Why is directionality needed? Sunlight, especially at tropical latitudes, is heavy in UV including at damaging (material-altering) UV-C wavelengths. Perhaps just a shadow mask and sunlight. What if a painter painted on sunscreen in the complimentary positions, exposed the resulting painting for a while, and then washed the sheet to free it from sunscreen?
The Shroud has no paint pigment penetrating the linen fibers. Something else formed the image.