Triangle

using your illustration as a guide

solid:

c=colour
x=pixelsFromLeft
y=pixelsFromTop
z=pixelsTall
PSET (x, y)
FOR i = 1 TO z
LINE (x - i, y + i)-(x + i, y + i), c
NEXT i

hollow:

c=colour
x=pixelsFromLeft
y=pixelsFromTop
z=pixelsTall
PSET (x, y)
FOR i = 1 TO z
PSET (x - i, y + i), c
PSET (x + i, y + i), c
NEXT i

LOL=P~