Hey all,
I'm trying to figure out the rendering toolchain, tracking particularly
three elements:
CRS (and projection): where these are determined, handled which
I believe I now underestand in general and will figure out
specifics later,
Resolution (display at 96dpi versus printing at 300+ dpi) and
Orientation(so I can pick how the map will appear on my
screen/page/powerpoint slide).
I presume that the rendering system must be capable of handling these
things. Below, you'll find Jesse's original email outlining the
rendering steps---all my understanding to date comes from that so if my
question seems confused, I am referring to that outline.
1) uDig vs. Geotools
As I understand things, uDig has a "meta" rendering system that embeds
various renderers internally. Is this meta system capable of handling
the issues above in a way that the individual renderers cannot?
2) Handling dpi
Does Geotools assume a screen resolution or is the rendering system
resolution independent? If the latter, where in the chain is the dpi
determined?
As I understand Jesse's explanation, there's a decimation step that
happens once the 'styled features' have been created. This implies that
the dpi has been set by that stage.
3) Handling rotation
As far as I can tell, any rotation stuff I have come across has been a
hack rather than a robust part of the renderering system. That is, we
can't rotate the output while still having the text labels appear
horizontal. Is that correct? Has anyone thought seriously about this
issue?
thanks,
adrian
P.S. Sorry to pester the list incessantly but I do hope to figure out
enough of what is going on so I can create docs to help a whole cadre of
programmers take the library and run with it.
**********************************************************************
On Tue, 2006-05-23 at 09:38 -0700, Jesse Eichar wrote:
> You are on the right track. Mind you it depends on the renderer you
> are using. Shapefile has a optimized renderer that has a different
> pipeline from the Streaming renderer which is used to render other
> datastores. I think I explain the StreamingRenderer as it is used
by
> more datastores.
>
> I'm going to take it from the client code as well. I'm assuming
that
> the geometry is either a line or a polygon. Points and
GridCoverages
> are handled in a similar manner but the styling and drawing section
> is slightly different.
>
> 1. Create a MapContext.
> 2. Add MapLayer(s) to context.
> 3. Set Area of Interest and CRS on the MapContext
> 4. Create renderer.
> 5. Set MapContext of renderer
> foreach( layer ) do {
> 6. FeatureType is obtained from Datastore. (Contains the CRS in
> the *current* Feature model)
> 7. Area of Interest is reprojected into the CRS of the layer.
> 8a. A query is constructed from the projected Area of Interest.
> 8b. The query is restricted to only request Attributes that are
> required for styling the feature. (required for thematic styling,
> and other non-basic styles)
> 9. A MathTransfrom is constructed that transforms from Layer CRS
> to Screen space
> foreach feature do {
> 9. Feature is obtained from the datastore.
> 9a. (Assuming shapefile) Geometry is created, no reprojection
> 9b. Attributes are read
> 9c. Feature is created.
> 10. Geometry is decimated. (Vertices that map to a single
pixel
> are collapsed to a single vertex)
> 11. Geometry is reprojected using the constructed MathTransform
> 12. A java.awt.Shape is constructed that wraps the Geometry
> 13. The graphics object is set according to the style. (Fill,
> color, pattern, etc... are all set)
> 14. Shape is drawn.
> 15. If labeling is done the label for the current feature is
> put in a cache.
> }
> }
> 16. Labels are drawn.
>
> Hope this helps.
>
> Jesse
>
> On 23-May-06, at 6:23 AM, Adrian Custer wrote:
>
> > Hey all,
> >
> > Could someone explain the uses of CRS and transforms in the
rendering
> > pipeline? I need a coarse overview of rendering as used by
> > uDig/streaming Renderer to understand:
> > (1) how we go from shapefile data to screen data
> > (2) what CRS's are stored in the rendering system
> > This is all prep work so I can compose an intro to CRS that will
> > work as
> > background for a future, more complex discussion of CRS with
features
> > and renderers.
> >
> > What I really want is a pipeline, from a UTM shapefile (as an
example)
> > to a displayed map on screen where we know what CRS's are in play
and
> > where the transformations are performed. The initial start of the
> > operations are:
> >
> > 10) Get Shapefile Metadata => CRS used for
> > geometryAttributeType
> > 20) Get Shapefile coordinates
> > 30) Build a geometry
> > 40) Build a feature
> > ...
> > ?) Create a Renderer (? with a crs?)
> > ?) add the feature to the chain
> > ...
> > ?) transform to projected coordinates (if in lat/long?) ?
> > ...
> > ?) transform to image coordinates
> > ...
> > ?) transform to screen coordinates
> >
> > If someone could flesh out the chain so I have a working idea of
> > what's
> > going on, I would appreciate it.
> >
> > thanks,
> > adrian
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