chapter 2 graphics programming with c++ and the dark gdk library

Addison Wesley is an imprint of

Chapter 2Graphics Programming with C++ and

the Dark GDK Library

Starting Out with

Games & Graphics in C++

Tony Gaddis

2.1 Getting Your Feet Wet with C++ and the Dark GDK Library

Concept:

All C++ programs that use the Dark GDK library start out with the same code. The first step in learning to write a graphics program is to learn how to start a Dark GDK program in C++.

Minimum framework of a C++/Dark GDK program

Figure 2-6 Summary of the skeleton

program

• Include directive – causes the contents of a file to be included in a program– DarkGDK.h contains the setup code

needed for the Dark GDK library to work correctly with our C++ programs

– We must include the DarkGDK.h file in every program that will use the Dark GDK library

• Function – group of statements that collectively has a name

– Function named DarkGDK is required by any program that uses the Dark GDK library

–DarkGDK function contains the statements that will be executed when program runs

• C++ is a case-sensitive language, which means it regards uppercase letters as being entirely different than their lowercase counterparts

• The name of the function DarkGDK must be written with

– uppercase D– lowercase ark– uppercase GDK

• Pay attention to the case!– C++ does not see

• darkgdk the same as DarkGDK• VOID the same as void

• Function Call – causes the statements in a function to execute

• dbWaitKey – when this function is called, it causes the program to pause until a key is pressed on the keyboard

• Comments are:– Short notes explaining how parts of a program

work– Not intended for the compiler– Intended for anyone who is reading the code

Comments

• Two types of comments:– Line comments

• begin with two forward slashes– // used to comment a single line

– Block comments• Begin with /* and end with */

– /* used to comment– multiple lines */

Comments

• An example of a program that contains line comments:

Comments• An example of how block comments may

be used:

• Use blank lines and indentations to create a sense of visual organization

• For example, one convention virtually all programmers follow is:– indenting statements inside a function

• Following conventions, such as indenting statements inside a function, is known as programming style

Programming Style:

Making Your Code Easier to Read

2.2 The Screen Coordinate System

Concept:

A system of X and Y coordinates is used to identify the locations of pixels in a window.

• The images that are displayed on a computer screen are made up of tiny dots called pixels

• The default Dark GDK window is 640 pixels wide and 480 pixels high

• It has a resolution of 640 by 480

Figure 2-3 The width and height of the default window

• A screen coordinate system is used to identify the position of each pixel in the window.

• Each pixel has – X coordinate

• Identifies the horizontal position

And– Y coordinate

• identifies the vertical position

• Coordinates are written in the form (X, Y)– For example:

• Upper-left corner pixel coordinates are (0, 0)– X is 0– Y is 0

• The X coordinates increase from left to right• The Y coordinates increase from top to bottom

– This is different from the Cartesian coordinate system you learned about in mathematics

• Coordinate numbering begins at 0 in the upper-left corner– Lower-right corner pixel coordinates are (639, 479)

• X is 639• Y is 479

Figure 2-4 various pixel locations in a 640 by 480 window

• The dbDot function draws a dot at a specific pixel location in the Dark GDK window.

• Here is the general format of how you call the dbDot function:

dbDot(x, y);

Drawing Dots with the dbDot Function

• In the general format, – the x argument is the X coordinate– the y argument is the Y coordinate

• For example, the following statement draws a dot at the X coordinate 319 and the Y coordinate 239:

Drawing Dots with the dbDot Function

dbDot(319, 239);

• The values that you write inside the function’s parentheses are called arguments

• Arguments are pieces of data that you send to a function when you call it

• When an argument is sent to a function, you are passing the argument to the function

• The dbWait function causes the program to wait for a specified amount of time before continuing

• Here is the general format of how you call the dbWait function:

dbWait(time);

The dbWait Function

• The value that you provide for the time argument is the number of milliseconds you want the program to wait.– There are 1000 milliseconds in a second

• The following statement will cause the program to wait for 1 second:

dbWait(1000);

The dbWait Function

• Dark GDK function names start with the letters db• For example:

– dbDot– dbWait– dbWaitKey

• The Game Creators, the software company that created the Dark GDK library, have also created a programming language called Dark BASIC– Most Dark GDK functions are the C++ equivalents of

Dark BASIC commands• For this reason, Dark GDK function names start

with the letters db, meaning Dark BASIC

2.3 Basic 2D Shapes

Concept:

The Dark GDK library contains several functions for drawing basic 2D shapes.

2.3 Basic 2D Shapes

• Objects that appear in 2D have only two dimensions:– Width– Height

Figure 2-6 A two-

dimensional game

character

2.3 Basic 2D Shapes

• The Dark GDK library provides several functions for drawing simple 2D shapes– Lines– Circles– Ellipses– Rectangles

2.3 Basic 2D Shapes

• The dbLine function draws a line between two points in the Dark GDK window.

