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Collision Detection behaving strange (jnrdev#1)

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After browsing these forums for about a month, I decided to create an account, because I'm hopelessly stuck, and you guys know your stuff.

I found a link a week ago (or so) to a web archive containing a "Jump n Shoot" tutorial written by jnrdev. I've been trying to build my own library based on his design on class interaction... Some parts are directly stolen, primarily because they work good, and I don't hold the skills to improve it much.

The main problems I am experiencing is that it detecs a collision when starting in the middle of the screen. Only the outer edges are covered in solid tiles. If I start to move using the arrow keys, the player suddenly drops through the map, until the y value exceeds what an int can hold. I've attached a screenshot showing the console and the main game window.

The Pmap class:
Contains an array of type TILE which contains a pointer to an SDL_Surface, and a boolean containing whether it's solid or not.
class Pmap{


//The only one worth noticing in Pmap
bool colmapxy( int x, int y )
return Mtile[x][y].solid;



Player class, main function:
Interacts with the Pmap class using the "collision_ver" and "collision_hor" functions, which can be seen below.

void Player::think(SDL_Event &event, Pmap& map, SDL_Surface &screen)

int tilecoord;

//X-axis first

//Moving right
if( xVel > 0)
printf("Moving right...xVel:%d, yVel:%d, x:%d, y:%d\n", xVel, yVel, x, y);
if(collision_ver( x+xVel+w, y, tilecoord, map) )
x = tilecoord*40 -w-1;
// else x += xVel;

//Moving left
if( xVel < 0)
printf("Moving left...xVel:%d, yVel:%d, x:%d, y:%d\n", xVel, yVel, x, y);
if( collision_ver( x+xVel, y, tilecoord, map))
x = (tilecoord+1) *40+1;
// else x += xVel;

//Caps the xVel at SPEED
if( xVel > SPEED ) xVel = SPEED;

//Then Y-axis
//Moving up
if( yVel < 0 )
printf("Moving up...xVel:%d, yVel:%d, x:%d, y:%d\n", xVel, yVel, x, y);

//Moving up & block in the way
if( collision_hor( x, y+yVel, tilecoord, map))
y = (tilecoord+1) *40+1;
yVel = 0;

//Moving up through blank spots
y += yVel;


//yVel > 0, hence falling or on the ground
printf("Moving down / on ground...xVel:%d, yVel:%d, x:%d, y:%d\n", xVel, yVel, x, y);

//Moving down, block in the way
if( collision_hor( x, y+yVel+h, tilecoord, map))
y = tilecoord*40-h -1; //On ground
yVel = 1;
//Falling / in air
printf("Falling...xVel:%d, yVel:%d, x:%d, y:%d\n", xVel, yVel, x, y);
y += yVel;

if( yVel >= TILESIZE )

jumpLock = true;

Collision checker function:
Note: there are two of these (_hor and _ver), but they are so similar I choose to only copy in one of them.
Also, this one is identical to jnrdev's function, except for variable names.

bool Player::collision_hor(int x, int y, int &tilecoordy, Pmap& map){
int tilexpixels = x-(x%40); //calculate the x position (pixels!) of the tiles we check against
int testend = x + w; //calculate the end of testing (just to save the x+w calculation each for loop)

tilecoordy = y/40; //calculate the y position (map coordinates!) of the tiles we want to test

int tilecoordx = tilexpixels/40; //calculate map x coordinate for first tile

//loop while the start point (pixels!) of the test tile is inside the players bounding box
while(tilexpixels <= testend){
if(map.colmapxy(tilecoordx, tilecoordy)) //is a solid tile is found at tilecoordx, tilecoordy?
printf("Collision(hor) at: X:%d, Y:%d\n", x, y);
return true;

tilecoordx++; //increase tile x map coordinate
tilexpixels+=40; //increase tile x pixel coordinate

cout<<"No hor coll\n";
return false;

For some reason, collision_ver() kept returning a wrong value, which I've corrected now, and it appears to be working. If possible, please delete this thread.
The lesson I've learned is that printf / cout will save you tons of time when debugging.

Edited by Admiral Pimms

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