Determining which side of platform Player collides with

This is a platform game

I have a player object ( blue square ) which moves along the x ( left/ right movement ) and y ( gravity/ jumps ) axis ( image below )
I also have a platform object ( black square ) which the Player should interact with, like a solid box ( image below )
http://i43.tinypic.com/j63q75.jpg
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My event editor should looks like this, with collition detection before objects move ( for specific reasons i wont get intoo now )

If Player overlaps Platform
- *reposition Player
Allways --->
- Player move X
- Player move Y
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What i cant grasp is how i can determine when Player is colliding with the topside og the Platform ( image below )
http://i43.tinypic.com/k2yc1h.jpg
and when Player is colliding with the left side of the Platform ( image below )
http://i40.tinypic.com/20pwkno.jpg
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Any sugestions or ideas on how to tackle this would help alot

Hell man... I was thinking about this the other day, and didnt come to a conclusion . _.'

Maybe, just maybe, you could make so that the platoform has 32 directions, (not visual only virtual), and determine witch direction will stop the player or not. Of corse, se it to "look always" to player.

Got it? if not I can send an example soon. (too busy right now)

Intersting idea, I think i understand your consept
It would be cool if you could make an example

I still welcome other suggestions and ideas

My method for doing platform games is slightly different, but the general theory I use for this is to separate horizontal and vertical movement into different events rather than having them both in one "always". That way, you know whether a player is moving vertically or horizontally, because you're only dealing with events that handle one axis at a time.

You need to create detectors. Detectors are invisible active objects (usually just a thin line, 1 or 2 pixel thick) on each side of your player. Always set their X and Y to that of the player. Then you can just test if "Right Dectector" is overlapping backdrop. It is pretty easy to figure out but I can make an example if you need it. Good luck!

On the contrary, you don't need detectors at all. I like to move the player around a bit and store the data in Alterable Values. Create four values, labeled Overlap Left, Overlap Up, Overlap Right, and Overlap Down. Then in your events, do

Always:
->Set Overlap Left to 0
->Set Overlap Up to 0
->Set Overlap Right to 0
->Set Overlap Down to 0
->Set X to X-1

Object is overlapping Backdrop:
->Set Overlap Left to 1

Always:
->Set X to X+1
->Set Y to Y-1

Object is overlapping Backdrop:
->Set Overlap Up to 1

Always:
->Set Y to Y+1
->Set X to X+1

Object is overlapping Backdrop:
->Set Overlap Right to 1

Always:
->Set X to X-1
->Set Y to Y+1

Object is overlapping backdrop:
->Set Overlap Down to 1

Always:
->Set Y to Y-1

As you can see, it basically moves the object in a circle, first checking the left, the top, the right, and then finally the bottom. The last event will put it back to the center. All of the positional events cancel out before the end of the event cycle, so you don't see any movement at all. Then, when you're right up against an object, you can see from the Overlap values what side you are colliding on. This is the fundamental aspect of all movements, whether platform, iso, 8dir, etc. You need to know where obstacles are in relation to the player. You can do it this way, or use detectors, or use fastloops, or even a separate object that tells you where the obstacles are (such as surface, an active, or an array). There are probably more ways, but once you have this, you can make any movement you want with little effort.

I have been avoiding detectors, since platform movement object is so effective...

King, I am kind in a rush right now, I may do your example soon, if you still need it.

First of all, thanks for all the ideas and suggestions people!
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ChrisBurrows
I know i dont need to use detectors to make a platform movement work. I thought i needed them but have later realized it was perfectly possible with just a collition mask ( like the blue box in the image in original post ). And also seeing that im working on this platform movement to work for both Player and Enemy object, adding 1 - 4 detector object to every Enemy in a Level with lets say 25 enemys would be overkill imo ( resulting in 25 - 100 extra objects ).
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Jacob
Indeed it is importaint to know where Obstacles are in relation to the Player. The example you gave above will do exactly this "when you're right up against an object" as you say yourself.
In my case however ( example first post ), moving my object around in acircle like in your example dont answer the question "is Player colliding with Left or Right side of Platform?".

Im not exactly sure about the code, and i know they are two different softwares which work in different ways, but i remeber a command in GameMaker wich could be applied to an object with speed and direction. The comand was called moveToContact i think, and moved the object in the direction untill its possition was right up against a Solid ( platform ) object.
Know its wishfull thinking, but i could need something like this right now.
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DavidN
I am familiar with your method of doing platform movement and have done it this way many times myself. However i am beginning to become more and more of a perfectioninst and freak for details, and so i see the the flaws of my old ways of doing things and want to perfect them.

