import java.awt.*;

public class BasicRayTrace extends BufferedApplet {

	int focalLength = 4;

	double[] v = new double[3];
	double[] w = new double[3];
	Color[][] image;
	Color bgColor = new Color(0, 0, 0);

	double[][] spheres = {
		{-0.85, -0.85,  -6.5, (double)2440/6378},	//MERCURY
		{-0.35, -0.35,  -7, (double)6052/6378},	//VENUS
		{0.35, 0.35,  -8, (double)6378/6378},	//EARTH
		{0.45, 0.35,  -7, (double)1738/6378},	//MOON
		{0.85, 0.85,  -8, (double)3397/6378},	//MARS
	};


	Color[] colors = {
		new Color (128, 0, 0),	//MERCURY
		new Color (0, 128, 128),	//VENUS
		new Color (0, 0, 255),	//EARTH
		new Color (128, 128, 128),	//MOON
		new Color (255, 0, 0),	//MARS
	};

	public void render(Graphics g) {
		int H = bounds().width;
		int W = bounds().width;
		g.setColor(bgColor);
		g.fillRect(0, 0, W, H);

		image = new Color[W][H];
		for (int i = 0; i < W; i++)
			for (int j = 0; j < H; j++)
				image[i][j] = bgColor;

		// image has height, width H, W; camera at origin
		v[0] = 0;
		v[1] = 0; // CAMERA EYEPOINT IS AT THE ORIGIN
		v[2] = 0;

		for (int i = 0; i < W; i++) // LOOP OVER IMAGE COLUMNS
			for (int j = 0; j < H; j++) { // LOOP OVER IMAGE ROWS

				w[0] = (double) (i - W / 2) / (W / 2);
				// COMPUTE RAY DIRECTION AT EACH PIXEL
				w[1] = (double) (H / 2 - j) / (W / 2); //
				w[2] = -focalLength; // PLACE IMAGE PLANE AT z=-focalLength

				image[i][j] = rayTrace(v, w);
				// COMPUTE COLOR AT PIXEL BY RAY TRACING
			}

		for (int i = 0; i < W; i++)
			for (int j = 0; j < H; j++) {
				g.setColor(image[i][j]);
				g.drawRect(i, j, 1, 1);
			}

	}

	public Color rayTrace(double[] v, double[] w) {
		double t = Double.MAX_VALUE;
		double newT;
		double A, B, C;
		Color color = bgColor;

		A = Math.pow(w[0], 2) + Math.pow(w[1], 2) + Math.pow(w[2], 2);

		for (int i = 0; i < spheres.length; i++) {
			B =
				2 * (v[0] - spheres[i][0]) * w[0]
			      + 2 * (v[1] - spheres[i][1]) * w[1]
			      + 2 * (v[2] - spheres[i][2]) * w[2];

			C =
				Math.pow(v[0] - spheres[i][0], 2)
			      + Math.pow(v[1] - spheres[i][1], 2)
			      + Math.pow(v[2] - spheres[i][2], 2)
			      - Math.pow(spheres[i][3], 2);

			double discriminant = Math.pow(B, 2) - 4 * A * C;

			if (discriminant >= 0) {
				// calculate the lesser possiblity for newT
				newT = (-B + Math.sqrt(discriminant)) / 2 * A;
				if ((-B - Math.sqrt(discriminant)) / 2 * A < newT) {
					newT = (-B - Math.sqrt(discriminant)) / 2 * A;
				}
				
				if (0 < newT && newT < t) {
					t = newT;
					color = colors[i];
				}
				
			}
		}

		return color;
	}

}