[Xastir] Slow raster map render

[Tom Hayward]

On Fri, Sep 7, 2012 at 11:30 AM, Tom Russo <russo at bogodyn.org> wrote:
> On Fri, Sep 07, 2012 at 10:07:14AM -0600, we recorded a bogon-computron collision of the <esarfl at gmail.com> flavor, containing:
>> Why not just
>> re-project the APRS stations and any shapefile overlay to match the
>> raster tiles? Wouldn't this be much faster?
>
> Yes, it would, but then you'd have to deal with the issue that many rasters
> aren't in the same projection, and the vector overlays are often in something
> else, too.

AFAIK, all the web tile services are Mercator. The issue here is
finding a way to make the web tile services render quickly. My hunch
was that vector overlays would be less computationally expensive to
reproject, so leaving the tiles in Mercator would be most efficient.

> So the approach is that the Xastir coordinate system is basically Platte Carre
> (equidistant cylindrical) with a scaling so that the entire globe is represented
> by long integer coordinates.  Everything is done in that coordinate system,
> and maps have to be reprojected.
>
>
>> I normally use Xastir with a USGS topo tiles base layer (from ESRI,
>> using the OSM tile loader) with an OSM Cloudmade shapefile overlay.
>
> If you really want topo maps as a base layer, you're better off getting
> USGS GeoTIFFs and doing the reprojection once, converting them to WGS84
> lat/lon instead of NAD83 or NAD27 UTM.  This way, no projection is done at
> render time.  You can do the reprojecting using gdalwarp.

I occasionally use USGS DRG GeoTIFFs, but I prefer the scanned topo
tiles. The DRGs don't look as nice because they're limited to ~5
colors (or something like that), not millions of colors, meaning
there's no antialiasing on the text and contour lines. This makes a
big difference in readability, at least for me.

Where would be a good place to improve efficiency? My two ideas so far:

1. Have Xastir reproject each tile before caching it (rather than
caching the original download), so they do not need to be reprojected
on each cache read. I don't know if this idea is compatible with how
Xastir stitches tiles for display.

2. Add a "fixed zoom Mercator" mode to Xastir where tiles are
displayed in their native size/shape and everything else gets
reprojected. This sounds really difficult to implement :-)


Tom KD7LXL