1.7 Scr0370: the repeat loop in scratch 2.0  (Page 4/7)

Animation

Since the earliest days of animation, two-dimensional (2D) animation has been created by presenting a series of drawings to the viewer in rapid succession where each drawing was similar to but slightlydifferent from the previous drawing. Because of the property of human vision to blur one Image Into the next, if the images are sufficiently similar and theyare presented in rapid succession, the human mind perceives the series of images as continuous motion. This is how motion pictures work, and it is also how thecartoons that you see on television work.

Costumes

Animation can be created in a couple of different ways in Scratch. One way is through the use of something called costumes . For example, when you createa new project in Scratch, an image of a cat having two costumes is automatically added to the project. Image D shows a screen shot of the center panel in the user interface for a new project with the Costumes tab selected.

Figure

Image d. the costumes tab for a new scratch project.

Image D shows a lot more than just the current costumes. The current costumes are shown in the upper-left corner of Image D .

The cat sprite has two costumes

As you can see, the cat sprite has two costumes, which are really two different images of the cat. The images are designed to make it look like thecat is walking if the two images are repeated in rapid succession. You can also see that each costume has a name, with the default names in this case being:

• costume1
• costume2

If you choose to do so, you can click in the boxes containing the names and modify them. Also, if you choose to do so, you can use the built-in drawing program to modifythe drawing. However, most of that is beyond the scope of this module so we won't get into it here.

With only two costumes for the cat, the walking motion is not very smooth. Somewhere on the Scratch website, I saw a project that had sixteendifferent costumes for the cat. When those sixteen different costumes are displayed in succession, the cat appears to walk very smoothly.

Loop blocks

If you click the tan Control button in the Scratch user interface, you will expose a large number of blocks that are generally used to control the flow of the program. The following control blocks that areexposed by that button can be used to construct loops:

The repeat (numeric value) block

The repeat (numeric value) block (referred to hereafter simply as the repeat block) provides the mechanism by which you can cause a set of actions to be executed a specified number of times. (See Image E .)

You create a repeat block that will do something useful by inserting programming blocks into the mouth of a repeat block. When you drag a raw repeat block into the center panel, it looks like the tan image shown in Image E .

Figure

Image e. various scratch blocks.

Setting the number of repeats (iterations)

By default, the white box in the repeat block contains a literal numeric value. You can modify that value and specify the number of times thatthe code in the repeat block will be executed. When you use a literal value, the code will be executed the same number of times every time you run theprogram.