Is is possible to eliminate space between non-stroked polygons in Processing? If so, how? - image-processing

Question

I'm working on a Processing project to simulate very basic hard-shadows. For the most part I've got it working; each edge of each object checks if its back is facing the light. If it is, a shadow polygon is added using that edge and others cast back away from the point directly away from the light.
However, when I tried to shift from solid shadows to transparent I ran into some problems. Namely, because the shadows are made of multiple shapes the borders tended to overlap, making them darker than everywhere else:
I disabled the stroke on the shadows, which improved the effect but left small lines between the shadows, despite the edges for the polygons being identical:
Is there a way to eliminate this artifact? If so, how?

Answer 1

The solution is to not draw the shadows as separate pieces, but to draw the combined polygon of all the shadow pieces as one polygon.
Here's a little example that exhibits your problem:
void setup(){
size(500, 500);
}
void draw(){
background(255);
noStroke();
fill(0);
ellipse(mouseX, mouseY, 10, 10);
fill(128, 128, 128, 128);
beginShape();
vertex(mouseX, mouseY);
vertex(0, height);
vertex(width, height);
endShape();
fill(128, 128, 128, 128);
beginShape();
vertex(mouseX, mouseY);
vertex(width, height);
vertex(width, 0);
endShape();
}
Notice the white line between the two polygons:
But if I instead draw the two polygons as one:
void setup(){
size(500, 500);
}
void draw(){
background(255);
noStroke();
fill(0);
ellipse(mouseX, mouseY, 10, 10);
fill(128, 128, 128, 128);
beginShape();
vertex(mouseX, mouseY);
vertex(0, height);
vertex(width, height);
vertex(width, 0);
endShape();
}
Then the white line goes away: