from png tile, create symmetrical screen image

[hsiangch_ong]

i have a png file.  that will serve as a tile.  this tile will be flipped.  horizontally and vertically.  to create a symmetric image.  that will be larger than my display screen.

in other words.  for 2x2 tile.  first the original tile.  then next to it horizontally.  it is flipped horizontally.  then the two tiles just below.  are the same thing.  but both flipped vertically.  so that the 2x2 image is symmetrical.  then this 2x2 image.  is used to create another image.  which is larger than the display screen.

the display screen is 1366x768.  the png image will be much smaller than this.  usually below 200 pixels in both dimensions.  i round off on purpose to even numbers.

how do i crop this large image.  so the display screen is symmetrical all around?  horizontally as well as vertically?

i have this program.  if i'm lucky.  it falls into place like expected.  but sometimes it doesn't do it right horizontally.  or doesn't do it right vertically.

Code: (Select All)

'by hsiangch_ong 20-aug-2025
OPTION _EXPLICIT

DIM AS LONG scr, scrout, scr1, wd, ht, wd1, ht1, xx, yy, u, j, ll
DIM AS _BYTE flipx, flipy
DIM AS INTEGER doxflip
DIM afile$, bfile$, apath$

REDIM sf(1 TO 1) AS STRING

'change the following path to what you want in your system.
'especially if you're on macos.  (i forgot which is the precise user home directory.)
'copy the provided png files to this directory.
'this program's output is the same directory.
$IF LINUX THEN
apath$ = ENVIRON$("HOME") + "/Pictures/backf"
$ELSE
apath$ = ENVIRON$("USERPROFILE") + "/Pictures/backf"
$END IF
afile$ = _FILES$(apath$)
DO UNTIL afile$ = ""
    IF RIGHT$(afile$, 4) = ".png" THEN
        ll = ll + 1
        REDIM _PRESERVE sf(1 TO ll) AS STRING
        sf(ll) = apath$ + "/" + afile$
    END IF
    afile$ = _FILES$
LOOP

FOR j = 1 TO ll
    scr = _LOADIMAGE(sf(j), 32)
    IF scr >= -1 THEN
        _MESSAGEBOX "Backflip", "Failed to load image:" + CHR$(13) + sf(j), "error"
        SYSTEM
    END IF

    wd = _WIDTH(scr)
    ht = _HEIGHT(scr)
    wd1 = wd
    ht1 = ht
    doxflip = 0
    DO UNTIL wd1 >= 1366
        doxflip = doxflip + 1
        wd1 = wd1 + wd
    LOOP
    DO UNTIL ht1 >= 768: ht1 = ht1 + ht: LOOP

    scr1 = _NEWIMAGE(wd1, ht1, 32)
    scrout = _NEWIMAGE(1366, 768, 32)
    IF scr1 >= -1 THEN
        _FREEIMAGE scr
        _MESSAGEBOX "Backflip", "There isn't enough memory for an image that large!", "error"
        SYSTEM
    END IF
    IF scrout >= -1 THEN
        _FREEIMAGE scr1
        _FREEIMAGE scr
        _MESSAGEBOX "Backflip", "There isn't enough memory for two images!", "error"
        SYSTEM
    END IF

    u = INSTR(sf(j), ".")
    afile$ = LEFT$(sf(j), u - 1) + "-neu.png"
    _TITLE "Backflip - SAVING " + afile$

    flipx = 0
    flipy = 0
    FOR yy = 0 TO ht1 STEP ht
        FOR xx = 0 TO wd1 STEP wd
            IF flipy THEN
                IF flipx THEN
                    _PUTIMAGE (xx + wd - 1, yy + ht - 1)-(xx, yy), scr, scr1
                ELSE
                    _PUTIMAGE (xx, yy + ht - 1)-(xx + wd - 1, yy), scr, scr1
                END IF
            ELSE
                IF flipx THEN
                    _PUTIMAGE (xx + wd - 1, yy)-(xx, yy + ht - 1), scr, scr1
                ELSE
                    _PUTIMAGE (xx, yy)-(xx + wd - 1, yy + ht - 1), scr, scr1
                END IF
            END IF
            flipx = NOT flipx
        NEXT
        IF doxflip MOD 2 > 0 THEN flipx = NOT flipx
        flipy = NOT flipy
    NEXT

    wd = (wd1 - 1366) \ 2
    ht = (ht1 - 768) \ 2
    _PUTIMAGE (0, 0)-(1365, 767), scr1, scrout, (wd, ht)-(wd + 1365, ht + 767)
    _SAVEIMAGE afile$, scrout
    _FREEIMAGE scrout
    _FREEIMAGE scr1
    _FREEIMAGE scr
    PRINT "Saved: "; afile$
NEXT
PRINT "COMPLETED"
_TITLE "Backflip - COMPLETED"
_DELAY 3
SYSTEM

use your own png files.  it's recommended they are smaller than 200x200.  unless you have a much larger screen than mine.

originally the output of this program.  was jpeg format.  converted with imagemagick.  because qb64 phoenix doesn't offer jpeg saving with compression.

  hsiangch_ong-backflip-batch.zip (Size: 128.21 KB / Downloads: 32)