Computer Graphics 1

01 Handout 1 - Introduction to Computer Graphics.

Computer Graphics

describes the use of computers to create and manipulate images

Modeling

deals with the mathematical specification of shape and appearance properties in a way that can be stored on the computer

Rendering

deals with the creation of shaded images from 3D computer models

Animation

a technique to create an illusion of motion through a sequence of images

Raster

an array of pixels (picture elements) displayed on a screen, arranged in a grid with two (2) dimensions

Pixels

specify colors using triples of floating-point numbers between 0 and 1, which represent the amount of red, green, and blue light existing in a color

RGB values

0 - indicates that no amount of color exists
1 - represents that color is displayed at full intensity

Resolution

is the number of pixels in the raster

Precision

is the number of bits used for each pixel

Buffer
(data buffer or buffer memory)

is a part of a computer’s memory that serves as temporary storage for data while being moved from one location to another

FrameBuffer

• pixel data is stored in a region of memory

• may contain multiple buffers that store different types of data for each pixel

Color Buffer

• stores RGB values

• at minimum, the framebuffer must contain this

Depth Buffer

• stores distances from points on scene objects to the virtual camera.

• determine whether the object’s points are in front of or behind other objects

  • from the camera perspective

Stencil Buffer

• store values used in generating advanced effects

  • such as shadows, reflections, or portal rendering

Frame

A single image that is displayed in animations

Frame Rate

• the speed or rate at which images appear in an animation

• measured in frames per second (FPS)

Graphics Processing Unit

(GPU)

features a highly parallel structure that makes it more efficient than CPUs for rendering computer graphics

Shaders

• programs run by GPUs

• used to perform many different computations required in the rendering process

Application Programming Interface

(API)

• implemented by Shader programming languages

• defines a set of commands, functions, and protocols that can be used in interacting with an external system such as the GPU

DirectX API & High-Level Shading Language (HLSL)

API and their shader language:

• used on Microsoft platforms, including the Xbox game console

Metal API & Metal Shading Language

API and their shader language:

• runs on modern Mac computers, iPhones, and iPads

OpenGL (Open Graphics Library) & OpenGL Shading Language (GLSL)

API and their shader language:

• a cross platform library

  • is the most widely adopted graphics API

Graphics Pipeline

• is an abstract model used to describe a sequence of steps needed in rendering a three-dimensional scene

• pipelining enables a computational task to be split into subtasks thus increasing overall efficiency

• increase the efficiency of the rendering process, enabling images to be displayed at faster rates

Application

4 stages of pipeline model used in OpenGL:

• initializing the window where rendered graphics will be displayed

• sending data to the GPU

Geometry Processing

4 stages of pipeline model used in OpenGL:

• determining the position of each vertex of the geometric shaped to be rendered

  • implemented by a program known as vertex shader

Rasterization

4 stages of pipeline model used in OpenGL:

• determining which pixels correspond to the geometric shapes to be rendered

Pixel Processing

4 stages of pipeline model used in OpenGL:

• determining the color of each pixel in the rendered image

  • involving a program called a fragment shader

Creating a window where the rendered graphics will be displayed

Stage 1: Application:

• the window must be initialized to read the graphics from the GPU framebuffer

• for animated and interactive applications, the main application contains a loop that repeatedly re-renders the scene, usually aiming for a rate of 60 FPS

Reading data required for the rendering process

Stage 1: Application:

• data may include vertex attributes which is stored in vertex buffer objects (VBOs).

• stores images to be used as textures in texture buffers

• source code for the vertex shader and fragment shader programs need to be sent to the GPU, compiled, and loaded

Vertex Attribute

describe the appearance of the geometric shapes being rendered

Vertex Buffer Objects

(VBOs)

a GPU memory buffers used to store vertex attribute data

Texture Buffers

stores images to be used as textures

Sending data to the GPU

Stage 1: Application:

• application needs to specify the associations between attribute data stored in VBOs and attribute variables in the vertex shader program.

• a single geometric shape may have multiple attributes for each vertex (such as position and color).

• the corresponding data is streamed from buffers to variables in the shader during rendering.

• frequently, it is also necessary to work with many sets of such associations.

• multiple geometric shapes may be rendered by the same shader program

• each shape may also be rendered by different program

• these set of associations can be managed using vertex array objects (VAOs)

Vertex Array Objects

(VAOs)

stores the information of the sets of associations and can be activated and deactivated as needed during the rendering process

Mesh

• the shape of a geometric object is defined by ___

• a collection of points that are grouped into lines or triangles

Vertex

• the properties or attributes that are specific to rendering each individual point are grouped together into a data structure called ___

• should contain the three-dimensional position of the corresponding point.

Includes:

• a color to be used when rendering the point

• Texture coordinates (or UV coordinates)

• a normal vector

Texture Coordinates

(or UV Coordinates)

indicate a point in an image that is mapped to the vertex

Normal Vector

indicates the direction perpendicular to a surface and is typically used in lighting calculations

Wireframe
Vertex Colors

Texture

with Lighting Effects 🍒

figures that illustrated different renderings of a sphere that make use of these attributes

Model Transformation

Stage 2: Geometry Processing

• the collection of points defining the intrinsic shape of an object may be translated, rotated, and scaled

• the object appears to have a particular location, orientation, and size with respect to a virtual three-dimensional world

• the coordinates expressed from this frame of reference are in world space

World Space

the origin is at the center of the scene

View Transformation

Stage 2: Geometry Processing

• coordinates in this context are said to be in view space

View Space

(or camera space, or eye space)

is the result when world-space coordinates are transformed to coordinates in front of the user’s view

Projection Transformation

Stage 2: Geometry Processing

• coordinates expressed at this stage are in clip space

Clip Space

any points outside the specified region are discarded or clipped from the scene

Stage 3: Rasterization

• once the vertex shader has specified the final positions of each vertex, the rasterization stage begins.

• the points themselves must first be grouped into the desired type of geometric primitive: points, lines, or triangles, consisting of sets of 1, 2, or 3 points

• for lines and triangles, additional information must be specified

  • For example: an array of points [A, B, C, D, E, F] is to be grouped into lines

  • they could be grouped in disjoint pairs, such as (A, B), (C, D), (E, F), producing a set of disconnected line segments

  • the could also be grouped in overlapping pairs such as (A, B), (B, C), (C, D), (D, E), (E, F)

Line Strip

• a set of connected line segments

• array of points are grouped in overlapping pairs

Primitive Assembly

process of grouping points into geometric primitives

Fragment

• is created for each pixel corresponding to the interior of a shape

• a collection of data used to determine the color of a single pixel in a rendered image

Raster Position

• also called pixel coordinates

• the data stored in a fragment always includes __

Fragment Shader

a program applied to each of the fragments to calculate their final color.

Stage 4: Pixel Processing

primary purpose of this stage is to determine the final color of each pixel, storing this data in the color buffer within the framebuffer.

• this calculation may involve a variety of data stored in each fragment, in combination with data globally available during rendering

  • a base color applied to the entire shape

  • colors stored in each fragment (interpolated from vertex colors)

  • textures (images applied to the surface of the shape), where colors are sampled from locations specified by texture coordinates

  • light sources, whose relative position and/or orientation may lighten or darken the color, depending on the direction the surface is facing at a point, specified by normal vectors