This is an illustration of a Player object ( blue box ) moving at a constant speed in the right direction and at an icreasing speed downwards ( simulating gravity )
http://i42.tinypic.com/29asa3l.jpg
The Player clearly never overlaps any of the Obstacles ( black boxes ) along its movement path


What happens when treating X and Y movement and collitions seperate in the same example i illustrate below
http://i42.tinypic.com/15qvqsh.jpg
As you see here the Player overlaps both Obstacles at Positions A and B when it realy should not, judging from the curved trajectory in the first illustration ( Hot Spot of Player located at bottom center in both illustrations )
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EdibleWare
Thats OK if you dont have time
I have been thinking about your idea a bit and am having troubble believing a single diection check ( alone ) between Platforms possition and Players position ( it being the Players current overlaped possition or possition prior to overlap ) would result in detecting which side of Platform the collition occures ( seeing that x and y movement are variable )
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I do have some half baked ideas on how to handle this issue myself, but they all seem overly complicated and im beginning to think im way too worried about detailes like this.
Is this realy that big of an issue? Am I exaggerating the problem? I hate to give up, though i could just go for one of the simpler but less precise solutions and move on

You should look into the Ball Bounce widget I posted here. It calculates the target position and then interpolates towards it, detecting collisions on every side. I'm sure it can be modified into a platform widget or anything else you need it to be.

However, I wouldn't worry too much about what you describe. Of course you're right about that sometime the player will collide with a wall in a few situations when it's not desireable, but unless you're working with really high speeds, it doesn't tend to cause any problems.

I usually use detectors, but they do require a lot of extra objects so I guess they aren't the best option.

I would definitely use Jacob's method if you want to avoid detectors though. It actually does answer the question you asked about colliding with the left or right side of a platform. The alterable values he declared called Overlap up, left, down, and right determine which sides of your object are colliding with an obstacle. Basically, every frame your object is moved 'in a circle' (one pixel in each of the four main directions) and sets a corresponding value if your object overlapped an obstacle after moving in that direction. The movement is instant so you don't actually see what's happening, but after the loop is done you'll know what side any obstacles are on. It's kind of difficult to explain but it does work.

Nifflas
Your widgets blow my mind ( beside the Bounce Ball widget i also had a look at the Basic Movement widget ). It will take me some time to fully understand those widgets, they look pretty advanced. What i think i understand from the Bounce Ball movement is that it enters a fast-loop which slowly moves the object towards its new possition ( is this interpolation maybe? ) with collition detection along the way.
Seing that i want to use my platform movement for both Player and Enemys, im not sure a large amount of fast-loops are desireable.

I have however found a solution ( in theory ) which does not involve loops or fast-loops, which i present further down in the post
It is also very comforting to hear i shouldnt worry about this too much, though i might sometime im not about to give up just yet
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Gkinfinity
I dissagree that Jacobs method solvs my problem
But before i reject it completly let me see if i have understood it correctly

Before Object ( Blue square ) moves it goes through a short loop or process where it ( Object ) is moved '1 pixle' in all four directions ( left, up, right, down ) like this
http://i43.tinypic.com/6f82s2.jpg
Have i missed anything or am i understanding the method correctly?
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I mentioned i had some half baked ideas on how to solv this before, and i have indeed found a solution ( in theory ) of how to solv this issue ( topic ) without going through numerous loops

When my objects ( Player and Enemies ) move, they move in a line from point A to point B

Before player moves it stores its X Y coordinates in alt values.
When Player overlaps an Obstacle, it needs to check in what area ( relative to the obstacle ) it was 'previously' possitioned in ( see illustration below )
http://i41.tinypic.com/29gj4eo.jpg
Off cource this means that when Player overlaps with an Obstacle, the program checks 8 conditions whereof 1 will trigger ( based on Players previous position )

The conditions for the center areas ( when Players previous position is in one of the center areas ) are easy because Player always moves in a straight line from A to B

On overlap + if Player previously was in 'top center' area --->
- Player collides with top of Obstacle
On overlap + if Player previously was in 'bottom center' area --->
- Player collides with bottom of Obstacle
On overlap + if Player previously was in 'left center' area --->
- Player collides with left of Obstacle
On overlap + if Player previously was in 'right center' area --->
- Player collides with right op Obstacle

The problematic part is when Players previous position is in one of the corner areas ( top left, top right, bottom left, bottom right )
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I have however found a solutio for this, illustrated in an .mfa attachment at the bottom of the post ( i wont go further intoo how i figured it out, but feel free ta ask ).

When Players previous position is in one of the corner areas, i simply calculate the volume ( in pixles ) of two specific square areas ( based on Players current possition, previous possition, and corner position of Obstacle ).
I then compare the volume of those two areas to determine which is bigger.

I have attached an example file which illustrates how the volume of the two areas are calculated, for a collotion originating from the top left area of the Obstacle.

On overlap + If volume of area1 ( green area ) is greater than volume of area2 ( purple area ), its a top collition.
On overlap + If volume of area2 ( purple area ) is greater than volume of area1 ( green area ), its a left collition.
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Im no expert, but I persoally belive this solution will work and is a better than a solution which move all objects ( Player and Enemies ) slowly using loops for currect collition detection ( resulting in lots of fast-loops per frame when having lots of enemies ).
My solution ( mentioned above ) calculate two values and compare them ( in 4 out of 8 possible situations ) which result in precice collition detection ( working for square objects ).

With Jacob's solution, you can get results such that both "Overlap Top" and "Overlap Right" are equal to 1, etc.

It will work for all 8 directions that way.