A slightly faster SoilDown.

Atari/gr_basics.asm

@@ -202,7 +202,7 @@ ClearPlot
 ; checks state of the pixel (coordinates in xdraw and ydraw)
 ; xdraw (word) - X coordinate
 ; ydraw (word) - Y coordinate
-; result is in A (zero or appropriate bit is set)
+; result is in A (0 - point is set;  appropriate bit is set - point is clear) INVERTED!
 
     ; let's calculate coordinates from xdraw and ydraw
 
@@ -226,7 +226,6 @@ ClearPlot
     ldx xdraw   ; optimization (256 bytes long bittable)
 
     lda (xbyte),y
-    eor #$ff
     and bittable1_long,x
     rts
 .endp

C64/gr_basics.asm

@@ -159,7 +159,7 @@ ClearPlot
 ; checks state of the pixel (coordinates in xdraw and ydraw)
 ; xdraw (word) - X coordinate
 ; ydraw (word) - Y coordinate
-; result is in A (zero or appropriate bit is set)
+; result is in A (0 - point is set;  appropriate bit is set - point is clear) INVERTED!
 
     ; let's calculate coordinates from xdraw and ydraw
 
@@ -182,7 +182,6 @@ ClearPlot
 
     ldy #0
     lda (xbyte),y
-    eor #$ff
     and bittable1_long,x
     rts
 .endp

grafproc.asm

@@ -1074,7 +1074,7 @@ DoNotClearParachute
     sta temp  ; Loop Counter
 ByteBelowTank
     jsr point_plot
-    beq EmptyPoint2
+    bne EmptyPoint2
     sec
     ror UnderTank2
     sec
@@ -1340,7 +1340,7 @@ NextColumn1
     mwa #0 ydraw
 NextPoint1
     jsr point_plot
-    bne FoundFirstPoint
+    beq FoundFirstPoint
 StillNothing
     inc ydraw
     lda ydraw
@@ -1392,7 +1392,7 @@ FalloutOfLine
     ; and checking if there is a pixel there
     sta ydraw
     jsr point_plot
-    bne ThereIsPixelHere
+    beq ThereIsPixelHere
     ; if no pixel we plot it
     mva #1 color
     jsr plot.MakePlot