• Here is the general format of how you call the dbLine function:

dbLine(x1, y1, x2, y2);

Drawing Lines: The dbLine Function

2.3 Basic 2D Shapes

• You pass four arguments to the dbLine function– X1 and Y1 are the coordinates for the starting

point of the line– X2 and Y2 are the coordinates for the line’s

ending point

2.3 Basic 2D Shapes

• For example, the following statement draws a line between the points (80, 120) and (400, 520):

dbLine(80, 120, 400, 520);

Figure 2-6 A line drawn from (80,

120) to (400, 520)

2.3 Basic 2D Shapes

• The dbBox function draws a filled rectangle– Filled means it is filled with color

• Here is the general format of how you call the dbBox function:

Drawing Rectangles: The dbBox Function

dbBox(x1, y1, x2, y2);

2.3 Basic 2D Shapes

• You pass four arguments to the dbBox function– X1 and Y1 are the X and Y coordinates for the

rectangle’s upper-left corner– X2 and Y2 are the X and Y coordinates for the

rectangle’s lower-right corner

2.3 Basic 2D Shapes

• For example, look at the following statement:

dbBox(100, 80, 540, 380);

Figure 2-12 A rectangle with corners at

(100, 80) and (540, 380)

2.3 Basic 2D Shapes

• The dbCircle function draws a circle

• Here is the general format of how you call the dbCircle function:

Drawing Circles: The dbCircle Function

dbCircle(x, y, radius);

2.3 Basic 2D Shapes

• The x and y arguments are the coordinates of the circle’s center point

• The radius argument specifies the circle’s radius– The radius is the distance, in pixels, from the

center point to the outer edge

2.3 Basic 2D Shapes

• Here is an example:

dbCircle(320, 240, 100);

Figure 2-16 A circle with its

center at (320, 240) and a

radius of 100

2.3 Basic 2D Shapes

• The dbEllipse function draws an ellipse– an ellipse is an oval shape

• Here is the general format of how you call the dbEllipse function:

Drawing Ellipses: The dbEllipse Function

dbEllipse(x, y, xrad, yrad);

2.3 Basic 2D Shapes

• The x and y arguments are the coordinates of the ellipse’s center point

• The xrad argument specifies the ellipse’s radius along the X axis

• The yrad argument specifies the ellipse’s radius along the Y axis

2.3 Basic 2D Shapes

• Here is an example:

Figure 2-18 An ellipse’s

center point, x-radius, and y-

radius

dbEllipse(320, 240, 140, 100);

2.3 Basic 2D Shapes

• Any point that has– X coordinate from 0 through 639– Y coordinate from 0 through 479

• Is visible in the Dark GDK window

• You can use points that have coordinates outside these ranges but…– They are not visible in the window

Drawing Outside the Dark GDK Window

2.3 Basic 2D Shapes

• For example, the following statement draws a dot at the coordinates (800, 600)

dbDot(800, 600);

• But because that location is outside the Dark GDK window, it will not be visible

2.3 Basic 2D Shapes

• Points that are above the top row of pixels in the window have a negative Y coordinate

• Points that are to the left of the leftmost column of pixels have a negative X coordinate

Figure 2-20 Negative coordinates

2.3 Basic 2D Shapes

• The following statement draws a circle with its center point located at (-50, -20) and with a radius of 200

dbCircle(-50, -20, 200);

Figure 2-21 Circle drawn partially off-screen

2.4 Displaying Text

Concept:

You can use the dbPrint, dbText, or dbCenterText functions to display text in the Dark GDK window. You can use the dbSetWindowTitle function to display text in the window’s title bar.

2.4 Displaying Text

• You can use these functions to display text in the Dark GDK Window– dbPrint– dbText– dbCenterText

Displaying Text Inside the Dark GDK Window

2.4 Displaying Text

• The dbPrint function displays a string of characters– String is a term used in programming to mean

“string of characters”

• Here is the general format of how you call the dbPrint function:

dbPrint(string);

2.4 Displaying Text

• The string argument is the string you want to display• The string is printed as a line of output in the Dark GDK

window• If no argument is passed

– a blank line will be displayed• The first call to the dbPrint function

– output is printed at the top of the window, justified along the left side

• Each subsequent call to the dbPrint function– prints a line of output below the previous line of output.

2.4 Displaying Text

• The dbText function displays a string of characters at a specific location in the window

• Here is the general format of how you call the dbText function:

dbText(x, y, string);

2.4 Displaying Text

• The x and y arguments are a set of coordinates

• The string argument is the string that is to be displayed

• When the string is displayed– The upper-left corner of the first character will

be positioned at the X and Y coordinates

2.4 Displaying Text

• For example, the following statement displays the string “Hello World” in the window

dbText(10, 10, “Hello World”);

Figure 2-23 Results of the

dbText function

2.4 Displaying Text

• Text can be centered horizontally with the dbCenterText Function• Here is an example:

dbCenterText(319, 239, “Game Over”);

Figure 2-25 A string centered

just below the point at (319,

2.4 Displaying Text

• The dbSetWindowTitle function displays text in the window’s title bar

• Here is the general format of how you call the dbSetWindowTitle function:

Displaying Text in the Window’s Title Bar

dbSetWindowTitle(string);

2.5 The Program Development Cycle

Concept:

When creating programs, programmers typically follow a process known as the program development cycle.

2.5 The Program Development Cycle

• Design the Program• Write the Code• Correct Syntax Errors• Test the Program• Correct Logic Errors• Repeat until error free

Figure 2-37 The program development cycle

Addison Wesley is an imprint of

Chapter 2Graphics Programming with C++ and the

Dark GDK Library

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chapter 2 graphics programming with c++ and the dark gdk library

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