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Movement control of NPC in XNA4.0 sprite RPG.


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#1 PaigeEX   Members   -  Reputation: 100

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Posted 12 March 2012 - 03:33 AM

I've been having a rough time getting an NPC sprite to move to random destinations that should occur sequentially as the sprite reaches each destination in a 2D top down tile-based map, ie: Sprite starts at a set point, logs that point, picks a random destination within a certain range that is not its current destination, moves towards it, and once it arrives at that destination, logs it, then randomly picks the next destination to move towards that is not its current position, or any of its previous positions (which it's keeping track of), and repeats this process.

The map that it's on is tile based, so I'm checking and using tiles to determine my destination coordinates. Right now, it's only setting destinations one tile width away (32 pixels), and moving towards those. I'm using a simple If-else statement to make sure it's arrived, and to continue setting its motion if it hasn't. If it has arrived, I've even tried making sure to set its motion vector to zero to make sure it doesn't carry over movement from the previous Update(). The check to make sure the sprite is at its destination currently looks like this:

if (Sprite.Position.X != (lastDestination.X * Engine.TileWidth) && Sprite.Position.Y != (lastDestination.Y * Engine.TileWidth) )
			{
				motion.X = (lastDestination.X * Engine.TileWidth - Sprite.Position.X);
				motion.Y = (lastDestination.Y * Engine.TileWidth - Sprite.Position.Y);
			}
			else
			{
				motion = Vector2.Zero;
				if (frameCount > maxFrameCount)
				{
					lastDestination = destination;
					frameCount = 0;
				}
			}

lastDestination is set in Tile coordinates, where as Sprite.Position returns pixel coordinates. In the call where the update is made to the sprite position, motion goes through motion.Normalize before Sprite.Position += motion * Sprite.Speed (which is 2).

Sprite.Position returns the pixel coordinates of the sprite, and not the tile coordinates (ie: Tile(1,1) is pixel coordinates (32, 32)). I've done versions of the check where I divide the Sprite.Position by the tile width and store it as an integer value, but the problem with that is as soon as the sprite is within the area of the tile, the constraints of using integer division to match pixels to tiles means that the sprite actually at (3.9, 3.9) is considered to be in tile (3, 3), when it's only barely there. I've tried instead multiplying the destination by the tile width (tile width and height are the same, so I'm just being lazy there) to make sure the pixel coordinates are the same.

The problem I get, and I'm betting this is just the math I'm using to create the motion vector, is sometimes the Sprite gets to its destination with either version (comparing tile coordinates or pixel coordinates), and then moves on, but it doesn't take long before it gets to a point where it appears to be vibrating, and my debug statements reveal that it's never fully arrived at its destination to move on. My best guess is its motion is continuously overcompensating to correct and try to reach its final destination and never quite gets there, so it's left bouncing back and forth by minimal pixel values, but I'm beginning to think there's some section of this where I may be going about it wrong entirely.

EDIT: I solved it with a friend's advice. I was just so caught up I forgot to think of the distance formula. I added in a method that returns a bool to check the distance between where the sprite is going to be based on its normal speed, and the distance of how far it needs to go. If the distance it's going to go is too far, the bool is returned true, and the motion is limited only to the distance remaining as opposed to the product of the motion and the sprite's speed.

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