Combining multiple geometries with Direct2D - c++

Question

When trying to create a compass with Direct2D, I was faced with the challenge of combining 72 individual lines. My question was: how do I combine a relatively larger number of ID2D1PathGeometries?

Answer 1

After disappointment with Direct2D's ability to combine only two geometries at one time, I decided to set out to create a modular way to combine multiple.
Since Direct2D DOES provide the "CombineWithGeometry" function, this function just manages that, as well as temporary resources, to create a final geometry.
A few notes: this function is fairly expensive, so it should not be run during a frame render, but instead, before if possible, and the result should be cached. This version only supports path geometry, however, adding support for other geometry is easy, just change the geometry type in the parameters.
Without further ado, here's the function:
ID2D1PathGeometry* combine_multiple_path_geometries(ID2D1Factory*& srcfactory, int geo_count, ID2D1PathGeometry* geos[]) {
ID2D1PathGeometry* path_geo_1 = NULL;
ID2D1PathGeometry* path_geo_2 = NULL;
srcfactory->CreatePathGeometry(&path_geo_1);
srcfactory->CreatePathGeometry(&path_geo_2);
for (short i = 0; i < geo_count; i++) {
ID2D1GeometrySink* cmpl_s1 = NULL;
ID2D1GeometrySink* cmpl_s2 = NULL;
if (i % 2 == 0) {
//copying into 1
path_geo_1->Open(&cmpl_s1);
if (i == 0)
geos[i]->CombineWithGeometry(geos[i], D2D1_COMBINE_MODE_UNION, NULL, cmpl_s1);
else
geos[i]->CombineWithGeometry(path_geo_2, D2D1_COMBINE_MODE_UNION, NULL, NULL, cmpl_s1);
cmpl_s1->Close();
cmpl_s1->Release();
if (i != 0) {
path_geo_2->Release();
srcfactory->CreatePathGeometry(&path_geo_2);
}
//cmpl_g1 now contains the geometry so far
}
else {
//copying into 2
path_geo_2->Open(&cmpl_s2);
geos[i]->CombineWithGeometry(path_geo_1, D2D1_COMBINE_MODE_UNION, NULL, cmpl_s2);
cmpl_s2->Close();
cmpl_s2->Release();
path_geo_1->Release();
srcfactory->CreatePathGeometry(&path_geo_1);
//cmpl_g2 now contains the geometry so far
}
}
if (geo_count % 2 == 0) {
if (path_geo_1)
path_geo_1->Release();
return path_geo_2;
}
else {
if (path_geo_2)
path_geo_2->Release();
return path_geo_1;
}
}
You can wrap this into a class, keep it as a standalone, or however you see fit. As mentioned before, you can easily support different geometry types, or, even with a little tweaking, multiple geometry types. As well, you can easily change the combination mode from union just by changing the D2D1_COMBINE_MODE_UNION to whatever you need.
MSDN - Direct2D Geometry Combination Modes: https://msdn.microsoft.com/en-us/library/windows/desktop/dd368083%28v=vs.85%29.aspx
MSDN - Direct2D Geometries: https://msdn.microsoft.com/en-us/library/windows/desktop/dd756653%28v=vs.85%29.aspx
MSDN - Direct2D Geometry Combinations: https://msdn.microsoft.com/en-us/library/windows/desktop/dd756676%28v=vs.85%29.aspx