mode 2 ' 320x240
Const SCREEN_HEIGHT = 480
Const Q = 3
Dim XP(Q), YP(Q), ZP(Q) As Integer
Dim MX(26,26), MY(26,26) As Integer
cls
' Define charge points
XP(1) = 11 : YP(1) = 6 : ZP(1) = 175
XP(2) = 17 : YP(2) = 18 : ZP(2) = -150
XP(3) = 7 : YP(3) = 19 : ZP(3) = 40
' Projection constants
Dim XS , YS  As Integer
XS = 25: YS = 200
' Fill matrix with transformed coordinates
For X  = 0 To 25
For Y  = 0 To 25
 UPJE = 0
For W  = 1 To Q
D = Sqr((X - XP(W))^2 + (Y - YP(W))^2)
inc UPJE, ZP(W) * Exp(-D / 2)
Next
' Fix grid orientation (X then Y)
MX(X,Y) = XS + 11*X + 11*Y
' Flip Y-axis (to match PC-BASIC lower-left origin)
MY(X,Y) = SCREEN_HEIGHT - (UPJE + YS - X*4 + Y*4)
' Optional: show control points
Pixel MX(X,Y)/2, MY(X,Y)/2, rgb(0,255,255)
Next
Next
' Draw the surface
E = 0
For Y = 25 To 1 Step -1
For X = 0 To 24
X1 = MX(X,Y)/2: Y1 = MY(X,Y)/2
X2 = MX(X+1,Y)/2: Y2 = MY(X+1,Y)/2
X3 = MX(X+1,Y-1)/2: Y3 = MY(X+1,Y-1)/2
X4 = MX(X,Y-1)/2: Y4 = MY(X,Y-1)/2
' Fill with alternating pattern
If (E Mod 2 = 0) Then
K = RGB(0,0,255)
else
K = RGB(0,128,255)
endif
TRIANGLE X1, Y1, X2, Y2,X3, Y3 ,k,k
TRIANGLE X1, Y1, X4, Y4,X3, Y3 ,k,k
Line X1,Y1 ,X2,Y2,, K
Line X2,Y2 ,X3,Y3,, K
Line X3,Y3,X4,Y4,, K
Line X4,Y4,X1,Y1,, K

XPc = (X1 + X3) \ 2
YPc = (Y1 + Y3) \ 2
' Highlight border
K = RGB(255,255,255)
Line X1,Y1,X2,Y2,, K
Line X2,Y2 ,X3,Y3,, K
Line X3,Y3,X4,Y4,, K
Line X4,Y4,X1,Y1,, K
inc E
Next
Next