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Chapter 7 The Game Loop and Animation

Chapter 7 The Game Loop and Animation. Starting Out with Games & Graphics in C++ Tony Gaddis. 7.1 The Game Loop. Concept: The game loop is a special loop used in games and animation programs. It synchronizes the refreshing of the screen with the program’s other operations.

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Chapter 7 The Game Loop and Animation

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  1. Chapter 7The Game Loop and Animation Starting Out with Games & Graphics in C++ Tony Gaddis

  2. 7.1 The Game Loop Concept: The game loop is a special loop used in games and animation programs. It synchronizes the refreshing of the screen with the program’s other operations.

  3. 7.1 The Game Loop • Virtually all games and animation programs have a loop of some sort that continuously performs operations such as • Calculations • Gathering input • Moving objects on the screen • Playing sounds • And so forth • In such a program, the loop must allow the screen to be updated at the appropriate time • In other words, you must synchronize the loop with the updating of the screen

  4. 7.1 The Game Loop The Dark GDK provides the following functions you can use for synchronizing the loop with the updating of the screen:

  5. 7.1 The Game Loop • Disable automatic screen refreshing • Establish a refresh rate • Test and synchronize the game loop • Display graphics • Refresh the screen • Here is the general format of the game loop: Figure 7-1 Game loop code

  6. 7.1 The Game Loop Giving Control Back to the Dark GDK • The Dark GDK library has a function called dbSyncOff that causes automatic screen updating to start again • Not needed very often • Can be helpful in some situations • For example: • When prompting the user to enter a value during the game loop • The prompt won’t display until after the screen is refreshed with the dbSync function • To display the prompt and resume the game loop: • Call dbSyncOff just before displaying the prompt • Call dbSyncOn after the user enters a value

  7. 7.2 Simple Animation Concept: A simple way to create an animation is to write a game loop that draws a shape at a different location during each iteration. Be sure to clear the screen of anything displayed during the previous iteration, though!

  8. 7.2 Simple Animation Program 7-3 (MovingBall.cpp) partial listing Figure 7-2 Output of Program 7-3 Clear the screen Draw the shape Calculate the new XY coordinates Refresh the screen

  9. 7.2 Simple Animation Clearing the Screen in the Game Loop Figure 7-3 Drawing the ball without clearing the screen Always clear the screen before drawing a shape Otherwise, all drawings of the shape will appear on the screen

  10. 7.3 Controlling Objects with the Keyboard Concept: The Dark GDK provides functions that let you know whether certain keys, such as the arrow keys, spacebar, Enter key, and so forth are being pressed. Many games and animation programs allow the user to control objects on the screen with such keys.

  11. 7.3 Controlling Objects with the Keyboard • Games commonly allow the player to use keys on the keyboard to control objects on the screen • The Dark GDK provides the functions listed in Table 7-1 for the purpose of detecting whether the user has pressed these keys

  12. 7.3 Controlling Objects with the Keyboard • If the user is pressing the spacebar • The dbSpaceKey function returns a value of 1 (true) • The message “You pressed the spacebar.” is displayed • If the user is not pressing the spacebar • The dbSpaceKey function returns a value of 0 (false) • The message is not displayed Here is an example that determines whether the user is pressing the spacebar:

  13. 7.3 Controlling Objects with the Keyboard • A message is displayed during each iteration of the game loop for the key the user is pressing Figure 7-4 Example output of Program 7-4

  14. 7.3 Controlling Objects with the Keyboard

  15. 7.3 Controlling Objects with the Keyboard Letting the User Move an Object • To move a circle with the arrow keys: • Prepare the circle: • Declare and initialize RADIUS constant • Declare and initialize x and y coordinate variables with starting values • Prepare the game loop: • Turn on manual refresh • Set the maximum refresh rate

  16. 7.3 Controlling Objects with the Keyboard Letting the User Move an Object • Inside the game loop: • Clear the screen • Draw the circle • Update x and y values • Check arrow keys: • If up arrow key is pressed • Decrement value of y • If down arrow key is pressed • Increment value of y • If left arrow key is pressed • Decrement value of x • If right arrow key is pressed • Increment value of x • Refresh the screen

  17. 7.3 Controlling Objects with the Keyboard Performing Other Operations with the Keyboard • Increasing and decreasing the radius of a circle

  18. 7.4 Sprites Concept: A sprite is a graphic image that is used as an element in a game. Sprites can be moved and manipulated in various ways.

