randomness from spritemap in spritebatch - xna

Question

Im sure this is easily fixed, and I do have searched both high and low, traversed the net both east, west, north and south but to no prevail...
My problem is this. Im in the middle of trying to make a bejeweled clone, just to get me started in xna. However im stuck on the random plotting of gems/icons/pictures.
This is what i have.
First a generated list of positions, a random and a rectangle:
List<Vector2> platser = new List<Vector2>();
Random slump = new Random();
Rectangle bildsourcen;
protected override void Initialize()
{
for (int j = 0; j < 5; j++)
{
for (int i = 0; i < 5; i++)
{
platser.Add(new Vector2((i*100),(j*100)));
}
}
base.Initialize();
}
Pretty straight-forward.
I also have loaded a texture, with 5 icons/gems/pictures -> 5*100px = width of 500px.
allImage = Content.Load<Texture2D>("icons/all");
Then comes the "error".
protected override void Draw(GameTime gameTime)
{
GraphicsDevice.Clear(Color.CornflowerBlue);
spriteBatch.Begin();
int x = slump.Next(2);
bildsourcen = new Rectangle((x * 100), 0, 100, 100);
for (int i = 0; i < platser.Count; i++)
{
spriteBatch.Draw(allImage, new Rectangle((int)platser[i].X, (int)platser[i].Y, 100, 100), bildsourcen, Color.White);
}
So, there is my code. And this is what happens:
I want it to randomly pick a part of my image and plot it at the given coords taken from the vector2-list. However, it puts the same image at all coords and keeps randomly replacing them, not with random images but with the same. So the whole board keeps flickering the same icons. Ie, instead of generating 15231 and keeping it frozen, it one second puts 11111 and the next second it puts 33333.
Does anybody understand what im trying to describe ? I'm almost at the point of pulling my own hair out. The cat's hair has already been pulled...
Thx in advance

Answer 1

The Draw function is called once each frame. This line:
int x = slump.Next(2);
Is generating a random number (either a 0 or a 1 in this case) each frame, hence the flicker.
The line after that selects a sprite from your sprite atlas based on that number (specifically it specifies the rectangle containing that sprite). And in the loop that follows you're drawing multiple copies of that sprite (always the same image).
You should be doing all of your game logic in your Update function. That function will give you a time and you will probably want to implement a method of waiting for a certain amount of time to pass before you generate a random block (so keep accumulating the time that passes between each Update, until it reaches some threshold). The exact mechanics of when you want to generate your random block is up to you.
Of course, that is not to mention that there are other flaws in the structure of your code. Bejewelled is played on a fixed-sized board with different coloured blocks (each block you could represent with a number from 1 to X). The location of the blocks should be be implicit in your data structure (so you don't need to generate your platser list).
So your Game class should have something like:
const int BoardWidth = 10;
const int BoardHeight = 10;
int[,] board = new int[BoardWidth, BoardHeight];
Then in your Initialize function you should fill board and perhaps use 0 as an empty space and 1 to X to represent your colours, like so:
for(int x = 0; x < BoardWidth; x++) for(int y = 0; y < BoardHeight; y++)
{
board[x,y] = slump.Next(1, 6); // gives 5 different sprites
}
Then in Update wait for user input or a time-out before modifying the board (depending on your gameplay).
Then in your Draw function do something like this:
for(int x = 0; x < BoardWidth; x++) for(int y = 0; y < BoardHeight; y++)
{
if(board[x,y] == 0) continue; // don't render an empty space
Vector2 position = new Vector2(100*x, 100*y);
Rectangle bildsourcen = new Rectangle(100*(board[x,y]-1), 0, 100, 100);
sb.Draw(allImage, position, bildsourcen, Color.White);
}