Drawing Projected Vertices/Faces In Order

Question

I recently created a 3d engine, and have gotten to what is (as I've found) the hardest part so far. I need to sort the objects and then draw them. I've looked at sorting algorithms, but my case is rather specialized. I have an array of vertices stored inside the Face3D object which I've created, and need to draw faces correctly when overlapping, Firstly, am I thinking about this correctly? Draw the back vertices first? Secondly, How would I sort this? Here's my code for the Face3D file:

package me.dylan.ThirdDimension.src;


import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Polygon;
import java.awt.Shape;
import java.awt.geom.Point2D;
import java.util.ArrayList;

public class Face3D {
    Polygon face=new Polygon();
    ArrayList Lines = new ArrayList();
    ArrayList Vertices = new ArrayList();
    Color color;
    public Face3D(int x,int y, int z, int width, int height, int depth,Main m,Color c) {
        color=c;
        Vertices.add(new Point3D(x, y, z, m));
        Vertices.add(new Point3D(x, y+height, z, m));
        Vertices.add(new Point3D(x+width, y+height, z+depth, m));
        Vertices.add(new Point3D(x+width, y, z+depth, m));
        for(int i=0;i
            Vertices.get(i).update();
            Point2D tmp = new Point2D.Double();
            tmp.setLocation((int)Vertices.get(i).x2d, (int)Vertices.get(i).y2d);
            face.addPoint((int)Math.round(tmp.getX()),(int)Math.round(tmp.getY()));
        }

}

public void paint(Graphics g) {
    g.setColor(color);
    Graphics2D g2d = (Graphics2D)g;
    face=new Polygon();
    for(int i=0;i<Vertices.size();i++) {            
        Vertices.get(i).update();
        Point2D tmp = new Point2D.Double();
        tmp.setLocation((int)Vertices.get(i).x2d, (int)Vertices.get(i).y2d);
        face.addPoint((int)tmp.getX(),(int)Math.round(tmp.getY()));
    }
    g2d.fillPolygon(face);

}

}

I'd just like to apologize if my question is unclear, it's difficult for me to explain this.

Answer 1

It appears that you are trying to write a 3D render engine, and you want the faces to be rendered from front to back so you don't get an x-ray effect. Your current method would not work with intersecting faces.

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Z-Buffering

A Z-buffer is an array that stores the z-distance of every pixel in the viewport:

_{Image from Wikipedia: Original page.}

When you are rendering, you calculate the Z-distance of the current pixel. You then compare that to the corresponding Z-distance in the buffer for that pixel. If the distance is less than that of the Z-buffer, you draw the pixel, and write the distance to the Z-buffer. If, however, the distance is greater, you skip that pixel and continue rendering.

For example:

drawPixel(int x, int y, int zDist, Color c)
{
    if(zDist < zBuffer[x][y])
    {
        pixel[x][y] = c;
        zBuffer[x][y] = zDist;
    }
}

I'm sure there are optimizations you could make, but this is the general way of doing it.