Video Game Design Foundations, 2nd Edition Page 136 (136 of 400)

136
Video Game Design Foundations
When the refresh rate is too low, the human eye will see flickering on
the screen.
Flickering
occurs because portions of the screen are not being
illuminated quickly enough and black from nonilluminated pixels shows.
This differs from tearing. Video
tearing
occurs when frames are out of sync
or are only partially refreshed. When the packets of video data are sent to
the monitor, some of them do not get to the monitor and the image is only
partially refreshed. The solution to tearing is using a multiple buffering
system, such as double buffering, to assure that the image displayed on the
screen is complete.
Double buffering
requires two buffers, called the front buffer and the
back buffer. The front buffer contains the composite image being displayed on
the screen. The back buffer feeds to the front buffer only the segments of the
composite image that have changed. Instead of having to rewrite the entire
front buffer data, only the changed pixels are uploaded to the front buffer. This
keeps the computer only reading front buffer data. The back buffer contains all
preloaded data that can be changed on the screen.
To understand this, think about a game like Pong. Most of the screen
remains the same throughout the game. CPU load will decrease if the back
buffer only sends information to change the front buffer, for example to add
white pixels where the ball moved, black pixels where the ball was before, new
pixels for the score, and new pixels for paddles.
Figure 4-15.
Goodheart-Willcox Publisher
As a video is downloaded for streaming in an Internet browser,
the data bar shows the current position of the video as well as the
downloaded amount in the buffer.

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136
Video Game Design Foundations
When the refresh rate is too low, the human eye will see flickering on
the screen.
Flickering
occurs because portions of the screen are not being
illuminated quickly enough and black from nonilluminated pixels shows.
This differs from tearing. Video
tearing
occurs when frames are out of sync
or are only partially refreshed. When the packets of video data are sent to
the monitor, some of them do not get to the monitor and the image is only
partially refreshed. The solution to tearing is using a multiple buffering
system, such as double buffering, to assure that the image displayed on the
screen is complete.
Double buffering
requires two buffers, called the front buffer and the
back buffer. The front buffer contains the composite image being displayed on
the screen. The back buffer feeds to the front buffer only the segments of the
composite image that have changed. Instead of having to rewrite the entire
front buffer data, only the changed pixels are uploaded to the front buffer. This
keeps the computer only reading front buffer data. The back buffer contains all
preloaded data that can be changed on the screen.
To understand this, think about a game like Pong. Most of the screen
remains the same throughout the game. CPU load will decrease if the back
buffer only sends information to change the front buffer, for example to add
white pixels where the ball moved, black pixels where the ball was before, new
pixels for the score, and new pixels for paddles.
Figure 4-15.
Goodheart-Willcox Publisher
As a video is downloaded for streaming in an Internet browser,
the data bar shows the current position of the video as well as the
downloaded amount in the buffer.

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137
Chapter 4 Perspective, Scene Design, and Basic Animation
Tile-Based Games
Tile-based video games that use a grid map, like checkers, would also
work well for blitting and double buffering. Only the tiles that contain a moved
checker or the tile where the checker was would need to be redrawn for each
turn. Tile-based games can be very complex and use standard sized tiles to
represent a portion of the game map, actor, or asset.
Tile-based games use standard-size tile images to make a composite
game map and other objects within the game. To save computer memory, the
computer only draws in the tiles that are composited as the display image that
the player sees. The other tiles are stored in the back buffer so they can be
quickly drawn into the next frame to be displayed.
Traditional tile based games, like Scrabble, use a square tile that is placed
on a grid game board. Tile-based video games work the same way by adding
square tiles to create a composite image of the game map. Characters, vehicles,
game pieces, and more are drawn as tiles on a layer above the game map to
move with the action of the game, as shown in Figure 4-16.
The video games SimCity and Civilization are examples of tile-based games.
If you use the map editor functions of these games, the tile-based aspect is
clear. Civilization 5 introduced hexagonal tiles to that series of video games.
Previously, the Civilization games had rectangular tiles.
Figure 4-16.
iralu/Shutterstock.com
A tile-based video game uses standard-size images, such as those
shown here, for the characters and objects within the game. The
chroma green marks areas of transparency.

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135
Chapter 4 Perspective, Scene Design, and Basic Animation
One solution to a device with low CPU power is to use blitting or double
buffering with bitmaps. The basic premise of blitting or double buffering a
bitmap is to significantly reduce the amount of CPU usage. Imagine running
the same scrolling image in one case where 90 percent of the CPU is used and
another that uses only 10 percent of the CPU—without loss of quality. This can
be achieved using blitting and double buffering.
Blitting
Blitting
is a computer operation that takes one or more bitmap images from
a memory block and displaying it on a display device or copying it into another
bitmap image. Blit stands for bit block transfer. When video games are designed
and programmed, several different images and objects must be displayed on
the screen. The computer tracks all of these images independently, combines
them, and displays them all as a single screen image. Blitting takes out all
the individual images and only sends a single rasterized image to the CPU
to display. The CPU has less work to do because only the pixels that change
from one screen to the next must be generated. The unchanged pixels remain
illuminated from the previous screen image.
Blitting works because the display refreshes very quickly. The
refresh
rate
is how often the screen is redrawn. Refresh rates are measured in Hertz
(Hz), which is how many times per second something occurs. A refresh rate
of 1 Hz refreshes the screen once every second. Refresh rate is different from
frame rate.
Frame rate
is the speed at which frames are played, while refresh
rate is the frequency that a displayed frame is redrawn. Video displays have a
refresh rate higher than the frame rate. For example, a computer typically has
a frame rate of 30 frames per second, but most monitors have a refresh rate
of at least 60 Hz. This combination means each frame is redrawn twice and
there are 60 calls to draw per second (30 × 2 = 60).
For digital video displays, each pixel must be drawn in at a refresh rate
of 60 Hz or higher. Because some television, movie, and HD monitors have
substantially higher refresh rates—as high as 600 Hz or more—blitting is not
needed for equipment with these high-end specifications.
Double Buffering
Blitting uses double buffering to combine the individual game objects into
a single bitmap that is rendered on the screen.
Buffering
is preloading data into
a section of memory called the buffer. Everyone who has downloaded a song,
game, movie, or other streaming video or audio has seen a playback bar at the
bottom of his or her screen, as shown in Figure 4-15. The data bar typically
shows the current position of the video and also the downloaded amount
in the buffer. The video display is reading the information directly from the
buffer memory location. If the playback rate is faster than the download rate,
the audio or video will stop, lag, pause, or even display an hourglass with a
message stating “Buffering…”.
Reducing CPU Usage

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