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

116 APPLE II COMPUTER GRAPHICScolored background mangled. You could erase it using the color of thefield, but that wou ld mess up the black section of the background. TheXDRAW command eliminates that problem. With Listing 1©-5 in memory,type171819 PRINT CLR$ (Cl)21 NEXT Cl3948 XDRAW 1 AT X ,YRun the modified program and notice the differences. Line 48© changesthe DRAW to XDRAW-, and the other statements eliminate C2 which representsthe color in which the apple was DRAWn. Where DRAW attempts toplot the apple in the specified color, XDRAW ignores HCOLOR and plotsthe shapes in colors which depend entirely on the background. XDRAWautomatically plots the shape in the complement of the background color.Black and white are complements, as are blue and green, orange andviolet. XDRAWing on the green field will give you a blue apple, and anyportion which overlaps into the black will be white. The usefu l aspect ofXDRAW is that when it XDRAWs the figure the second time, it erasescleanly and completely, regard less of the background. This attribute is veryhandy when animating-a subject we will cover in later chapters.RecappingWe have shown you how to design and digitize shapes, and at the sametime we have hopefu lly shown you the value of a good Hi-Res editor. Ifyou are planning to do any serious work in Hi-Res, an editor is a necessarytool. We have also demonstrated how to use BSAVE or the Monitor "W"to save the table, and BLOAD or SHLOAD to recover it. You have alsobeen introduced to the commands used to plot, rotate, and scale theshape. The initial work involved with shapes is wearisome and frustrating,but when the shape is done it is a simple yet powerfu l element of yourprogramming arsenal.VocabularyDigitizeDRAWOffsetPoint of OriginROTSCALEShapeShape TableSH LOADVectorVector DiagramXDRAW