bends a different amount depending on the material

- Modeling this phenomenon accurately is difficult
- Hence, most real-time, interactive applications favor opaque, reflective objects
- Less interactive applications can use special techniques to get impressive results

Dealing With Complex Scenes

- To render an image we need to determine:
- What an object looks like when viewed from a certain camera angle
- How light reflects from that surface, at that angle, based on multiple light sources

- Move the camera 1 millimeter to the left
- Have to do all that calculating all over again!

- Conclusion:
- Rule out as much unnecessary work as possible!

WARNING! The following material is pretty dense. Focus on the concepts, instead of the details and you’ll be fine. And please ask questions if something is unclear!!!

Clipping

- Extend the view volume from the camera position using simple geometry
- Anything outside the view volume will not be drawn, analyzed

Scan Conversion

- Draw a line through each pixel until you reach an object
- Easy for one object
- Trickier: what if objects overlap?

Hidden Surface Removal

- When you take a picture of someone’s face:
- Can you see the back of their head?
- Can you see what is on the wall behind their chest?
- What about layered objects?

- Hidden Surface Removal = determining which polygons are actually visible, throwing out the rest
- Saves lots of time!

Painter’s Algorithm

- Sort all polygons from back to front, then draw the ones in the back first
- Those overlapping in front will ‘paint over’

Z-Buffer Algorithm

- Similar to Painter’s algorithm, but instead of drawing whole objects, focus on determining what is drawn in each pixel
- Start back to front again
- For each object:
- Check to see if it intersects with that pixel
- Check to see if anything in front of it also intersects with that pixel
- If not, then that polygon determines the color of that pixel

- Shoot a bow and arrow, draw the first thing it hits!
- But we don’t program it like this because it’s less efficient

Images below provided by Penny Rheingans at UMBC

Shading

- Flat Shading: Add coloring effects to give depth to each individual polygon
- Creates faceted appearance

- Gouraud and Phong Shading: Use mathematics to estimate the original shape
- Creates smooth, rounded appearance

- Bump Mapping: Creates bumpy, rounded appearance

A sphere as it might appear when rendered by flat shading

A sphere as it might appear when rendered by Phongshading

A sphere as it might appear when rendered using bump mapping

Graphics Accelerators

- Most computer graphics calculations involve a great deal of multiplication and addition
- Transformation/Rotation
- Lighting/Shading effects
- Z-buffer computation

- A standard processor, fast as it is, can only do 1-2 simple operations per clock cycle
- But it’s also general-purpose

- Graphics Cards have 100’s or 1000’s of tiny parallel processors
- But all those processors can do is multiply and add!

- Result: instead of doing several pixels every second, we can do several screens every second!

Advanced Lighting Models

- Local Lighting Model: Does not account for light interactions among objects
- Global Lighting Model: Accounts for light interactions among objects
- Ray Tracing
- Radiosity

Ray tracing

Ray Tracing and Radiosity Examples

http://www.oyonale.com/modeles.php?lang=en&page=40

http://blogs.intel.com/research/2007/10/real_time_raytracing_the_end_o.php

Animation: Simulating Motion

- Dynamics: Applies laws of physics to determine position of objects
- Kinematics: Applies characteristics of joints and appendages to determine position of objects
- Avars
- Motion Capture

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