0.13 Xna0124-using background images and color key transparency  (Page 3/9)

The UFO image

The UFO image is shown in Figure 2 . Note the green background. All of the green pixels in the UFO image will be replaced by transparent black pixels whenthe program runs. I took advantage of the Color Key transparency feature by setting the properties for the UFO image as shown in Figure 1 .

Figure 2 . The UFO image.

Image of the planet

Figure 3 shows a reduced view of the image showing the planet that I used as a background image.

Figure 3 . Reduced version of the alien planet image.

The version that I used in the program was 640 pixels wide and 480 pixels high. However, that is too large to publish comfortably in this narrow publication format,so I reduced the image in Figure 3 to 450x338 for publication purposes only. The source code that you will see later is based on an image size of 640x480.

The screen output

Figure 4 through Figure 12 show nine screen shots taken while the animation was running.

In Figure 4 , the UFO is approaching the planet from some distance away. Note that the green portion of the UFO image has become transparent. Also note the sizeof the planet and its moon for comparison with subsequent screen shots.

Figure 4 . The UFO approaching the planet from some distance away.

Flying a relatively straight path

In Figure 5 through Figure 7 , the UFO continues along a relatively straight path flying over the planet and getting closer to the surface of the planet all thetime. Note how the planet increases in size from one image to the next, giving the illusion that the UFO is getting closer to the surface of the planet.

The background is moving

Note also that the position of the UFO in the game window is not changing. Instead an illusion of motion is created by causing the background to change.This is a technique that was used for many years in the movies to create the illusion that actors were in a car driving along the highway when in fact, thecar was standing still inside a studio. The image of the stationary car was superimposed on a moving background image.

Figure 5 . Flying over the planet.

Figure 6 . Flying over the planet.

Figure 7 . Flying over the planet.

Preparing to change course

In Figure 8 , the UFO has gone about as far as it is going to go along the original course. It is preparing to change course and come back towards thecamera getting ever closer to the surface of the planet.

Figure 8 . Preparing to change course.

Flying towards the camera

In Figure 9 and Figure 10 , the UFO has changed course. It is now flying towards the camera very close to the surface of the planet. You can even see some surfacefeatures on the planet (due to the distortion that is produced when a bitmap image is enlarged).

Figure 9 . Flying towards the camera very close to the surface of the planet.

Figure 10 . Still flying towards the camera very close to the surface of the planet.

In Figure 11 , the UFO is almost touching the rings around the planet.

Figure 11 . Almost touching the rings around the planet.

Heading for home

In Figure 12 , the UFO leaves the planet and heads for home. Shortly after this, the program will reset and repeat the animation sequence.