How does a lightmap work on an object without uv2 prior to r151


I’m trying to understand something. Prior to the introduction of multiple uvs support in r151, light and AO maps were by default used with the “uv2” attribute in the obejct’s geometry.

However, I have an object here which has a diffuse and a light map, and only one “uv” attribute in its geometry. Although the lightmap is barely just a white image, it seems that the “light map logic” works as you would expect (I guess fallbacks to use the “uv” attribute…?). Evidently, if you download the model and open it in a pre r151 editor, you see the model correctly, and if you switch the lightmap off, the model goes black as expected (without any dynamic lighting).

Why does this happen? Why don’t I see a shader error about uv2 not being defined or something like that?

model.json (740.9 KB)