You have noticed it correctly; my filtering is confined to a single layer only. But I have a reason for that. In this application, layers do represent the same spatial area, but the imagery can be totally different, ranging from satellite imagery to aero-photo, from near infrared (false coloring) to true color. There are gaps in layers. Also, they are made in different date/time intervals, hence some features do not exist in some layers, or are differently shaped. Using trilinear/anisotropic method to choose texels from different layers for the same spatial areas makes “islands” of “false texturing”.
Nevertheless, some kind of spatial coherency is achieved by using blending. What I mean will be (probably) clearer after seeing the following images: EC-1, EC-2, EC-3, EC-4, EC-5, EC-6, EC-7 and EC-8.
For example, take a look at image EC-6. The airport runaway is from year 2000. The texture is a near infrared satellite image that I’ve recolored. The runaway is shorter with bomb craters. There is also some ghosting effect (a ghost-runaway parallel to the real one). On the aero-photo, the runaway is fixed, longer and with wider facilities around it. Blending according to spatial distance from the viewer is the only solution that gives more or less acceptable transition (fig. EC-7 and EC-8).
I did, and made a short document for myself about anisotropic filtering.
There is one question left. Maybe this is a right place to discuss about.
In the spec, Px denotes distance in texture space in screen x-direction. Py is a distance in texture space down screen y-direction. Anisotropy-factor is calculated as:
AF = N = min( ceil( max(Px,Py) / min(Px,Py) ), maxAniso)
and Pmin is used for layer selection (hence, more detailed layer is used for texels fetch).
On the other hand, in a NV example, anisotropy-factor is calculated as:
AF = max(Px,Py) * max(Px,Py) / det
where det is a determinant ( det = abs(dx.x * dy.y - dx.y * dy.x) ) of partial derivatives. Also, there are differences in layer selection.
Both approaches come from NV, but the second is about 8 years “newer”. Is there any official definition of anisotropy-factor, or it can be loosely interpreted?
P.S. EXT_texture_filter_anisotropic spec contains false pronouncing of Greek letter lambda. Just a remark for younger readers.