williams-et-al-1983-apple-ii-computer-graphics

CHAPTER 10-SHAPING UP 103In the first byte, section A contains the PU code, section B the code forSR, and section C is set to (/)(/) to be ignored. The next PU code is in sectionA of byte number 2, followed by the SL in section B, and (/)(/) in C, which isagain ignored. The SU is in section A of byte number 3, followed by PRand then (/)(/). Byte number four is fi lled with zeros and marks the end ofthe shape. Any shape is ended by the fi rst zero byte encountered; therefore,if you intend to put the skip-up code ((/)(/)(/)) in sections A and B,section C MUST NOT BE ZEROS!Following these rules, Figure 1 (/)-4 shows the entire shape encoded infour bytes. The shape may not be displayed until it is entered in a shapetable, but before we do that, let's digitize one more shape fo r practice.UFO TimeIf you did not believe in flying saucers before, you will soon, as you areabout to create one. Figure 1 (/)-SA contains the block diagram of thesaucer you will create and Figure 1(/)-SB contains its vector diagram.ABFigure 1 Cll-5. Saucer.It is necessary to plot each of the 23 dots shown in Figure 1 (/)-SA, butthe order in which they are traced is not important. However, changingthe pattern of the vectors will also alter the values used in the shape, so forthe sake of consistency we suggest that you use the vectors shown inFigure 1 (/)-SB. Follow the vectors and write down a list of their functions asyou go. Your list will look like this:PR, PR, PR, SR, SD , PL, PL, PL, PL,PL, PL, PD, PR, SR, SR, PR, SR, SR,PD, PL, PL, PL, PL, PL, PL, PD, SR,SR, PR, PR, PRGo back and double check your list; accu racy is tedious but imperative!!The next step is to encode the instructions as bytes. We have done thisin Figure 1 (/)-6. To be on the safe side, the encoding is done in a series ofsmall steps, each of which is easily checked. The extra care taken now will