Monkey-X - Fill Triangle with Bresenham Algorithm - code example

'
' Fill triangles using the bresenham algorithm
'

Import mojo

Class filledtriangle
    ' These variables are the points of
    ' the triangle.
    Field x1:Int,y1:Int
    Field x2:Int,y2:Int
    Field x3:Int,y3:Int
    ' This variable is the y of the triangle
    ' that has the lowest value.
    Field lowesty:Int
    ' This is the total height of the triangle.
    Field sizey:Int
    ' These arrays hold the x coordinates with
    ' which we draw the lines.
    Field lefty:Int[]
    Field righty:Int[]
    ' Here we create and draw the triangle.
    Method New(x1:Int,y1:int,x2:Int,y2:int,x3:int,y3:int)
        ' In order to draw below ZERO we add a value to the inputted
        ' coordinates. We decrease this amount when drawing the
        ' actual triangles on the canvas.
        Local offscreen:Int=10000
        x1 += offscreen
        y1 += offscreen
        x2 += offscreen
        y2 += offscreen
        x3 += offscreen
        y3 += offscreen
        ' We need to know which coordinate has the lowest
        ' y value and we put that in the lowesty variable.
        'find lowest
        If y1<y2 And y1<y3 Then lowesty = y1
        If y2<y1 And y2<y3 Then lowesty = y2
        If y3<y1 And y3<y2 Then lowesty = y3
        ' We also need to know the total height of the
        ' triangle so we can create a array of that size
        ' where we store the left and right side line
        ' coordinates in.
        'find height
        If y1>y2 And y1>y3 Then sizey = y1-lowesty
        If y2>y1 And y2>y3 Then sizey = y2-lowesty
        If y3>y1 And y3>y2 Then sizey = y3-lowesty
        '
        ' If there is nothing to draw then
        ' exit this method.
        If sizey = 0 Then Return
        ' Create two arrays which will hold the coordinates
        ' for the lines inside the triangles. The x coordinates.
        lefty = New Int[sizey+1]
        righty = New Int[sizey+1]
        ' Here we fill the lefty and righty arrays with the
        ' x coordinates of the lines inside the triangles.
        bline(x1,y1,x2,y2)
        bline(x2,y2,x3,y3)
        bline(x1,y1,x3,y3)
        ' Here we draw the lines inside the triangles. Filling
        ' it. You can do per pixel for colloring ect.
        For Local y:Int=0 until sizey
            DrawLine(lefty[y]-offscreen,lowesty+y-offscreen,righty[y]-offscreen,lowesty+y-offscreen)
        Next
    End Method
    '
    ' This is the bresenham algorithm. It is modified so
    ' it fills two arrays with the x coordinates of the
    ' lines inside the triangles.
    Method bline:Void(x4:Int,y4:Int,x5:Int,y5:Int)
        Local dx:Int, dy:Int, sx:Int, sy:Int, e:Int
        dx = Abs(x5 - x4)
        sx = -1
        If x4 < x5 Then sx = 1      
        dy = Abs(y5 - y4)
        sy = -1
        If y4 < y5 Then sy = 1
        If dx < dy Then 
            e = dx / 2 
        Else 
            e = dy / 2          
        End If
        Local exitloop:Bool=False
        While exitloop = False
            ' Here we fill the left and right sides arrays.
            ' we draw lines between these later on to fill the triangle                            
              
            If lefty[y4-lowesty] = 0 Then 'If left not used then fill left
                 lefty[y4-lowesty] = x4
            Elseif righty[y4-lowesty] = 0 'if right not used then fill right
                   righty[y4-lowesty] = x4    
             Else 'if both sides are filled
                If lefty[y4-lowesty] = x4 Then  'overwrite same value
                    lefty[y4-lowesty] = x4
                Else 'write new value
                    righty[y4-lowesty] = x4    
                End If       
              End If
      

          If x4 = x5 
              If y4 = y5
                  exitloop = True
              End If
          End If
          If dx > dy Then
              x4 += sx ; e -= dy 
               If e < 0 Then e += dx ; y4 += sy
          Else
              y4 += sy ; e -= dx 
              If e < 0 Then e += dy ; x4 += sx
          Endif

        Wend
    
    End Method
        
End Class

Class MyGame Extends App
    ' cnt is used for the seed
    Field cnt:Int
    ' mytriangle is used to draw a triangle
    Field mytriangle:filledtriangle

    Method OnCreate()
        SetUpdateRate(10)
    End Method
    
    Method OnUpdate()
        ' If pressed space/touch/lmb then new set of triangles
        If KeyHit(KEY_SPACE) Or MouseHit(MOUSE_LEFT) Then cnt+=1    
       End Method    
        
    Method OnRender()
        Cls 0,0,0
        ' Always draw the same using this seed
        Seed = cnt
        ' Draw 100 triangles
        For Local i:=0 Until 100
            Local x1:Int=Rnd(0,DeviceWidth)
            Local y1:Int=Rnd(0,DeviceHeight)
            Local x2:Int=x1+Rnd(-80,80)
            Local y2:Int=y1+Rnd(-80,80)
            Local x3:Int=x1+Rnd(-80,80)
            Local y3:Int=y1+Rnd(-80,80)        
            SetColor(Rnd(255),Rnd(255),Rnd(255))
            mytriangle = New filledtriangle(x1,y1,x2,y2,x3,y3)
        Next
        ' Draw some text
        SetColor 255,255,255
        DrawText("Press the space bar/lmb/touch to draw new set.",0,0)
    End Method    
    
End    Class

Function Main()
    New MyGame()
End Function