使用一个带有四个方向的子画面图纸的子画面二维运动动画

Question

I'm making a 2D tower defense game for learning purposes, and can't make the enemies(sprites) to face the correct direction when moving.

Heres how the map is built for clarification:

http://i.imgur.com/ivO8kWe.png

And here is the sprite sheet I'm using to test:

http://i.imgur.com/2h4fSL3.png

The first sprite at the top left is the frame 0, the next on the right is the frame 1 and so on. As you can see the sprite is already looking to the wrong direction. The map have a start point(the first brown tile on the top) and and end point(the last brown tile in the end), and only the brown tiles are walkable, so with the start point and an end point it will calculate the shortest valid path for the sprites to walk in order to reach the end point.

Said that, every sprite that spawn will have a pre determined path to walk, from that i try to find the direction the sprite is facing by checking the last X or Y position and the X or Y current position, with that i choose which line of the sprite sheet i will use for the walk animation. For example, let's say the sprite is moving south, it should use the sprites at the bottom of the sprite sheet(frames 15 to 19), but it's not working.

This is the animation class I'm using for the enemies:

 public class AnimatedSprite : Sprite
    {
        public int Lines { get; set; }
        public int Columns { get; set; }
        protected int currentFrame;
        protected int totalFrames;
        protected int timeSinceLastFrame = 0;
        protected int milisecondsPerFrame = 50;


        public AnimatedSprite(Texture2D texture, int lines, int columns, Vector2 position)
            : base ( texture, position)
        {
            this.texture = texture;
            this.position = position;
            Lines = lines;
            Columns = columns;
            totalFrames = Lines * Columns;
        }

        public override void Update(GameTime gameTime)
        {
            base.Update(gameTime);

           //Here i check if the sprite sheet have more than 1 line because if it have, 
           //it must use a different update method.
            if (Lines > 1)
            {
                // Down
                if (lastPostion.Y < position.Y)
                {
                    AnimateDown(gameTime);
                }
                // Up
                if (position.Y < lastPosition.Y)
                {
                    AnimateUp(gameTime);
                }

                // Right
                if (position.X > lastPosition.X)
                {
                    AnimateRight(gameTime);
                }

                // Left
                if (position.X < lastPosition.X)
                {
                    AnimateLeft(gameTime);
                }
            }

            if (Lines == 1) {
            timeSinceLastFrame += gameTime.ElapsedGameTime.Milliseconds;
            if (timeSinceLastFrame > milisecondsPerFrame)
            {
                timeSinceLastFrame -= milisecondsPerFrame;
                currentFrame++;
                if (currentFrame == totalFrames)
                {
                    currentFrame = 0;
                }
            }
        }                        
           center = new Vector2(position.X + texture.Width / Columns, position.Y + texture.Height / Lines);
        }

        public void AnimateUp(GameTime gameTime)
        {
            timeSinceLastFrame += gameTime.ElapsedGameTime.Milliseconds;
            if (timeSinceLastFrame > milisecondsPerFrame)
            {
                timeSinceLastFrame -= milisecondsPerFrame;
                currentFrame++;
                if (currentFrame > 14)
                    currentFrame = 10;
            }
        }

        public void AnimateDown(GameTime gameTime)
        {
            timeSinceLastFrame += gameTime.ElapsedGameTime.Milliseconds;
            if (timeSinceLastFrame > milisecondsPerFrame)
            {
                timeSinceLastFrame -= milisecondsPerFrame;
                currentFrame++;
                if (currentFrame > 19)
                    currentFrame = 15;
            }
        }           

        public void AnimateLeft(GameTime gameTime)
        {
            timeSinceLastFrame += gameTime.ElapsedGameTime.Milliseconds;
                if (timeSinceLastFrame > milisecondsPerFrame)
            {
                timeSinceLastFrame -= milisecondsPerFrame;
                currentFrame++;
                if (currentFrame > 4)
                    currentFrame = 0;
            }
        }

     public void AnimateRight(GameTime gameTime)
        {
            timeSinceLastFrame += gameTime.ElapsedGameTime.Milliseconds;
                if (timeSinceLastFrame > milisecondsPerFrame)
            {
                timeSinceLastFrame -= milisecondsPerFrame;
                currentFrame++;
                if (currentFrame > 9)
                    currentFrame = 5;
            }
        }


        public override void Draw(SpriteBatch spriteBatch)
        {
            int width = texture.Width / Columns;
            int height = texture.Height / Lines;
            int line = (int)((float)currentFrame / (float)Columns);
            int column = currentFrame % Columns;

            Rectangle originRectangle = new Rectangle(width * column, height * line, width, height);
            Rectangle destinationRectangle = new Rectangle((int)position.X, (int)position.Y, width, height);          

            spriteBatch.Draw(texture, destinationRectangle, originRectangle, Color.White);
        }
    }
}

Edit: I made a test in a straight line level(start point at left), it starts facing left (frame 0), but when it reaches the third brown tile, it fixes and face the correct direction:

i.imgur.com/3FsGhuY.png

Edit2: I made a test with straight line levels in all four directions(starting down and going up, start right and going left and vice versa), and in all of them, it starts at frame 0 and when it reaches the third tile it fixes and face the correct direction.

Answer 1

You're not checking whether the current frame is below the minimum value for the desired animation loop; you're only checking it against the maximum value. Also, there's some repetition in your code that should probably be factored out to make it easier to read and work with.

I would replace all of your AnimateXXXX methods with a single method:

public void AnimateLoop(GameTime gameTime, int loopFirstFrame, int loopLastFrame)
{
    timeSinceLastFrame += gameTime.ElapsedGameTime.Milliseconds;
    if (timeSinceLastFrame > milisecondsPerFrame)
    {
        timeSinceLastFrame -= milisecondsPerFrame;
        currentFrame++;
    }
    if (currentFrame > loopLastFrame || currentFrame < loopFirstFrame)
        currentFrame = loopFirstFrame;
}

And then call them like this:

// Down
if (lastPostion.Y < position.Y)
    AnimateLoop(gameTime, 15, 19);
// Up
if (position.Y < lastPosition.Y)
    AnimateLoop(gameTime, 10, 14);
// Right
if (lastPosition.X < position.X)
    AnimateLoop(gameTime, 5, 9);
// Left
if (position.X < lastPosition.X)
    AnimateLoop(gameTime, 0, 4);