191 plots causes crash

[slenkar]

Code:

Function distance (x1 as Integer, y1 as Integer, x2 as Integer, y2 as Integer) as Integer
Dim x as Integer
x=Abs(x1-x2)
Dim y as Integer
y=Abs(y1-y2)
Return x+y
End Function

Dim bresx(0 to 255) as Integer
Dim bresy(0 to 191) as Integer

Function line(X1 as Integer,Y1 as Integer,X2 as Integer,Y2 as Integer) as Integer
'Draws a line of individual pixels from X1,Y1 to X2,Y2 at any angle

Dim dist as Integer
dist=distance(X1,Y1,X2,Y2)
dim bresiter as Integer
For bresiter=0 To dist
bresx(bresiter)=0
bresy(bresiter)=0
Next

Dim steep as Integer
if Abs(Y2-Y1) > Abs(X2-X1) then
steep=1
else
steep=0
end if

PRINT AT 1,0 ; PAPER 1 ; INK 0 ; "steep "+STR(steep)
    
If steep =1 Then
Dim Temp as Integer
Temp=X1
X1=Y1
Y1=Temp
Temp=X2
X2=Y2
Y2=Temp
End If
    Dim DeltaX as Integer
    DeltaX=Abs(X2-X1)
    Dim DeltaY as Integer
    DeltaY=Abs(Y2-Y1)
    Dim IError as Integer
    IError=0
    Dim DeltaError as Integer
    DeltaError=DeltaY
    Dim x as Integer
    x=X1        'Start at X1,Y1
    Dim y as Integer
    y=Y1        
    Dim XStep as Integer
    Dim YStep as Integer
    If X1<X2 Then
    XStep=1
    Else
    XStep=-1
    end if
    If Y1<Y2 Then
    YStep=1
    Else
    YStep=-1    'Direction
    end if
    Dim iter as Integer
    iter=1
    
    While x<>X2
        x=x+XStep        'Move in X
        IError=IError+DeltaError        'Add to counter
        If (IError SHL 1)>DeltaX then        'Would it overflow?
        y=y+YStep        'Move in Y
        IError=IError-DeltaX        'Overflow/wrap the counter
        End if
    
    If steep then
        bresx(iter)=y
        bresy(iter)=x
    Else
        bresx(iter)=x
        bresy(iter)=y
    End If
        iter=iter+1
        
    If iter>dist then
    Print  at 3,0;PAPER 1;INK 0;"bresenham over distance "+STR(dist)
    Return dist
    End If
    
    Wend
    Return dist
End Function
Dim dist as Integer
dist=line(1,1,100,95)-1
PRINT AT 0,0 ; PAPER 1 ; INK 0 ; "dist "+STR(dist)

for x=0 to dist
    Plot bresx(x),bresy(x)
next
'PRINT AT 2,0 ; PAPER 1 ; INK 0 ; "bresx "+STR(bresx(108))
'PRINT AT 3,0 ; PAPER 1 ; INK 0 ; "bresx "+STR(bresx(57))
'PRINT AT 4,0 ; PAPER 1 ; INK 0 ; "bresx "+STR(bresx(18))
'PRINT AT 5,0 ; PAPER 1 ; INK 0 ; "bresy "+STR(bresy(108))
'PRINT AT 6,0 ; PAPER 1 ; INK 0 ; "bresy "+STR(bresy(59))
'PRINT AT 7,0 ; PAPER 1 ; INK 0 ; "bresy "+STR(bresy(1))
'main loop
100  LET j$ = INKEY$
GOTO 100

Im implementing a bresenham thingy for line of sight calculations,
whenever the length of the line exceeds 191 the screen gets corrupted,
191 just happens to be the height of the screen, but I dont see the relation.

[boriel]

I will try to debug.
Also, you might want to have a look at DRAW asm routine, which already uses Bresenham's algorithm.
Maybe you have some overflow which leads to a stack or ROM variables (e.g. 23659) to be accidentally overwritten?

[slenkar]

It was my fault, it was an array over-run
:oops:
here is a distance function to make up for it:

Code:

Function distance (x1 as Integer, y1 as Integer, x2 as Integer, y2 as Integer) as Integer
Dim x as Integer
x=ABS(x1-x2)
Dim y as Integer
y=ABS(y1-y2)

Return Sqr((x^2)+(y^2))
End Function

this bresenham line drawing algorithm lets you do line-of-sight to see if 2 units can see each other in a game