[mapserver-dev] Precision about geotransform

Mohamed Saâd HESSANE saad.hessane at gmail.com
Thu Apr 21 15:04:32 EDT 2011


That exactly what I guessed. But it isn't true. If it was, a rotated map at
45°, is represented by an extent having de double of the initial size
extent. A 600x600 (360000 pixels) rotated image correspond to a 848x848
(~720000) normal image. But the rotated image take 0.21s, and the normal
image take 0.32s. The difference is huge.
So I read the part of the source code which is responsible of rotation.
There's nothing about this. And i don't find the solution to this (I don't
undestund the rotation algorithm too).


2011/4/21 thomas bonfort <thomas.bonfort at gmail.com>

> Mohamed,
> I'm guessing that as the underlying data drivers only accept
> rectangular extents oriented northwards, they have to return more data
> if there is a rotation implied. And mapserver also has to filter the
> extra features out at some point, even if they do not fall on the
> final map image. This could explain the overhead.
>
> --
> thomas
>
> 2011/4/21 Mohamed Saâd HESSANE <saad.hessane at gmail.com>:
> > I don't think so. If it was, juste when a cos or a sin vanish.
> > My first thought was that mapserver have to calculate more pixels in a
> > rotated map than a normale map. But when i read the source code i found
> that
> > it's the projection which rotate, not the map. So why this discimination?
> > The same projection
> >
> >
> > 2011/4/21 Stephen Woodbridge <woodbri at swoodbridge.com>
> >>
> >> On 4/21/2011 10:32 AM, Mohamed Saād HESSANE wrote:
> >>>
> >>> Hy list,
> >>> I have a question. I'm doing a benchmarking test of a rotated map, and
> i
> >>> get the result in the attached file.
> >>> My question is why it take more time to draw a map rotated at 45° than
> a
> >>> map rotated at 90° or 183° ???
> >>> Thank you !
> >>
> >> I'm only guessing here but could it be that the math required to compute
> >> cos and sin is more costly in cpu cycles at those angles.
> >>
> >> Regardless, this is an interesting question and I love the graphic, nice
> >> job on that.
> >>
> >> -Steve W
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> >
> >
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> >
>
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