# C++ Platformer Collision

This is very simple, but I have spent nearly 6 months messing with this and it is always messed up.

This is a tile platformer, everything is "normal", x and y correspond to tile. Tiles are 32 x 32 pixels.

// Get all tile numbers, top left, top right, bottom left, bottom right
int X = 63; // and in this, say the player is moving right
int Y = 50;
int Width = 33; // This is the problem, if it is < 32, the collisions are all over the place, in the open, where there shouldn't be.
int Height = 50;

int tile_x = floor( (double) X/32);
int tile_y = floor( (double) Y/32);
int tile_x2 = floor( (double) (X + Width)/ 32);
int tile_y2 = floor( (double) (Y + Height)/ 32);
bool open = true;

// This is meant to catch cases where width < 32
if (tile_x == tile_x2) {
if (IsOpenPt(tile_x, tile_y)) { return true; }
}

// loop through tiles
for (int y = tile_y; y < tile_y2; y++) {

for (int x = tile_x; x < tile_x2; x++) {

// this spot is not open
if (!IsOpenPt(x,y)) { open = false; }

}
}

return open;


This works if the width is greater than 32, but if it anything less, everything becomes random open point/ not.

See anything wrong with my formula? I have exhausted searches, nothing. Once again I emphasize, the entire project, I have been trying to get this part to work. I have literally a 100 page notebook of attempts at visualizing and grids to put it on paper.

• What space are X, Y, Width and Height in? Pixels? Is 32 your tile width in pixels? – Tyyppi_77 Apr 11 '16 at 10:17
• Tiles are 32 by 32. X and Y are the top left point of the player. The player stands on the ground (Y + Height), at X (with a width of Width.) So, with the code above, it works perfect if the width is greater than 32. If the width is less than, the entire thing breaks. By that I mean, I can not figure out what it is doing, I get very mixed results, seemingly almost random. – Evan Carslake Apr 11 '16 at 16:39
• The question as phrased is for me hard to understand. If you could structure it more like: 1. Here is a collision test method. 2. When I give it this input, I would expect the following output, "<output>". 3. But what I got instead was "<wrong output>". The code here looks like the middle of a function, but not the whole thing... – david van brink Apr 11 '16 at 17:04
• Won't your tile_x == tile_x2 check break anyways if height > 32? But yes, please add more information as mentioned above. – Tyyppi_77 Apr 11 '16 at 18:23

I think you need some documentation about collisions. It seems you want to check the collision between 2 hitbox on a 2D game:

if (rect1.x < rect2.x + rect2.width &&
rect1.x + rect1.width > rect2.x &&
rect1.y < rect2.y + rect2.height &&
rect1.height + rect1.y > rect2.y) {
// collision detected!
}


But you're trying to make a pixel perfect collision, which is needed only when you have check a collision with this code.

This article seems to make a good summarize of the topic.

Your issue seems to be that you are only checking a single tile if your player is 32 or less pixels wide. That obviously does not work, because your player is taller than one tile. Hence, you need to remove the first easy-oit as it breaks the detection, and simply just rely on the looping method.

It's not clear what you are trying to achieve, but changing the < operators to <= might be a step in the right direction.