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• Steps of Image Generation– Create a model of the objects– Create a model for the illumination of the

objects– Create an image (render) the result

I Computer Graphics Background

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Image Terminology• Pixel – picture element

– One location in a computer image– Gray scale image has one value– Color image has three values for red, green,

and blue

• Frame buffer– The two-dimensional grid of pixels for an

image

• A window is an area of the computer screen where the image is displayed

I-3

Graphics Pipeline

• Three main stages– Modeling– Rendering– Display

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Modeling Stage

• Specifies details of objects in scene– Shape– Materials– Color– Shininess, transparency, translucency– Texture

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Rendering Stage

• Simulates the flow of light

• Models the way light interacts with objects– Different materials react with light differently

• Determines visibility of objects

I-6

Image Display Stage

• Image display is supported by graphics hardware

• Different devices output images differently– Manufacturers use different components,

which accounts for the difference

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View Information• The viewpoint gives the location of the

viewer• The view direction is a vector specifying

the direction the viewer is looking

I-8

Coordinate Systems

• Object coordinate system– Used to define object shape

• World coordinate system– Used to position various objects in the scene

• View coordinate system– Defined by the viewpoint, view direction, and

view up direction

• Window coordinate system– Defines the part of the world that is visible

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Local or object, world, view, and window coordinate systems

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• Right-handed vs. left-handed coordinate systems

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Homogeneous Coordinates

• Locations are typically specified through the (x, y, z) coordinate

• Homogeneous coordinates add a fourth component to get (x, y, z, w)

• Typically, the w term will be 1 but transformations may change this

• If w in (x, y, z, w) is not 1, the actual (x', y', z') coordinate is (x/w, y/w, z/w)

I-12

Color Models

• Used to specify a color

• There are many different color models that have different purposes

• Color can be defined by wavelength– Wavelengths can be combined to produce

color

I-13

RGB Color Model

• Based on red, green, and blue

• An additive color model– Red, green, and blue light is combined to

create different colors

• Can be shown on a cube with red, green, and blue on the three major axes

• Shades of gray appear along the main diagonal

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RGB Color Model

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CMY Color Model• Based on cyan, magenta, and yellow

• A subtractive color model– Cyan, magenta, and yellow ink or toner

subtract color from white

• Closely related to the RGB model– C=1.0-R, M=1.0-G, Y=1.0-B

• CMYK model also includes black to create richer output– K=minimum(C, M, Y), C=C-K, M=M-K, Y=Y-K

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HSV Color Model

• Based on hue, saturation, and value

• An artistic model

• Hue determines color

• Saturation determines the amount of color– How much white is added to the full color

• Value indicates the lightness of the color– How much black is added to the full color

I-17

HSV Color Model

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HLS Color Model

• Based on hue, lightness, and saturation

• Similar to the HSV color model

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Displaying Images

• Output devices include:– Monitors– Flat panel displays– Printers– Three-dimensional printers

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