Chap 3 More Tools page 1CS 367

Coordinate Systems (3.1-3.2)

• Device coordinates, or screen coordinates (pixels)put limitations on programmers and the code is not portable

generally expressed in integers

• World coordinatesan effort was made to hide the actual device coordinates from the

programmer and allow ANY coordinate to be used

generally expressed in floats

programmer defines world space

• Mappingthe mathematical transformation from one coordinate system to

another

involves relatively simple algebra

See page 85 for one option

Chap 3 More Tools page 2CS 367

Mapping

• Convert from world coordinates to viewport coordinates

• Proportional along the x-axis and y-axis

• Values neededWindow left, right, bottom, top

Viewport left, right, bottom, top

Sx = VL + ((Wx - WL)/Wr - WL) * Vr - VL)

Sy = Vb + ((Wy - Wt)/Wt - Wb) * (Vt - Vb)

• ExampleDevice coordinates of 640 x 480

World coordinates of 5000 x 4000

Map (2500, 2000) to ___________

Map (1000, 1000) to ___________

Chap 3 More Tools page 3CS 367

Viewing

• Viewing rectangle or windowitems within the window will be seen and others will be clipped

• 3D Viewing VolumeglOrtho(left,right,bottom,top,near,far)

• Be aware of right handed coordinate systempositive Z comes out from screen

• Viewporta rectangular region within the window

glViewport(x,y,width,height)

• Aspect Ratiothe ratio of the viewing rectangle should be the same as the viewport

Chap 3 More Tools page 4CS 367

Mapping in OpenGL

• Primitives are multiplied by matrices before being displayed.

• OpenGL has two important matricesMODELVIEW - for viewing parameters

PROJECTION - for display parameters

• Typical initializationglMatrixMode(GL_PROJECTION);

glLoadIdentity();

glOrtho(left, right, bottom, top);

glViewport (left, bottom, width, height);

glMatrixMode(GL_MODELVIEW);

• Leave in MODELVIEW mode

• OpenGL handles the mapping and clipping of points

• Demoviewport.c

Chap 3 More Tools page 5CS 367

Animation 3.2

• repeatedly draw images to create the illusion of motion

• this may cause flicker though

• solution is double bufferingdraw images in the back buffer

display images in the front buffer

• swap when ready to displayglutSwapBuffers();

• Let OpenGL knowglutInitDisplay (GLUT_DOUBLE | GLUT_RGB);

• Demosingle_double.c

• Zooming and PanningAdjust window coordinates as needed for different effects

Chap 3 More Tools page 6CS 367

Canvas class (3.4)

• Author’s class to support to 2D rendering

• My version is available at~grissom/367/Canvas.c

• Must create a global variableCanvas cvs (640, 480, “title);

• Instance VariablesCP - current position (x, y)

CD - current direction (angle)

• MethodssetWindow (left, right, bottom, top)

setViewport (left, right, bottom, top)

clearScreen ( )

setBackgroundColor(r, g, b)

setColor (r, g, b)

moveTo (x, y)

lineTo (x, y)

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Relative Drawing (3.5)

• MethodsmoveRel (x, y)

lineRel (x, y)

• Group Activitydraw stairs

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Turtle Graphics (3.5)

• MethodsturnTo (angle)

turn (angle)

forward (dist, isVisible)

• Anglespositive rotation is CCW

starts a degree zero on X-axis

human uses degrees

computer uses radians (2 π)

• Group Activitydraw a star

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Polygons

• Calculate the points based on angle

• Convert degrees to radiansformula?

• x = radius * cos(angle) + center.x

• y = radius * sin(angle) + center.y

• Group Activitydraw a hexagon

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Input Devices

• Physical devicesPointing devices - Mouse, ligth pen, microphone, trackball, joystick

Keyboard device - keyboard

3D input - laser scanners, VR helmets, data gloves

• Logical devicesnot related to a specific physical device

device independent just like cout statements

String - sequence of ASCII characters

Locator - returns XY position

Pick - returns ID of a selected object

Choice - one of a limited number of options (menu)

Dial - valuator, returns analog values (slidebars)

Stroke - a series of points or vectors

Chap 3 More Tools page 11CS 367

Usability Design Guidelines

• Usability Engineering

• ObjectivesReduce errors

Improve performance

Ease of use

Attractive

• ExamplesNothing is every as easy to use as the designer thinks it is

EDIT (select everything, delte, insert T)

Delete everything or cancel this request?

YES or NO

Remove the diskette from the protective cover

Press any key to continue

• Usability Testing is a mustWatch typical users

It can be frustrating

Chap 3 More Tools page 12CS 367

Usability Measures

• Number of errors

• Performance time

• User satisfaction

• Ability to customize