TYPO3 Core

The following two warnings are thrown with php 8.1 in the GifBuilder:

(1/1) #1476107295 TYPO3\CMS\Core\Error\Exception
PHP Warning: Undefined array key "_GIFBUILDER." in /var/www/html/typo3/sysext/frontend/Classes/Imaging/GifBuilder.php line 140

(1/1) #1476107295 TYPO3\CMS\Core\Error\Exception
PHP Warning: Undefined array key "mask." in /var/www/html/typo3/sysext/frontend/Classes/Imaging/GifBuilder.php line 215

How to test:

LocalConfiguration:
displayErrors = 1
exceptionalErrors = 12290

Typoscript Setup:

lib.test = IMAGE
lib.test {
  file = GIFBUILDER
  file {
    XY = 1024,768
    format = jpg
    quality = 88

    10 = IMAGE
    10.file = fileadmin/backimage.jpg

    15 = IMAGE
    15.offset = 420,18
    15.file = fileadmin/overlay.png
    15.mask = fileadmin/mask.png
  }
}

page = PAGE
page.10 < lib.test

page.config.contentObjectExceptionHandler = 0

You can download the files to test from the documentation:
https://docs.typo3.org/m/typo3/reference-typoscript/11.5/en-us/Gifbuilder/Examples.html