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

7Preserving YourPicturesObjectivesAfter reading Chapter 7 you should be able to:• Save the Low-Res screen using BSAVE, print a list of values, write thelist of values to a text file on disk, or create a text fi le of DATAstatements on disk.• Replace a figure on the Low-Res screen by using BLOAD, read the textfile of values from disk and POKE them into screen memory, POKEvalues from DATA statements-either keyed or EXECed nto the program,or load the screen values into a safe memory area and thencopy those values to Low-Res screen memory.In Chapter 6 you discovered how to save the Low-Res screen usingBSAVE or the Monitor WRITE command. Both methods were simple, butthey both have the same drawback-they often save a great deal of emptyspace. Since disk space is a precious commod ity, we will devote thischapter to discussing alternate methods for saving your pictures.Scanning MemoryOne alternate method of saving your picture is to record which bytes ofLow-Res memory are turned "on," and what values they contain. In theapple picture in Chapter 6, only about one of every ten bytes contains anon-zero value. That is, nine-tenths of the screen is blank, and we areinterested only in the non-blank portions-those that are displaying a53