' text$ is the text to save
DIM txtmatrix(LEN(textin$) * 9, 13)
' we must dim the matrix to keep it in memory
LOCATE 1, 1: PRINT text$
' the text is printed on the screen so that it can be saved. If the text you want to save
' is already there, you can remove this line !
FOR i = 0 TO LEN(textin$) * 8
' in screen 9, the characters width is 8
FOR j = 1 TO 13
' in screen 9, the characters height is a little less than 13,
'but if the text is
'already modified (with underlining, for example), the best thing to do is to reserve
'more memory for the matrix
txtmatrix(i, j) = POINT(i, j)
' here is the way to save each dot of the text in the matrix.
NEXT j, i

Now that your text is saved, you can modify it !
For example, to flip the text horizontaly, add this code after saving the text matrix :

FOR i = 630 TO 630 - (LEN(textin$) * 8) STEP -1
' we must start from the right of the screen (640*350 in screen 9)
FOR j = 1 TO 13
IF txtmatrix((630 - i), j) <> 0 THEN PSET (i, j), txtmatrix((630 - i), j)
' I add the test because on slow computers, the program printed the black dots too and it
' was a waste of time !
NEXT j, i

To flip the text vertically, you can draw a separation line using this line of code :

LINE (0, 14)-(LEN(textin$) * 8, 14)

To flip the text, that's the same sub as previously but now J is reversed, not I.
So you must use that sub :

FOR i = 1 TO LEN(textin$) * 8
FOR j = 14 TO 1 STEP -1
IF txtmatrix(i, 14 - j) <> 0 THEN PSET (i, j + 14), txtmatrix(i, 14 - j)
NEXT j, i
And if you want to create a nice effect on the vertical flipping, transform the third line of code to the following :

IF txtmatrix(i, 14 - j) <> 0 THEN PSET (i + j, j + 14), txtmatrix(i, 14 - j)