PHP: imagecolorat(): -1,0 is out of bounds

Question

I have this code working before (6 years ago) while upgrading to the latest PHP8, it doesn't work anymore, well it does when using absurd way putting "error_reporting(0);"

    $a = imagecolorat($img, $x + $dx , $y);
    $b = imagecolorat($img, $x, $y + $dy);
    $c = imagecolorat($img, $x + $dx, $y + $dy);

I am getting this error message.

Notice: imagecolorat(): -1,0 is out of bounds on line 34

Notice: imagecolorat(): 0,-1 is out of bounds on line 35

Notice: imagecolorat(): -1,-1 is out of bounds on line 36

Do I miss something?