Collision-Detection methods in C++ - c++

Question

I am new to c++ and I have been practicing collision in a small game program that does nothing and I just can't get the collision right
So I use images loaded into variables
background = oslLoadImageFile("background.png", OSL_IN_RAM, OSL_PF_5551);
sprite = oslLoadImageFile("sprite.png", OSL_IN_RAM, OSL_PF_5551);
bush = oslLoadImageFile("bush.png", OSL_IN_RAM, OSL_PF_5551);
While there are variables stored like
sprite->x = 3;
if ( (sprite->x + spritewidth > bush->x) && (sprite->x < bush->x + bushwidth) && (sprite->y + spriteheight > bush->y) && (sprite->y < bush->y + bushheight) )
{
bushcol = 1;
}
else
{
bushcol = 0;
}
So when i press a button
if (osl_keys->held.down)
{
if (bushcol == 1)
{
sprite->y = bush->y + 38;
}
else
{
sprite->y += 3;
}
}
if (osl_keys->held.up)
{
if (bushcol == 1)
{
sprite->y = bush->y - 23;
}
else
{
sprite->y -= 3;
}
}
if (osl_keys->held.right)
{
if (bushcol == 1)
{
sprite->x = bush->x - 28;
}
else
{
sprite->x += 3;
}
}
if (osl_keys->held.left)
{
if (bushcol == 1)
{
sprite->x = bush->x + 28;
}
else
{
sprite->x -= 3;
}
}
i was thinking of things like
sprite->y = bushheight - 24;
but it doesnt work
Any suggestions?

Answer 1

I'd suggest making a function solely for the purpose of bounding box colision detection.
It could look like
IsColiding(oslImage item1, oslImage item2)
{
/* Perform check */
}
in which you perform the check if there is a collision between image 1 and image 2.
As for the algorithm you're trying to use check out this wikipedia for example AABB bounding box
Especially this part:
In the case of an AABB, this tests
becomes a simple set of overlap tests
in terms of the unit axes. For an AABB
defined by M,N against one defined by
O,P they do not intersect if (Mx>Px)
or (Ox>Nx) or (My>Py) or (Oy>Ny) or
(Mz>Pz) or (Oz>Nz).

Answer 2

I think you have the basic idea. Just check your work. Here is a simple version which compiles:
#import <stdlib.h>
typedef struct {
// I'm going to say x, y, is in the center
int x;
int y;
int width;
int height;
} Rect;
Rect newRect(int x, int y, int w, int h) {
Rect r = {x, y, w, h};
return r;
}
int rectsCollide(Rect *r1, Rect *r2) {
if (r1->x + r1->width/2 < r2->x - r2->width/2) return 0;
if (r1->x - r1->width/2 > r2->x + r2->width/2) return 0;
if (r1->y + r1->height/2 < r2->y - r2->height/2) return 0;
if (r1->y - r1->height/2 > r2->y + r2->height/2) return 0;
return 1;
}
int main() {
Rect r1 = newRect(100,200,40,40);
Rect r2 = newRect(110,210,40,40);
Rect r3 = newRect(150,250,40,40);
if (rectsCollide(&r1, &r2))
printf("r1 collides with r2\n");
else
printf("r1 doesnt collide with r2\n");
if (rectsCollide(&r1, &r3))
printf("r1 collides with r3\n");
else
printf("r1 doesnt collide with r3\n");
}

Answer 3

First of all i suppose you mean
sprite->y = bush->**y** - 3;
second i dont know what platform you are using, but often times the y-coordinates are inverted. i.e., y=0 corresponds to top of the screen. In that case your comparisons might not work.
third collision checking can quickly become complicated when you add rotation and non-rectangular objects to it. You should consider using CGAL or some other computational geometry algorithms library, which can handle polygon intersection.