Question regarding 3D middle textures

[Ænima]

Does marking a line as a 3D middle texture (in UDMF) check the dimensions of the texture image (example, the image is 64 pixels high, therefore the 3D bridge will be 64 mapunits tall), OR does it actually check for where transparency starts in the texture?


I assume it's the former but I wasn't sure, so I figured I'd ask.