  19. 7.4 Sprites • The graphic images that perform actions in a computer game are commonly known as sprites • To create a sprite, you perform two actions: • Load an image into memory with the dbLoadImage function • Designate the image as a sprite and display it with the dbSprite function • Here is the general format of how you call the dbSprite function:

  20. 7.4 Sprites Creating a Sprite • SpriteNumber • Is an integer • Assigned to the sprite • In the range 1 through 65,535 • Used to identify the sprite in subsequent operations • X and Y • Are integers • Specify the screen coordinates where the sprite’s upper-left corner will be positioned • ImageNumber • Is an integer • Number of the image you want to use for the sprite

  21. 7.4 Sprites Creating a Sprite • The first statement loads the LadyBug.bmp file as image number 1 • The second statement designates that image as sprite number 1, and positions it at the screen coordinates (320, 240) • When this statement executes, the sprite will be displayed on the screen

  22. 7.4 Sprites Creating a Sprite By default, black (RGB = 0, 0, 0) is used as the key color for sprite transparency You can use the dbSetImageColorKey function to designate a different key color Program 7-7 demonstrates how we can set the key color to green, and then display the UFO image on top of the space image

  23. 7.4 Sprites Figure 7-6 The space.bmp and UFO.bmp images Figure 7-7 Output of Program 7-7

  24. 7.4 Sprites Moving a Sprite You can move an existing sprite to a new location on the screen by calling the dbSprite function and passing different values for the X and Y coordinates

  25. 7.4 Sprites Getting a Sprite’s X and Y coordinates • You can get the current screen coordinates of an existing sprite by • Calling the dbSpriteX and dbSpriteY functions • Passing the sprite number as an argument

  26. 7.4 Sprites Getting the Width and Height of a Sprite • You can get the width and height of an existing sprite by • Calling the dbSpriteWidth and dbSpriteHeight functions • Passing the sprite number as an argument

  27. 7.4 Sprites Rotating a Sprite • SpriteNumber is the number of the sprite you want to rotate • Angle is a floating-point value indicating the angle of rotation, in degrees dbRotateSprite rotates a sprite around its insertion point, which by default is the sprite’s upper-left corner 0 through 359 degrees

  28. 7.4 Sprites Rotating a Sprite Figure 7-8 UFO.sprite rotated at different angles

  29. 7.4 Sprites • You can get the angle of a sprite by calling the dbSpriteAngle function with the sprite number you want to get the angle for • For example, the following statement: • declares a float variable named angle • Initializes it with sprite number 1’s current angle of rotation

  30. 7.4 Sprites Offsetting a Sprite’s Insertion Point • SpriteNumber is the number of the sprite you want to offset • XOffset is an integer value for the amount you want to offset from the insertion point along the X axis • YOffset is an integer value for the amount you want to offset from the insertion point along the Y axis By default, a sprite’s insertion point is its upper-left corner You can change the sprite’s insertion point by calling the dbOffsetSprite function Here is the general format of how you call the function:

  31. 7.4 Sprites Offsetting a Sprite’s Insertion Point Figure 7-9 A sprite before and after it has been offset

  32. 7.4 Sprites Offsetting a Sprite’s Insertion Point • You can get the current Y offset of a sprite by calling the dbSpriteOffsetY function, passing the sprite number as an argument • For example, the following statement stores the Y offset of sprite 1 in the variable sprite1OffsetY: You can get the current X offset of a sprite by calling the dbSpriteOffsetX function, passing the sprite number as an argument For example, the following statement stores the X offset of sprite 1 in the variable sprite1OffsetX:

  33. 7.4 Sprites Showing and Hiding Sprites • The dbShowSprite function causes a hidden sprite to be displayed • You can tell if a sprite is visible with the dbSpriteVisible function • Pass sprite number as an argument • Returns 1 (true) if visible • Returns 0 (false) if hidden • You can hide all sprites with the dbHideAllSprites function • And display all hidden sprites with the dbShowAllSprites function The dbHideSprite function prevents a sprite from being displayed

  34. 7.4 Sprites Resizing a Sprite • dbSizeSprite(SpriteNumber, Xsize, Ysize); • Sprite Number • Size in pixels along the X axis • Size in pixels along the Y axis • dbStretchSprite(SpriteNumber, Xstretch, Ystretch); • Sprite Number • Percentage to scale along the X axis • Percentage to scale along the Y axis • Get the X and Y scale amounts of a sprite with: • dbSpriteScaleX(SpriteNumber); • dbSpriteScaleY(SpriteNumber);

