Opacity. Specifies material opacity.
Material opacity (percent); 0…100
The following material/object combinations support variable opacity:
If the material includes an image with an alpha channel,
opac= can be used to make the image more transparent, but not more opaque.
Material attribute. Ignored if the current object selection or the material does not support opacity.
opac=100, for a fully opaque material (if the image does not include an alpha channel).