bilinear thing

SCREEN _NEWIMAGE(640, 480, 32)
DIM c(32, 24) AS SINGLE

FOR x = 0 TO 31
  FOR y = 0 TO 23
    c(x, y) = RND
  NEXT
NEXT

m = 20: m1 = m - 1


'nearest neighbour
FOR x = 0 TO 639: ix = x \ m
  FOR y = 0 TO 479: iy = y \ m
    i = c(ix, iy) * 255
    PSET (x, y), _RGB(i, i, 255 - i)
  NEXT
NEXT
SLEEP

'bilinear
FOR x = 0 TO 639: ix = x \ m: rx = x / m - ix: ix1 = (ix + 1) MOD 32: rx1 = 1 - rx
  FOR y = 0 TO 479: iy = y \ m: ry = y / m - iy: iy1 = (iy + 1) MOD 24: ry1 = 1 - ry
    c00 = c(ix, iy) * 255
    c10 = c(ix1, iy) * 255
    c01 = c(ix, iy1) * 255
    c11 = c(ix1, iy1) * 255
    i = (c00 * rx1 + c10 * rx) * ry1 + (c01 * rx1 + c11 * rx) * ry
    PSET (x, y), _RGB(i, i, 255 - i)
  NEXT
NEXT
SLEEP

'cosine
FOR x = 0 TO 639: ix = x \ m: rx = cs(x / m - ix): ix1 = (ix + 1) MOD 32: rx1 = 1 - rx
  FOR y = 0 TO 479: iy = y \ m: ry = cs(y / m - iy): iy1 = (iy + 1) MOD 24: ry1 = 1 - ry
    c00 = c(ix, iy) * 255
    c10 = c(ix1, iy) * 255
    c01 = c(ix, iy1) * 255
    c11 = c(ix1, iy1) * 255
    i = (c00 * rx1 + c10 * rx) * ry1 + (c01 * rx1 + c11 * rx) * ry
    PSET (x, y), _RGB(i, i, 255 - i)
  NEXT
NEXT
SLEEP

'smoothstep
FOR x = 0 TO 639: ix = x \ m: rx = sm(x / m - ix): ix1 = (ix + 1) MOD 32: rx1 = 1 - rx
  FOR y = 0 TO 479: iy = y \ m: ry = sm(y / m - iy): iy1 = (iy + 1) MOD 24: ry1 = 1 - ry
    c00 = c(ix, iy) * 255
    c10 = c(ix1, iy) * 255
    c01 = c(ix, iy1) * 255
    c11 = c(ix1, iy1) * 255
    i = (c00 * rx1 + c10 * rx) * ry1 + (c01 * rx1 + c11 * rx) * ry
    PSET (x, y), _RGB(i, i, 255 - i)
  NEXT
NEXT
SLEEP

'the smoothstep function
FUNCTION sm (t)
  sm = t * t * (3 - 2 * t)
END FUNCTION

' the "cosine" function
FUNCTION cs (t)
  cs = SIN(t * 1.5707963): cs = cs * cs
END FUNCTION