Pacman remake with SdlDotNet -- reading colour information

[pymatio]

Hi, I'm trying to remake pacman with Mono & SdlDotNet, but I've hit a problem. I'm reading colour information off the screen that should stop pacman from going into the walls by setting the xSpeed & ySpeed to 0... But sometimes it goes into the walls and sometimes it stops you from moving (when you are travelling up to the top right hand corner).

using System;
using System.IO;
using System.Drawing;
using System.Collections.Generic;
using SdlDotNet.Core;
using SdlDotNet.Graphics;
using SdlDotNet.Graphics.Primitives;
using SdlDotNet.Input;

namespace PMAN
{
	public class PMAN
	{
		Surface sfcGameWindow;
		Surface map = new Surface ("media/background.jpg");
		const int gameWidth = 800;
		const int gameHeight = 800;
		const int blockSize = 40;

		int score;

		Surface point = new Surface ("media/point" + Convert.ToString (blockSize) + ".png");

		pacman pman;
		List<point> points = new List<point> ();

		public PMAN ()
		{
			string[] lines = File.ReadAllLines ("map" + Convert.ToString (blockSize) + ".txt");
			
			short x, y;
			x = y = 0;
			foreach (string line in lines) {
				foreach (char c in line) {
					if (c == '1') {
						short xp = Convert.ToInt16 (x + blockSize);
						short yp = Convert.ToInt16 (y + blockSize);
						map.Draw (new Box (x, y, xp, yp), Color.Blue, false, true);
					} else if (c == '0') {
						points.Add (new point (blockSize, x, y));
					} else if (c == 'p') {
						pman = new pacman (blockSize, x, y);
					}
					
					x += blockSize;
				}
				
				x = 0;
				y += blockSize;
			}
		}

		public void Run ()
		{
			sfcGameWindow = Video.SetVideoMode (gameWidth, gameHeight);
			
			Events.Quit += new EventHandler<QuitEventArgs> (Events_Quit);
			Events.Tick += new EventHandler<TickEventArgs> (Events_Tick);
			Events.KeyboardDown += new EventHandler<KeyboardEventArgs> (Events_KeyboardDown);
			Events.Run ();
		}

		void Events_Quit (object sender, QuitEventArgs e)
		{
			Events.QuitApplication ();
		}

		void Events_MouseDown (object sender, MouseButtonEventArgs e)
		{
		}

		void Events_KeyboardDown (object sender, KeyboardEventArgs e)
		{
			switch (e.Key) {
			case Key.DownArrow:
				pman.xSpeed = 0;
				pman.ySpeed = 200;
				break;
			case Key.UpArrow:
				pman.xSpeed = 0;
				pman.ySpeed = -200;
				break;
			case Key.RightArrow:
				pman.xSpeed = 200;
				pman.ySpeed = 0;
				break;
			case Key.LeftArrow:
				pman.xSpeed = -200;
				pman.ySpeed = 0;
				break;
			}
		}

		void Events_Tick (object sender, TickEventArgs e)
		{
			Update (e.SecondsElapsed);
			Draw ();
		}

		void Update (float elapsed)
		{
			pman.update (elapsed, map);
		}

		void Draw ()
		{
			sfcGameWindow.Blit (map);
			for (int i = points.Count - 1; i >= 0; i--) {
				point p = points[i];
				Rectangle prec = new Rectangle (p.x, p.y, p.pic.Width / 2, p.pic.Height / 2);
				Rectangle parec = new Rectangle (pman.x - 10, pman.y, pman.pic.Width, pman.pic.Height);
				
				if (prec.IntersectsWith (parec)) {
					points.RemoveAt (i);
				} else {
					sfcGameWindow.Blit (p.pic, new Point (p.x, p.y));
				}
			}
			sfcGameWindow.Blit (pman.pic, new Point (pman.x, pman.y));
			sfcGameWindow.Update ();
		}
	}
}

Pacman class:

using System;
using System.Drawing;
using SdlDotNet.Core;
using SdlDotNet.Graphics;

namespace PMAN
{
	public class pacman
	{
		public Surface pic;
		public int x,y;
		public int xSpeed,ySpeed;
		
		public pacman (int _mode, int _x, int _y)
		{
			x = _x;
			y = _y;
			
			pic = new Surface("media/pacman"+Convert.ToString(_mode)+".png");
			xSpeed = 200;
			ySpeed = 0;
		}
		
		public void update(float elapsed, Surface gmwindow){
			if(xSpeed == 200){
				if(gmwindow.GetPixel(new Point(x+pic.Width, y+3)).ToArgb() 
				   == Color.Blue.ToArgb()){
					xSpeed = 0;
					ySpeed = 0;
				}
			}else if(xSpeed == -200){
				if(gmwindow.GetPixel(new Point(x+15, y)).ToArgb() 
				   == Color.Blue.ToArgb()){
					xSpeed = 0;
					ySpeed = 0;
				}
			}else if(ySpeed == 200){
				if(gmwindow.GetPixel(new Point(x+15, y+pic.Height)).ToArgb() 
				   == Color.Blue.ToArgb()){
					xSpeed = 0;
					ySpeed = 0;
				}
			}else if(ySpeed == -200){
				if(gmwindow.GetPixel(new Point(x+15, y)).ToArgb() 
				   == Color.Blue.ToArgb()){
					xSpeed = 0;
					ySpeed = 0;
				}	
			}
			x += Convert.ToInt32(xSpeed * elapsed);
			y += Convert.ToInt32(ySpeed * elapsed);
		}
	}
}