Thanks for the information. I never had the intension to generalise on the fly. I want to implement the same principle I used with the imagemosaic-jdbc module, storing pyramid levels in the datastore.
All the pre generalized geometries have to be valid, because WFS should also be supported. As you pointed out, I think there are 2,perhaps 3 interesting cases 1) The native store contains multiple generalizations of the same geometry . In the case of a shape file, each generalized shape file has the same number of records and the same attributes, but different geometries. In the case of a JDBC database one could make it more intelligent by using views ( I did it this way). 2) The native store has the possibility to generalize before sending the data (e. g. DB2 has a SQL function st_generalize(...) using Douglas Peuker ) 3) Another idea would be to generalize a Feature Source at system startup and put the generalized geometries in a temp directory. These generalizations MUST be executed in a background thread at low priority and should not influence the response time of the system. If a generalization is finished, use it , otherwise fall back to an already finished generalization or the base geometries. At any point in time, the FeatureSource is fully functional, only the returned geometries my differ. But anyway, I do not want any generalization on the fly and 1) is my first target. Passing difference/offset in the native CRS as a hint would make me happy :-) If the hint is missing, you always get the base geometries, the same holds true if the value of the hint is 0. I would like to avoid CRS transformations to be consistent with other FeatureSources. Pleasy give me a hint and I would start. If possible in 2.5.x and 2.6.x, I need this feature in geoserver. christian Andrea Aime writes: > Christian Müller ha scritto: >> Last week I posed a question about having a FeatureSource which has also >> some generalizations of the vector data to speed up response time and >> reduce data transfer. >> >> While it is is surely not the problem to implement such a FeatureSource, >> I need the ScaleDenominator from the caller. >> >> I studied the source and I want to ask the specialists about the >> following assumptions: >> >> 1) StreamingRenderer and ShapeFileRenderer use >> FeatureSource>>getFeatures(queryObject) >> 2) The Query interface offers the possibility to pass Hints >> >> My idea would be to pass the scaleDenominater as a hint to the >> queryObject. This makes sense since >> the scaleDenominator is only usefull for some use cases and should not be >> part of the FeatureSource API. >> >> I am not sure here if the ScaleDenomintar is enough, perhaps a unit of >> measure is also required. On the other side, the Query interface offers >> a getter for the CRS, I think I can get the unit from the CRS, yes or no >> ? >> >> Anyway, if my assumptions come close to the truth, I would do such an >> implementation and insert one line in the mentioned renderer classes to >> test it. >> >> opinions ? > > If you're trying to speed up the rendering process by doing generalization > in memory inside a FeatureSource wrapper, hem, sorry, > you're on the wrong path, at least for WMS. > > The renderer is already doing that, and in a very efficient way, so > if you implemented it only in memory, as a wrapper, you would at > best get a small slowdown, because you'd be doing the generalization > inside the wrapper, and the renderer would try to do it again (uselessly > since you already did it). > > Different is the case in which you're not using a wrapper, but you have > some native way to access pre-decimated data in your datastore, for > example if: > - your native store contains multiple version of the same geometry at > different generalization levels > - your native store has way to efficiently generalize the data on the > fly and this can reduce network traffic and the associated data > encoding/decoding path > > Algorithm wise, there are many, but I've found that using generic and > well behaved algorithms like Douglas-Peuker results in a _slowdown_ > when performed inside the renderer, the recursive nature of the > algorithm makes generalization more expensive than the speedup gained > by rendering generalized data (if you are also reprojecting it may > be you still get a speedup, as reprojection might be more expensive > that generalization, don't know exactly because I did not try this > case out). So the current renderer uses a very simple one pass algorithm > that avoids expensive calculations, and it's based on pure offset: > a point is skipped if the deltaX, deltaY are less than one pixel size > from the last chosen point. > > So, if you have some kind of native support it's worth experimenting > with it. I can add one hint, the generalization distance/offset, > that will be specified in the same unit as the native data, and you > can leverage it inside your datastore to decide how you want to > generalize. > > When mixing generalization and transformation things get more complex. > As a rule of thumb, the renderer now never asks the datastore to > transform data, for two reasons: > - the datastores have been historically very unreliable at reprojecting > data > - the renderer does reprojection after generalizing (this is > very important to get good performance, as reprojection is expensive) > and datastores so far did not have any generalization capability > > Jody mentioned generalizing before/after reprojection. > This is an interesting topic too. The renderer always transforms > after generalization to get a good speedup of the whole rendering > process, this is in the common case good, but it's not in less > well behaved cases. > The issue is related to how the generalization distance is picked up. > Now the renderer gets the pixel size in the rendering CRS, and back > transforms it into the native CRS by placing a small segment in > the middle of the rendered area. If the transformatio is within > its "well behaved" area the result is going to be a good distance > for the whole rendering area, but if the linear deformation varies > wildly along the rendered area you may over generalize the input > data, resulting in bad rendering afterwards. > Two examples I can make are: > - rendering a polar stereographic so that the pole is in the middle > and the rendered area goes well below 80° latitude. In this case > you'll see ruined polygons (e.g., they were touching in the > native data, but they will be rendered as disconnected) at > the borders of the map > - rendering a UTM zone well beyond 6° from the central meridian, > with the same effect > > If you value accuracy more than speed even in those badly behaved > cases you should do the reprojection before doing the generalization. > A compromise could be performing some sampling of the area being > rendered and pick the lowest generalization distance, or generate > a grid of generalization distances so that each area uses > a more appropriate generalization. > Both of these approaches would fail anyways if the projection > has singularities, as there is not grid sampling good enough to > cope with a linear deformation that diverges close to a line > or a point inside (or at the border) or your map. > To be fair thought, you should not render such map to start with, > as you've gone way past any reasonable standard of good cartographic > representation. > > Jody's point about the choice of algorithm is also another > interesting point. The algorithm I described above is good > for rendering assuming the renderer can cope with invalid > geometries, as the algorithm gives no guarantees the result > will be topologically correct. However, for the rendering > case that is good enough. > > JTS has a topology preserving Douglas-Peuker implementation, > as that's what I would choose if I had to deal with a WFS output, > where speed is not such an issue (XML encoding is so expensive > that the difference in the generalization algorithm speed > will be un-noticeable anyways) but you want to give your > clients good data. > > Hope this helps > Cheers > Andrea > > > -- > Andrea Aime > OpenGeo - http://opengeo.org > Expert service straight from the developers. ------------------------------------------------------------------------------ _______________________________________________ Geoserver-devel mailing list Geoserver-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/geoserver-devel