  35. 7.4 Sprites Setting a Sprite’s Priority • Higher priority sprites are drawn last, regardless of where they appear • In the code above, sprite 1 will be drawn last and appear on top of sprites 2 and 3 Priority controls the order sprite’s are drawn All sprites have a default priority of 0 Drawn in order they appear in code Change priority with the dbSetSpritePriority function

  36. 7.4 Sprites Determining Whether a Sprite Exists • Determine whether a sprite exists by calling the dbSpriteExist function • Accepts a sprite number • Returns 1 (true) if sprite exists • Returns 0 (false) if sprite does not exist

  37. 7.4 Sprites Changing the Sprite Image • Call the dbSprite function • Sprite number • X coordinate • Y coordinate • New image number • Call the dbSetSpriteImage function • Sprite number • New image number • The dbSpriteImage function can get the current image number

  38. 7.4 Sprites Flipping and Mirroring a Sprite Figure 7-10 A flipped sprite Figure 7-11 A mirrored sprite • The dbFlipSprite function flips a sprite vertically • The dbMirrorSprite function mirrors a sprite horizontally • To determine if a sprite is mirrored or flipped call: • dbSpriteFlipped • dbSpriteMirrored • Returns 1 (true) • Returns 0 (false)

  39. 7.4 Sprites Setting the Back Save and Transparency Features Back Save Enabled, Transparency Disabled Back Save Disabled, Transparency Enabled • By default a sprite is set to • Restore its background • Use transparency • To disable one or both of these features • Call the dbSetSprite function • Sprite Number • Back Save • Transparency • 0 disables the feature • 1 enables the feature

  40. 7.4 Sprites Using a Sprite’s Alpha Value to Change its Opacity Figure 7-12 Example output of the stealthJet program • Sprites are normally opaque • Use the alpha value to make the sprite semitransparent • Call the dbSetSpriteAlpha function to change the alpha value • 0 through 255 • Call the dbSpriteAlpha function to get the alpha value

  41. 7.4 Sprites Deleting a Sprite from Memory Remove a sprite from memory by calling the dbDeleteSprite function For example, this statement removes sprite number 10 from memory.

  42. 7.4 Sprites Pasting and Cloning Sprites • Two ways to copy a sprite: • Pasting and cloning • Pasting is dependent on the original sprite • Pasted sprites are drawn to the screen immediately • Cloning is independent of the original sprite • Cloned sprites must be drawn with the sprite function

  43. 7.5 Cel Animation and Sprite Sheets Concept: You can create a simple animation by displaying a sequence of images one after the other. This can be done by manually loading and displaying separate images, or via an animated sprite sheet.

  44. 7.5 Cel Animation and Sprite Sheets Figure 7-17 Cel animation images Simple cel animations are created by displaying a sequence of images, one after the other, in the same location on the screen When played in order, slight changes in each cel create the illusion of movement

  45. 7.5 Cel Animation and Sprite Sheets Simplifying Animation with Sprite Sheets Figure 7-18 A sprite sheet Figure 7-19 A sprite sheet with two rows and four columns A sprite sheet contains all the frames of an animation sequence in one file Simpler way to store animations Organized into rows and columns Images are displayed from left to right Images are numbered, starting with 1, from left to right

  46. 7.5 Cel Animation and Sprite Sheets Playing Sprite Animations with the Dark GDK • Once you have created an animated sprite, the dbPlaySprite function can be used to play the animation by specifying a sprite number, start frame, end frame, and delay between frames • The dbPlaySprite function does not display the sprite • Call the dbSprite function to display an animated sprite You use the dbCreateAnimatedSprite function to create a single animated sprite from a sprite sheet by specifying a sprite number, file name, columns, rows, and image number

  47. 7.6 Sprite Collision Detection Concept: A collision between sprites occurs when one sprite’s bounding rectangle comes in contact with another sprite’s bounding rectangle. Collisions between sprites can be detected.

  48. 7.6 Sprite Collision Detection Figure 7-20 A sprite displayed inside its bounding rectangle Sprites have a bounding rectangle that is the width and height of the image used to display them A collision occurs when bounding rectangles overlap The dbSpriteCollision function can be used to detect collisions between two sprites by passing their sprite numbers as arguments Returns 1 (true) if collisions occur or 0 (false) otherwise Passing 0 as the second argument can detect a collision between the sprite and any other sprite

  49. Chapter 7The Game Loop and Animation QUESTIONS ?

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