Now: what color was that triangle? How many centimeters across was the base? Was it a solid, or a line enclosing an area, and if the latter how thick was the line? Did it have a matte finish, or glossy? Was it opaque, transparent, or translucent? If opaque, did it cast a shadow? Where was the light source, and how tall was the triangle, and what was the color of the light... for example, was the shadow cool or warm?

Most people's imagined triangles simply won't have those visual properties, even though triangles they actually see do have those properties, because imagination isn't a matter of re-presenting things to our visual systems. It's something else, though it has aspects of that.

You might be generalizing from one example. There are plenty of games asking people to imagine (say) a cube, then asking them about various properties of the cube, and then purporting to relate them to features of the subject's personality, and I can recall very few people answering “I don't know” to any such question.