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I am using Slick2d to create a top-down game, however an rectangle than I am drawing to the display is moving faster to the left than to the right, and faster up than down.

I am updating frames like so:

public void update(GameContainer gameContainer, int delta) throws SlickException { 
    player = new Rectangle(playerX, playerY, 10, 20);

    if (gameContainer.getInput().isKeyDown(Input.KEY_LEFT)) {
        x -= delta * 0.1f;
    }
    if (gameContainer.getInput().isKeyDown(Input.KEY_RIGHT)) {
        x += delta * 0.1f;
    }
    if (gameContainer.getInput().isKeyDown(Input.KEY_UP)) {
        y -= delta * 0.1f;
    }
    if (gameContainer.getInput().isKeyDown(Input.KEY_DOWN)) {
        y += delta * 0.1f;
    }
}

I then simply draw the rectangle to the display. It is noteworthy that the rectangle class is aorg.newdawn.slick.geom.Rectangle.

Why are the speeds for the movement different, and how can I fix this?

EDIT: I have fixed the problem by slowing the movement to the left and up, but this does not explain why the problem occurs

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  • \$\begingroup\$ The code seems to be fine. You can put debug statements inside each if block to check just in case any value is not getting set properly. \$\endgroup\$ – Hash Buoy Aug 22 '15 at 21:33
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This is most likely a rounding error. I'm guessing that x and y are ints.

Keeping track of the x and y values as floats, I expect will fix the issue.

Consider if the values are being floored:

Moving Left 10 - 1.6 = 8

Moving Right 10 + 1.6 = 11

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  • 1
    \$\begingroup\$ Don't ints round towards 0? I ran this code System.out.println((int) -1.7); on my machine and it printed -1. But still, he shouldn't be storing coordinates as ints. \$\endgroup\$ – Kyranstar Aug 25 '15 at 2:12
  • \$\begingroup\$ Yes, they will but screen coords are never negative.. so bad example on my part. Try 5-1.6 = 3, not 4 \$\endgroup\$ – Fuzzy Logic Aug 25 '15 at 2:29
  • \$\begingroup\$ Oh, good point. \$\endgroup\$ – Kyranstar Aug 25 '15 at 2:29
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    \$\begingroup\$ Keep in mind, they do not "round toward 0," they truncate. This distinction is important. For instance, 1.9999 truncates to 1 and 2.0001 truncates to 2. That's where this unexpected behavior is most likely occurring, in the truncation. \$\endgroup\$ – BennyMathison Sep 8 '15 at 2:40

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