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Going crazy, collision detection

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Hey guys! I posted here yesterday, and it led me to a solution, so I'd like your help again. Now, I'm really going crazy with this, I can't even think properly. My problem is, I'm making a platformer and I'm having a *hard* time getting the collision detection right. What I want is, platforms are bypassable from the bottom and the side, but if you jump through them and your legs are on top of it, they 'become solid'. I spent hours trying, no solution so far. Here is the code:

[CODE]
# This is a part of the Avatar class, which has a map in self.map which has platforms (their pygame.rect's) in it.
for platform in self.map.platforms:
left = self.rect.x
right = self.rect.x + self.rect.w
top = self.rect.y
bottom = self.rect.y + self.rect.h
platformLeft = platform.rect.x
platformRight = platform.rect.x + platform.rect.w
platformTop = platform.rect.y
platformBottom = platform.rect.y + platform.rect.h
# If there is a collision
if (bottom >= platformTop) and \
(top <= platformBottom) and \
(right >= platformLeft) and \
(left <= platformRight):
pass
# NO IDEA :(
[/CODE]

Help is very much appreciated, thanks for reading!

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The collision model for the "bypassable" platforms should not apply to objects that don't have their feet above the platform. In other words: For all objects below the platform, that platform doesn't exist.

So, however you're actually doing your overall collision detection, modify the algorithm to include a special pass for that particular circumstance.

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A quick thought is, if you you get a new collision (ie, it wasn't colliding in the last frame), then check the vertical velocity. If it's positive (moving up), ignore the collision. If it's negative (falling), then you need to check where the collision occurs on your player.

If it collides at the feet, then you can make the collision stick, and keep the player on top of the platform. If you're player collidies mid-body (or head, etc.), then he's probably moving left to right and falling, and needs to fall through.

That is just a quick thought about how I might try it, but i don't have any experience directly with this.

Good Luck!

EDIT: beat by Goran, but his idea is better than mine, to ensure the feet are above the top of the platform in question. Edited by BeerNutts

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[quote name='Goran Milovanovic' timestamp='1344455632' post='4967502']
The collision model for the "bypassable" platforms should not apply to objects that don't have their feet above the platform. In other words: For all objects below the platform, that platform doesn't exist.

So, however you're actually doing your overall collision detection, modify the algorithm to include a special pass for that particular circumstance.
[/quote]
[quote name='BeerNutts' timestamp='1344455708' post='4967504']
A quick thought is, if you you get a new collision (ie, it wasn't colliding in the last frame), then check the vertical velocity. If it's positive (moving up), ignore the collision. If it's negative (falling), then you need to check where the collision occurs on your player.

If it collides at the feet, then you can make the collision stick, and keep the player on top of the platform. If you're player collidies mid-body (or head, etc.), then he's probably moving left to right and falling, and needs to fall through.

That is just a quick thought about how I might try it, but i don't have any experience directly with this.

Good Luck!

EDIT: beat by Goran, but his idea is better than mine, to ensure the feet are above the top of the platform in question.
[/quote]

Thank you both, solid ideas, I hope I will get it right!

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Now I must be either a terrible thinker or I just went full crazy but I cannot find out how to solve this one:

[CODE]
# If there is a collision
if (bottom == platformTop) and \
(left >= platformLeft) and \
(right <= platformRight):
self.rect.y = platformTop - self.rect.h
#if self.falling:
# self.vector.y = 0
self.falling = False
# ^^^ this doesn't work, you cannot jump from the platform
# this is how you fall down from it
elif (bottom == platformTop) and \
(left <= platformLeft) or \
(right >= platformRight):
self.falling = True
[/CODE]

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Here's how i'd do it (pseudo code):
[code]
// store off where the player is before moving him
PreviousLoc = Player.Location;

// move Player
Player.Location.x += Player.Velocity.x;
Player.Location.y += Player.Velocity.y;

// Check for Collisions with platforms
foreach (Platform in Platforms) {
// Ignore platform if the previous position is below the platform's top
if (PreviousLoc.y > Platform.Location.y) {
continue;
}
// Player was above platform, check if it's current location hit it now
// Intersect checks the location and height/width of each and sees if it collides
if (Intersects(Player, Platform)) {
// Collided, so it must have hit the top
Player.OnGround = true;
// Move player back to previous location, and clear Y velocity
Player.Location = PreviousLoc;
Player.Velocity.y = 0;
}
}
[/code] Edited by BeerNutts

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