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The description in the documentation doesn't make a lot of sense to me. I believe it is safe to say, however, that in this XNA program, the viewport is synonymous with the gamewindow. However, that is not always the case. A game window can contain more than one viewport. See How To: Use Viewports for Split Screen Gaming for an example.

Compute the position of the UFO relative to the game window

The code in Listing 4 computes a position vector which, when applied during the drawing of the UFO, will cause the UFO to occupy a position approximatelylike that shown in Figure 4 .

Listing 4 . Compute the position of the UFO relative to the game window.

ufoPosition.X = viewport.Width / 2; ufoPosition.Y = viewport.Height - 70;

I say approximately because one additional adjustment to the position of the UFO will be made later using the origin parameter of the SpriteBatch.Draw method. If that adjustment were not made, the upper left corner of the rectangle that contains the UFO would beplaced at the position computed in Listing 4 . The origin property will be used to center the UFO on the position that is computed in Listing 4 .

Compute the base scale factor for the background image

Listing 5 computes a scale factor which, when applied to the background image during the drawing process, will cause the 640x480 background image to just fitthe game window. Recall that the size of the game window was set in the constructor of Listing 2 taking the aspect ratio of the background image into account.

Listing 5 . Compute the base scale factor for the background image.

backgroundBaseScale = (float)(450.0 / 640.0); }//end LoadContent

Not the only scale factor

The scale factor computed in Listing 5 will be applied to the background image when the program first starts running. However, during most of the runningof the program, a different scale factor will be applied to the background image and the scale factor computed in Listing 5 will be only one component of that overall scale factor.

An optical illusion

The application of a scale factor to the background image that changes with time is what causes the planet to grow in size giving the illusion that the UFOis approaching the surface of the planet.

Listing 5 signals the end of the LoadContent method.

The overridden Update method

The overridden Update method begins in Listing 6 .

Listing 6 . Beginning of the overridden Update method.

protected override void Update(GameTime gameTime) { //Compute the elapsed time since the last update.// Draw new data only if this time exceeds the // desired frame interval given by msPerFramemsElapsed += gameTime.ElapsedGameTime.Milliseconds; if(msElapsed>msPerFrame) { //Reset the elapsed time and draw the frame with// new data. msElapsed = 0;

Controlling the animation speed

The code in Listing 6 is not new to this module. I explained code like this in an earlier module.

Briefly, the Update method is called sixty times per second by default (I didn't change the default). That is too fast to produce ananimation that looks the way I wanted it to look.

Questions & Answers

How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
How can I make nanorobot?
Lily
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
how can I make nanorobot?
Lily
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
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Source:  OpenStax, Xna game studio. OpenStax CNX. Feb 28, 2014 Download for free at https://legacy.cnx.org/content/col11634/1.6
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