It’s just that if my interpretation is correct, the quality value gives you only an idea of how your current texel size (texels = pixels on the triangular faces) relates to optimal texel size. So it’s tied to the current unwrap and not to the source images. It often happens that if the alignment isn’t ideal (or something else I don’t know about is wrong), then the texture does not resemble the max resolution of the images but some badly washed out imitation with a lot less details than on the source images. And in my opinion that is then bad quality, as in a worse result that can be achieved. Whereas now it just tells you a geometric relation to something called Optimal Texel Size, of which we don’t know how it is determines (but is in my view directly attached to the unwrap and not the source images).
Look here, the first attached file:
I figured out now that the reason for this is indeed that the texels are bigger than my orthophoto resolution, that’s why they are represented by several texels. So texture quality only tells me how far off my current tesel size is from the ominous “optimal” texel size. From my example I know that the resolution of my source images would allow for a MUCH higher resolution (or smaller texel size) as you can see in the attachement of the link. But still my example says tx quality 62% and determines the optimal size at 0.0011 instead of the 0.0018 of my current unwrap. Guessing from my screenshot, I would say the images would allow for a texel size of at least 0.0008. That again is relatively close to 0.0011, so maybe the optimal texel size is just exactly what I am looking for…