msClipPolygonRect() / msClipPolylineRect()

[Stephen Woodbridge]

Frank,

Excellent! Performance has been an issue discussed on the list in resent
history, any chance of getting this fix back ported to a dot release of
4.4.x

I think that we should defer to the shapefile spec on bounds being set
correctly. It is possible to have "bad" shapefiles so so some sanity
checks should be in the code to trap bogus or out of bounds values and
then die with some useful message about which file was bad. I would then
add a bounds check to the verify or check shape utility to report and
fix things.

I would rather assume that my data is good and get the extra rendering
speed and if I have bogus data then running lots of files through the
checker is the price I have to pay.

-Steve

Frank Warmerdam wrote:
> Folks,
>
> As part of an optimization project, funded by the Department of
> Fisheries and Oceans I am digging through a few things.  I found
> that in my test rendering something 15% of my time was spent in
> msClipPolygonRect() as part of normal rendering.
>
> A quick review highlighted that many of the features being put
> through clipping were completely within the clip rectangle, so alot
> of work was being done to accomplish no change.  I have modified
> msClipPolygonRect() and msClipPolylineRect() in 4.5 CVS to return
> immediately if they determine that the bounds of the shape are
> completely within the clip rectangle.
>
> In my case this cut the clipping time by about 50%.
>
> My only concern with this change is whether there is any risk of
> shapeObj bounds not being correct.  Does anyone know of circumstances
> when the bound might not be in sync with the actual contents of the
> shape?
>
> I have applied the same change to msClipPolygonRect() though I
> don't use it much in my test case.
>
> I suspect that there may be futher opportunities for optimization on
> this function. For instance, it doesn't seem like there is any point
> in computing intersects for line segments where both end points are
> within the search rect.
>
> Best regards,
> --
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent
>