bar code generator in c# PIXEL-PERFECT COLLISION AND DESTRUCTIBLE ENVIRONMENTS in Font

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//The distance vector between the lander and the asteroid _v0.update ( _lander.xPos + _lander.width * 0.5, _lander.yPos + _lander.height * 0.5, _asteroidBitmap.x + _asteroidBitmap.width * 0.5, _asteroidBitmap.y + _asteroidBitmap.height * 0.5 ); //Check for a collision var loopCounter:int = 0; while (loopCounter++ != 10) { if(landerBitmap.bitmapData.hitTest ( new Point(_lander.xPos, _lander.yPos), 255, _asteroidBitmap, new Point(_asteroidBitmap.x, _asteroidBitmap.y), 255 ) ) { //Switch off gravity _lander.gravity_Vy = 0; //Move the lander out of the collision in the direction //of the distance vector _lander.setX = _lander.xPos - _v0.dx; _lander.setY = _lander.yPos - _v0.dy; _lander.vx = 0; _lander.vy = 0; } else { break; } } For a precise-looking collision, the code switches off the lander s gravity and set its velocity to 0. If this isn t done, the lander will appear to wobble slightly when it s resting on the asteroid. This is because the forces that are pushing and pulling it are not quite equal. If we switch gravity off when the lander is on the asteroid, we need to switch it back on again when it leaves. But there s a problem: when the lander is resting on the asteroid, it s actually not touching the asteroid any more. It s just slightly above it. Remember that the while loop pushes the lander and asteroid apart. The loop quits when they re no longer touching each other. That means that you can t run the same collision test to find out if

the lander is actually on the asteroid, because it will always return false. The loop has done its job and separated the objects perfectly. The lander and asteroid will be directly adjacent to each other, but they won t overlap by even 1 pixel. The problem is that because they re completely separated, we have no way to detect when the lander is resting on the asteroid. To solve this, we need to run a second collision check that tests to see if there is any ground directly below the lander. It checks to see if 1 pixel below the lander is in contact with the asteroid. If it is, then we know the lander is still on the asteroid. If there s no ground directly below it, then we know the lander isn t in contact with the ground, and we can switch gravity back on again. This will be easier for you understand if you can visualize it before you look at the code. Figure 58 illustrates the logic of what we want to accomplish.

Do a second collison test where the lander s collision area is shifted down by 1-pixel. This 1-pixel margin will tell you if the lander is touching the surface of the asteroid.

Figure 5-8. Check whether the lander is on the surface of the asteroid. The code for this second collision test is saying, If an area 1 pixel below the lander isn t touching the asteroid, turn gravity on. Here s what it looks like:

if(!landerBitmap.bitmapData.hitTest ( new Point(_lander.xPos, _lander.yPos + 1), 255, _asteroidBitmap, new Point(_asteroidBitmap.x, _asteroidBitmap.y), 255 ) ) { _lander.gravity_Vy = 0.1; } Note that it checks whether a collision is not happening. if(!... The + 1 is highlighted in the code so that you can clearly see where it shifts the collision area down by a pixel. new Point(_lander.xPos, _lander.yPos + 1), In environments where game objects rest on steep surfaces, you will need a bit more clearance. You ll be able tell this if the object appears to wobble slightly when it should be resting. If that s the case, use +2, +3, +4 keep increasing that number until the wobble disappears.