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

CHAPTER 12-BYTE-MOVE SHAPES 139l!ill!il REM CALCULATE Yl!iJ2!il REM COORDINATESlYJJ!il REM1YJ4!il DIM Y%( 192)l!il5!il FOR I = 1 TO 185 STEP 8: READ SA%l!iJ6!il FOR J = !il TO 7:Y%(I + J) = SA% +J * lYJ241YJ7!il NEXT J,I1YJ8!il DATA 8192,8J2YJ, 8448,8576, 87YJ4,88J2,896YJ,9YJ881YJ9!il DATA 82J2,8J6YJ, 8488, 8616 , 8744,8872,9!il!iJYJ, 9128ll!il!il DATA 8272,84YJYJ,8528, 8656,8784, 8912 ,9YJ4YJ,9168lllYJ RETURNListing 12-2.Lines 1 ©©© th rough 111 fl) cal cu late the starting addresses for each of the192 screen lines by taking the base address of the box that the line lies inand adding the position address to it. That position address is calculated asa multiple of 1©24. If that algorithm escapes you, try reviewing the portionof Chapter 8 where we talked about calculating addresses fo r Hi-Res memory.Line 245 is executed after each frame is displayed, and its purpose is toincrement each of the seven Y coordinates so that the prop will be plottedone line lower the next time.Run the mod ified program. You will see the propeller dropping downthe left edge of the screen as it spins. You will also see a trail of garbageleft behind as the shape progresses. Most of each frame is wiped out whenthe next one plots over it, but since we are moving down between frames,the top line of each frame remains on the screen to haunt you.When moving a shape using byte-move, it is often neccesary to makespecial provisions for erasing the left-overs.We all know that your first try never runs perfectly anyway, and theproblem is easily remedied by adding:215 POKE Y%(YYJ%) ,YJ244 YYJ% = Y1%Line 244 stores the Y-coordinate for the old top line as Y©%, and line 215POKEs a zero there to erase that byte.Getting It AcrossSo far, you have performed a stationary and a vertical animation using