A gobo is a picture placed in the key light. White passes the light, black blocks it, grey passes part of it, so the light lands on the model and the floor already patterned. Heimdall3D projects the mask from the key light's direction, so the pattern bends over whatever geometry it crosses. Scale, offset, rotation, softness and repeat place it.
Download one: blinds.png · window.png · slats.png · branches.png · cloudbreak.png · caustics.png · grid.png · breakup.png · foliage.png
Nine masks, 1024 x 1024, 8-bit greyscale PNG. All nine come out of Heimdall3D's own pattern code, so there is no photograph and no third party in any of them. Every one is periodic, so Repeat above 1 tiles it without a join showing.
Save a PNG into %APPDATA%\Heimdall3D\gobos. Add on the Lighting card's gobo row opens that folder, and the list is rescanned when you come back to the card.
CC BY 4.0, Heimdall3D. This line meets the licence, with the mask's name filled in:
Gobo mask "blinds" by Heimdall3D, https://heimdall3d.com, licensed CC BY 4.0, https://creativecommons.org/licenses/by/4.0/