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support faster wgs84->merc transforms, a very common reprojection scenario in web mapping - added benefit of this approach is easy clipping for robust handling of out of bounds mercator coords - closes #839

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This commit is contained in:
Dane Springmeyer 2011-09-12 02:10:58 +00:00
parent 7c72c31951
commit ac3e43e5a4
2 changed files with 40 additions and 1 deletions
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1
include/mapnik/proj_transform.hpp
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@ -56,6 +56,7 @@ private:
bool is_source_longlat_;
bool is_dest_longlat_;
bool is_source_equal_dest_;
bool wgs84_to_merc_;
};
}

40
src/proj_transform.cpp
View file

@ -33,16 +33,31 @@
// stl
#include <vector>
static const float MAXEXTENT = 20037508.34;
static const float M_PI_by2 = M_PI / 2;
static const float D2R = M_PI / 180;
static const float R2D = 180 / M_PI;
static const float M_PIby360 = M_PI / 360;
static const float MAXEXTENTby180 = MAXEXTENT/180;
namespace mapnik {
proj_transform::proj_transform(projection const& source,
projection const& dest)
: source_(source),
dest_(dest)
dest_(dest)
{
is_source_longlat_ = source_.is_geographic();
is_dest_longlat_ = dest_.is_geographic();
is_source_equal_dest_ = (source_ == dest_);
if (source.params() == "+init=epsg:3857" && dest.params() == "+init=epsg:4326")
{
wgs84_to_merc_ = true;
}
else
{
wgs84_to_merc_ = false;
}
}
bool proj_transform::equal() const
@ -52,9 +67,21 @@ bool proj_transform::equal() const
bool proj_transform::forward (double & x, double & y , double & z) const
{
if (is_source_equal_dest_)
return true;
if (wgs84_to_merc_) {
x = (x / MAXEXTENT) * 180;
y = (y / MAXEXTENT) * 180;
y = R2D * (2 * atan(exp(y * D2R)) - M_PI_by2);
if (x > 180) x = 180;
if (x < -180) x = -180;
if (y > 85.0511) y = 85.0511;
if (y < -85.0511) y = -85.0511;
return true;
}
if (is_source_longlat_)
{
x *= DEG_TO_RAD;
@ -85,6 +112,17 @@ bool proj_transform::backward (double & x, double & y , double & z) const
if (is_source_equal_dest_)
return true;
if (wgs84_to_merc_) {
x = x * MAXEXTENTby180;
y = log(tan((90 + y) * M_PIby360)) / D2R;
y = y * MAXEXTENTby180;
if (x > MAXEXTENT) x = MAXEXTENT;
if (x < -MAXEXTENT) x = -MAXEXTENT;
if (y > MAXEXTENT) y = MAXEXTENT;
if (y < -MAXEXTENT) y = -MAXEXTENT;
return true;
}
if (is_dest_longlat_)
{
x *= DEG_TO_RAD;