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## Grid Based Collision System Problems

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### #1anthnintendofan  Members

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Posted 07 January 2014 - 12:38 PM

So I am writing a game in C++, and its a lot like  Zelda  . The way I am storing the maps information is through an array which contains which blocks are walkable and which are not. It rounds the players position to the nearest multiple of 32 which is the size of a tile/square.

    void walk(int i, int j,char* collide)
{
int tempx,tempy;
animate = true;
//DOWN LEFT RIGHT UP
tempx=x+hspeed*i;
tempy=y+vspeed*j
int boundboxes [8];
if(j < 0)
{
face = UP;
}
else if(j > 0)
{
face = DOWN;
}
else if (i < 0)
{
face = LEFT;
}
else if (i > 0)
{
face = RIGHT;
}
{
x = tempx;
y = tempy;
}

}



So the problem i am having is here I cannot walk through the gap between the tree and the mountains, and there are several other false detections like being able to walk through trees if you are technically not on the tree tile. Can I have some help with this please?

#### Attached Thumbnails

Edited by anthnintendofan, 07 January 2014 - 12:39 PM.

### #2phil_t  Members

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Posted 07 January 2014 - 01:29 PM

Why are you using floating point numbers if all positions are snapped to a grid? That seems just ripe for problems.

What are i and j? They look like variables representing x and y movement direction, in which case they should have more descriptive names that properly describe their purpose.

It should be fairly straightforward to put a breakpoint on this code and step through it to see where you're getting unexpected values. Have you tried that yet?

### #3Servant of the Lord  Members

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Posted 07 January 2014 - 01:41 PM

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Here's a thought: If the player is at (90,90), he's basically almost entirely covering tile (3,3), but dividing by 32 will give you (2,2) - the wrong tile.

I think you'd get better results if you divided from the center of the player's sprite, rather than the top-left corner.

So you'd go:

const int TileWidth = 32;
const int TileHeight = 32;

const int PlayerWidth = 32;
const int PlayerHeight = 32;

int tileX = ceil((posX + (PlayerWidth/2)) / TileWidth);
int tileY = ceil((posY + (PlayerHeight/2)) / TileHeight);


After you get that working, I'd even suggest tweaking it to be the horizontal center of the sprite, but 3/4 of the height of the sprite, so the "center" of the sprite is closer to the sprite's feet.

const int PlayerWidth = 32;
const int PlayerHeight = 32;

const int SpriteOriginX = (PlayerWidth/2);
const int SpriteOriginY = int(float(PlayerHeight) * 0.75f);

int tileX = ceil((posX + SpriteOriginX) / TileWidth);
int tileY = ceil((posY + SpriteOriginY) / TileHeight);

It's perfectly fine to abbreviate my username to 'Servant' or 'SotL' rather than copy+pasting it all the time.
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### #4anthnintendofan  Members

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Posted 07 January 2014 - 06:53 PM

Thank you both for your suggestions, when I go back to work on it I will take it into account. I will return to show you guys how goes.

### #5ferrous  Members

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Posted 08 January 2014 - 04:14 PM

Why are you using floating point numbers if all positions are snapped to a grid? That seems just ripe for problems.

What are i and j? They look like variables representing x and y movement direction, in which case they should have more descriptive names that properly describe their purpose.

It should be fairly straightforward to put a breakpoint on this code and step through it to see where you're getting unexpected values. Have you tried that yet?

I'm assuming it's because they want to show movement between tiles, though for the actual "Can I move to grid space X,Y" check, you're right, that should be integer, though I'm guessing they wrapped this conversion from the units floating point position to integer values into their check.

### #6anthnintendofan  Members

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Posted 18 January 2014 - 05:57 PM

So I have sort have fixed the problem, a big thank you to Servant of the Lord. So there is solid collision detection which is all fine and dandy, the only problem is with walking towards a tile from the left,right and on top what happens is Link overlaps the tree, which could be any non-walkable tile. How can I fix my code, that way I can have Link not overlap on the tile from any direction other than approaching a non-walkable tile from the bottom. The image attached will illustrate what I mean. And here is the code.

void walk(int i, int j,char* collide)
{
const int PlayerWidth = 32;
const int PlayerHeight = 32;
const int TileWidth = 32;
const int TileHeight = 32;
const int SpriteOriginX = (PlayerWidth/32);
const int SpriteOriginY = int(float(PlayerHeight) * 0.50f);
int xpos,ypos;
animate = true;
//DOWN LEFT RIGHT UP
xpos=x+hspeed*i;
ypos=y+vspeed*j;
int xtile = ceil((xpos + SpriteOriginX) / TileWidth);
int ytile = ceil((ypos+ SpriteOriginY) / TileHeight);

if(j < 0)
{
face = UP;
}
else if(j > 0)
{
face = DOWN;
}
else if (i < 0)
{
face = LEFT;
}
else if (i > 0)
{
face = RIGHT;
}
if(collide[int(xtile+(ytile)*20)]==0)
{
x = xpos;
y = ypos;
}

}


#### Attached Thumbnails

Edited by anthnintendofan, 18 January 2014 - 05:58 PM.

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Posted 19 January 2014 - 07:24 AM

Since Link's sprite is the same height as that bush, all four of those screenshots show that it's not working how you intend (it looks like he's still half overlapping with the bush).

Part of the problem, I think, is that you're representing your character by only one point (the center), and so, if you look at your screenshots, it's working correctly--the center point isn't going int the non-walkable tiles. I don't know how the original Zelda game handled this situation, but this is pretty easily handled by basic aabb collision-checks.

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### #8anthnintendofan  Members

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Posted 26 January 2014 - 03:19 PM

Well guys, I have resolved the issue. First off I would like to thank Servant of the Lord for helping me with collision and NoAdmiral for giving me advice on with aabb collision checks. However rather than using the edges of the boxes for collision I used the four corners of the box to check for collision, so here is what I have for the collision check,

void walk(int i, int j,char* collide)
{
bool collision = false;
const int TileWidth = 32;
const int TileHeight = 32;
int xpos,ypos; //Players actual x position and y position
int xtile,ytile;//Player's position with respect to tile
animate = true;
//DOWN LEFT RIGHT UP
xpos=x+hspeed*i;//moving x
ypos=y+vspeed*j;//moving y
xtile = ceil((xpos + 4) / TileWidth);//figures out which tile top left corner is in(x component)
ytile = ceil((ypos + 16) / TileHeight);//figures out which tile top left corner is in(ycomponent)
if(collide[int(xtile+(ytile)*20)]!=0)//checks if the tile is occupado
{
collision = true;
}
xtile = ceil((xpos + 28) / TileWidth);//figures out which tile top right corner is in(x part)
if(collide[int(xtile+(ytile)*20)]!=0)
{
collision = true;
}
ytile = ceil((ypos + 28) / TileHeight);//figures out which tile bottom right corner is in(y component)
if(collide[int(xtile+(ytile)*20)]!=0)
{
collision = true;
}
xtile = ceil((xpos+4) / TileWidth);//figures out which tile bottom left corner is in(x component)
if(collide[int(xtile+(ytile)*20)]!=0)
{
collision = true;
}
if(j < 0)
{
face = UP;
}
else if(j > 0)
{
face = DOWN;
}
else if (i < 0)
{
face = LEFT;
}
else if (i > 0)
{
face = RIGHT;
}
if(!collision)//if there is not a collision the player can move
{
x = xpos;
y = ypos;
}
}


Once again thank you guys for the help.

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