; GUNREAD.ACT›;›;      CHECKSUM DATA›;[6A BA 3A 75 52 ]››; Read the Atari light gun›; and convert the readings›; of LPENH & LPENV to current›; graphics mode screen coordinates›;›; Algorithm developed by:›; Matthew J. W. Ratcliff›; Ratware Softworks›; (c) 1989›;›; For Analog Computing›;›; Algorithm:›; The DELTA-X gun readings were›; apparently DESIGNED to be 160›; with DELTA-Y at 96.  These values›; work out to be multiples of two,›; by powers of two, for each and›; EVERY possible Atari graphics mode›; 0 through 15 (full screen modes).›; ›; The X reading starts at about 89›; at the far left of the display,›; increases to 227 at about text›; column 34, then drops to zero.›; It increases to about 22 at the›; far right of the display.›;›; The Y reading starts at about 17›; at the top to 112 at the bottom.›;›; GunRead normalizes the X reading›; to 0-159, inclusive and Y to a›; range of 0-95.  Then the XSHIFT›; and YSHIFT tables are accessed,›; based on the current graphics mode.›; If the value is less than 128, it›; is a right shift count (divide).›; A value of 128 indicates a single›; left shift (multiply by 2).›; ›; The end result is a valid X,Y›; coordinate reading of the light›; gun for the present graphics›; mode.  It is up to the user›; to assure the screen intensity›; (COLOR*16+INTENSITY) is at a level›; to get valid gun readings.  A value›; of at least 10 is recommended.›; A "flash" technique may work best›; Set all playfield intensities to ›; 14, call GunRead, and restore the›; original playfield colors.›; ›    ›PROC GunRead( CARD POINTER xx,›              BYTE POINTER yy)››BYTE ARRAY xshift=[2 3 3 1 1 1 0 0 128 1 1 1 2 2 0 0]› ›BYTE ARRAY yshift=[2 2 3 2 1 1 0 0 128 128 128 128 2 3 128 128]››CARD GunX›BYTE GunY››BYTE DINDEX= $57›BYTE LPENH = 564›BYTE LPENV = 565››BYTE shift, index›     ››GunX  = LPENH›GunX  = GunX & $FF›GunY  = LPENV›index = DINDEX››IF GunX <= 40 THEN› GunX = GunX + 227› IF GunX > 255 THEN›  GunX = 255› FI›FI››IF GunX <= 90 THEN›  GunX = 90›FI››GunX = GunX - 90››IF GunX > 159 THEN› GunX = 159       ›FI››shift = xshift(index)››IF shift = 128 THEN›  GunX = GunX LSH 1›ELSE›  GunX = GunX RSH shift›FI››GunY = GunY - 17››  ›IF GunY > 127 THEN›  GunY = 0›FI›     ›IF GunY > 95 THEN›  GunY = 95›FI››shift = yshift(index)››IF shift = 128 THEN›  GunY = GunY LSH 1›ELSE›  GunY = GunY RSH shift›FI››xx ^= GunX›yy ^= GunY››RETURN›››