strokeを使わずに直線を描いてください。

Bresenhamの線描画アルゴリズムを利用。

import processing.core.PApplet;

public class ProcessingApp extends PApplet {

    private static final long serialVersionUID = 1L;

    @Override
    public void draw() {
        myline(mouseX, mouseY, width / 2, height / 2);
    }

    @Override
    public void init() {
        super.init();
    }

    @Override
    public void setup() {
        size(200, 200);
        line(0, 0, 200, 200);
    }

    void myline(int x0, int y0, int x1, int y1) {

        int E, x, y;
        int dx, dy, sx, sy, i;

        sx = (x1 > x0) ? 1 : -1;
        dx = (x1 > x0) ? x1 - x0 : x0 - x1;
        sy = (y1 > y0) ? 1 : -1;
        dy = (y1 > y0) ? y1 - y0 : y0 - y1;

        x = x0;
        y = y0;

        if (dx >= dy) {
            E = -dx;
            for (i = 0; i <= dx; i++) {
                point(x, y);
                x += sx;
                E += 2 * dy;
                if (E >= 0) {
                    y += sy;
                    E -= 2 * dx;
                }
            }
        } else {
            E = -dy;
            for (i = 0; i <= dy; i++) {
                point(x, y);
                y += sy;
                E += 2 * dx;
                if (E >= 0) {
                    x += sx;
                    E -= 2 * dy;
                }
            }
        }

    }
}

Bresenham's line algorithm - Wikipedia, the free encyclopedia
あとで、修整。