Poly Anomaly

[dave625]

When using the poly object, I'm getting a strange display anomaly when trying to draw a segment.  Here is the code:

void setup()
{
 GD.begin();
}

int f(int val) {
  return val * 16;
}

void shape(int x, int y) {
 Poly po;
 GD.ColorRGB(200,0,0);
 po.begin();
 po.v(f(x + 0), f(y + 15));
 po.v(f(x + 15),f(y + 0));
 po.v(f(x + 110), f(y + 0));
 po.v(f(x + 125), f(y + 15));
 po.v(f(x + 110), f(y + 30));
 po.v(f(x + 15), f(y + 30));
 po.draw();  
}

void loop()
{
 GD.Clear();
 shape(100,100);
 GD.swap();
}
all points appear to render correctly, with the exception of the top left most point. Here is a facsimile of the vertex in question:



Is there a way around this? Thanks in advance.

[jamesbowman]

Yes, the GPU's edge filling seems to sometimes not get every pixel exactly right, so you sometimes get a 2-pixel step at the end of an edge.

The easiest way to hide this - and all the other jaggies - is to draw an antialias line around the edge. Page 151 in the book covers this. If you draw it in the same color as the polygon interior, you get this:



The code is:

void shape(int x, int y) {
 Poly po;
 GD.ColorRGB(200,0,0);
 GD.SaveContext();
 po.begin();
 po.v(f(x + 0), f(y + 15));
 po.v(f(x + 15),f(y + 0));
 po.v(f(x + 110), f(y + 0));
 po.v(f(x + 125), f(y + 15));
 po.v(f(x + 110), f(y + 30));
 po.v(f(x + 15), f(y + 30));
 po.draw();
 GD.RestoreContext();

 GD.LineWidth(1 * 16);
 po.outline();
}

Alternatively, if you really want pixel-perfect diagonals - maybe for a retro game look - a bitmap might be a better approach. Creating an L1 bitmap that covers the screen and drawing the objects into it works well. The "space invaders" demo does this.

[dave625]

Thank you James, that's just the ticket...

Dave