C++ 2D polygon Collision Detection - c++

Question

I'm trying to implement 2D collision detection with two rectangles constructed using a graphics package.
Unfortunately, I'm beginning to think I don't understand the logic needed to write a function that will handle this.
Below is my code that draws a small sprite and a couple of other rectangles. My sprite moves with keyboard inputs.
I've used several books and also tried sites like Nehe etc and although they're really good tutorials, they only seems to deal directly with 3D collision.
Can someone please show me an efficient way of implementing collision detection using my rectangles above? I know you need to compare the coordinates of each object. I'm just unsure how to track the position of the objects, checking collision and stopping it moving should it collide.
I am self learning and seem to have come to a stop for days now. I'm totally out of ideas and searched more google pages than I care to remember. I'm sorry for my naivety.
I'd appreciate any constructive comments and example code. Thank you.
void drawSprite (RECT rect){
glBegin(GL_QUADS);
glColor3f(0.2f, 0.2f, 0.2f);
glVertex3f(rect.x, rect.y, 0.0);
glColor3f(1.0f, 1.0f, 1.0f);
glVertex3f(rect.x, rect.y+rect.h, 0.0);
glColor3f(0.2f, 0.2f, 0.2f);
glVertex3f(rect.x+rect.w, rect.y+rect.h, 0.0);
glColor3f(1.0f, 1.0f, 1.0f);
glVertex3f(rect.x+rect.w, rect.y, 0.0);
glEnd();
}
void drawPlatform (RECT rect){
glBegin(GL_QUADS);
glColor3f(0.2f,0.2f,0.0f);
glVertex3f(rect.x, rect.y, 0.0);
glColor3f(1.0f,1.0f,0.0f);
glVertex3f(rect.x, rect.y+rect.h, 0.0);
glColor3f(0.2f, 0.2f, 0.0f);
glVertex3f(rect.x+rect.w, rect.y+rect.h, 0.0);
glColor3f(1.0f, 1.0f, 0.0f);
glVertex3f(rect.x+rect.w, rect.y, 0.0);
glEnd();
}

Answer 1

You can use this collide function with an AABB struct (AABB stands for Aligned Axis Bounding Box) before drawing.
AABB.c
AABB* box_new(float x, float y, float w, float h, int solid)
{
AABB* box = 0;
box = (AABB*)malloc(sizeof(AABB*));
box->x = (x) ? x : 0.0f;
box->y = (y) ? y : 0.0f;
box->width = (w) ? w : 1.0f;
box->height = (h) ? h : 1.0f;
return(box);
}
void box_free(AABB *box)
{
if(box) { free(box); }
}
int collide(AABB *box, AABB *target)
{
if
(
box->x > target->x + target->width &&
box->x + box->width < target->x &&
box->y > target->y + target->height &&
box->y + box->height < target->y
)
{
return(0);
}
return(1);
}
AABB.h
#include <stdio.h>
#include <stdlib.h>
typedef struct AABB AABB;
struct AABB
{
float x;
float y;
float width;
float height;
int solid;
};
AABB* box_new(float x, float y, float w, float h, int solid);
void box_free(AABB *box);
int collide(AABB *box, AABB *target);
I hope it will help ! :)

Answer 2

You won't get that far by detecting a collision since you will have issues of floating-point precision. What you can do is detect overlaps between the rects, and if such an overlap occurs the collision has already happened so you can bump the rects out of each other.
Also, you need to split the engine in two states:
Rects get moved by input
Overlaps are detected and if found the rects get moved out of each other
Display the scene
As to detecting whether two rects overlap see this question:
Determine if two rectangles overlap each other?