Detect if "player" jumped on a box in AS3 - actionscript-3

Question

I'm working on a simple game in AS3 where a player should be able to jump on a box.
How to detect if the player landed on the box and didn't run into it?
AS3 to make the player jump:
var grav:Number = 10;
var jumping:Boolean = false;
var jumpPow:Number = 0;
stage.addEventListener(KeyboardEvent.KEY_DOWN, onDown);
stage.addEventListener(Event.ENTER_FRAME, update);
function onDown(evt:KeyboardEvent):void
{
if(evt.keyCode == Keyboard.UP)
{
//umbau das mehrfach tab bis höhe erreicht?
if(jumping != true)
{
jumpPow = -10;
jumping = true;
}
}
}
function update(evt:Event):void
{
if(jumping)
{
player_mc.y += jumpPow;
jumpPow += grav;
if(player_mc.y >= stage.stageHeight)
{
jumping = false;
player_mc.y = stage.stageHeight;
}
}
}
This is how the game layout looks like: (the grey box is moving from right to left, the player's position is fixed)

Answer 1

You should start with a simple rectangle collision function:
/* Test collision between a rectangle and another rectangle. */
/* right() and bottom() return x+width and y+height of the parent rectangle. */
Rectangle.prototype.collideRectangle=function(rectangle_){
if (this.x>rectangle_.right()||this.y>rectangle_.bottom()||this.right()<rectangle_.x||this.bottom()<rectangle_.y){
return false;
}
return true;
}
if you are using the gray rectangle as a platform, you can do a simple test to see if the red rectangle is landing on the gray one by using its current and last position:
/* If true, the two rectangles are definitely colliding. */
if (red_rect.collideRectangle(gray_rect)){
/* Get the last position of the bottom of the red rectangle. */
var old_bottom=red_rect.bottom()-red_rect.velocity.y;
/* If the old bottom was above the gray rectangle on the last frame and is colliding in this frame, you know that the red rectangle is landing on the gray rectangle. */
if (old_bottom<gray_rect.y){
/* Place the red rectangle on top of the gray rectangle. */
red_rect.y=gray_rect.y-red_rect.height;
}
}
This is assuming you move the red rectangle by adding the value of velocity.y to its y position on each frame. Also, I'm using JavaScript, but AS3 and JavaScript aren't so different.
If you want more than a platform, and you want full collision, you should consider the penetration depth of the red rect on the gray rect and separate them on the axis with the smallest penetration depth. There are a lot of resources about this stuff online, and AS3 in particular has a lot of game related tutorials with a focus on do it yourself physics. Some really good collision methods are separation of axes theorem (SAT) and GJK (I'm not going to try spelling the names in GJK).