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DrawPolygon doesn't seem to work for me. Where is my error?
Posted: 2009-09-08T14:16:33-07:00
by JohnF
I'm trying to port an old program of mine to use MagickWand for drawing, but I can't get DrawPolygon to work.
The following code:
Code: Select all
#define WIDTH 4000
#define HEIGHT 3000
void mwtest(void)
{
MagickWand *image;
PixelWand *colour;
DrawingWand *drawing;
PointInfo ptest[5] = {{1000, 750}, {3000, 750}, {3000, 2250}, {1000, 2250}, {1000, 750}};
MagickWandGenesis();
image = NewMagickWand();
colour = NewPixelWand();
PixelSetColor(colour, "rgb(200, 200, 255)");
if (!MagickNewImage(image, WIDTH, HEIGHT, colour)) return;
drawing = NewDrawingWand();
PixelSetColor(colour, "black");
DrawSetStrokeColor(drawing, colour);
PixelSetColor(colour, "white");
DrawSetFillColor(drawing, colour);
DrawPolygon(drawing,5,ptest);
MagickWriteImage(image, "mwtest.png");
MagickWandTerminus();
}
doesn't do what I expect. I get the image created and filled with the background colour, but I don't see the polygon.
I'm sure I'm making some fundamental mistake, but I can't see what it is ...
Re: DrawPolygon doesn't seem to work for me. Where is my error?
Posted: 2009-09-08T14:38:56-07:00
by JohnF
I was right - it was something fundamental
There was no association between the drawing commands and the image.
A call to MagickDrawImage solved that.
Re: DrawPolygon doesn't seem to work for me. Where is my error?
Posted: 2009-09-09T00:12:16-07:00
by JohnF
Further update - there appears to be an n^2 algorithm somewhere in the DrawPolygon/MagickDrawImage process.
I've got several thousand polygons that I want to render (some with many thousands of points).
If I call DrawPolygon multiple times, and only call MagickDrawImage once at the end, it takes forever
(I don't know how long - I don't have the patience to find out).
If, however, I call MagickDrawImage once every couple of hundred polygons or so (and then
re-initialise the drawing wand) everything completes in a reasonable amount of time.
Is this to be expected? Is the subdivision approach that I use (breaking the full list into smaller chunks)
a reasonable workaround? And if so, are there any guidelines as to what is a reasonable number of
polygons (or line segments) to render with a call to MagickDrawImage?
Re: DrawPolygon doesn't seem to work for me. Where is my error?
Posted: 2009-09-09T05:30:31-07:00
by magick
The rendering algorithm is O(n^2). The only expected speed-ups are shorter point lists in the polygon or use a multi-core system where ImageMagick renders one point per core simultaneously. An alternative is to generate SVG polygons and render with rsvg which is likely O(n).