Intersection of CGRect and CGPath - ios

Question

Is there an efficient way to detect if a CGPath and a CGRect intersect?
I've thought about looping through every point inside the CGRect sort of like this:
for (CGPoint point in rect) {
if (CGPathContainsPoint(path, nil, point, NO)) {
intersects = YES;
break;
}
But I wanted to know if there was a better way.
In my app there are many CGrects and one CGPath that may change its shape. I just need to check which rects are intersecting with the path, as shown in the image bellow.

Answer 1

Draw the path in a bitmap (white on alpha)
then check the Rectangle part of bitmap. check if there is any white in that area which would mean overlapping
-- to make it more performant draw only the part of the bitmap that is in the rectangle.
I havent tried this and it wont offer real-time performance but it sounds ok to me

Answer 2

The only completely accurate way that I've found of doing this is to loop through every point, something like this (I haven't tested this exact code):
for (NSValue *rectValue in rects) {
CGRect rect = rectValue.CGRectValue;
// Set the initial point values to the top left of the current rect
CGFloat x = CGRectGetMinX(rect);
CGFloat y = CGRectGetMinY(rect);
BOOL intersects = NO;
BOOL morePoints = YES;
// Loop until there are no more points to check in this rect
while (morePoints) {
CGPoint point = CGPointMake(x, y);
if (CGPathContainsPoint(path, nil, point, NO)) {
intersects = YES;
break;
}
// Adjust the x and y values to check all points
if (x < CGRectGetMaxX(rect)) {
x++;
} else if (y < CGRectGetMaxY(rect)) {
x = CGRectGetMinX(rect);
y++;
} else {
morePoints = NO;
}
}
if (intersects) {
// The path intersects the current rect
} else {
// The path does not intersect the current rect
}
}
But this is the most inefficient way.
You could optimize this solution by looping asynchronously:
[rects enumerateObjectsWithOptions:NSEnumerationConcurrent usingBlock:^(NSValue * _Nonnull rectValue, NSUInteger idx, BOOL * _Nonnull stop) {
CGRect rect = rectValue.CGRectValue;
...
}];
I only needed to know if the path intersected the edge of a rect so I did something similar to this:
for (NSValue *rectValue in rects) {
CGRect rect = rectValue.CGRectValue;
// Set the initial point values to the top left of the current rect
CGFloat x = CGRectGetMinX(rect);
CGFloat y = CGRectGetMinY(rect);
BOOL intersects = NO;
// top edge
for (; x < CGRectGetMaxX(rect); x++) {
CGPoint point = CGPointMake(x, y);
if (CGPathContainsPoint(path, nil, point, NO)) {
intersects = YES;
break;
}
}
if (intersects) {
// The path intersects the current rect on the top edge
}
// right edge
for (; y < CGRectGetMaxY(rect); y++) {
CGPoint point = CGPointMake(x, y);
if (CGPathContainsPoint(path, nil, point, NO)) {
intersects = YES;
break;
}
}
if (intersects) {
// The path intersects the current rect on the right edge
}
// bottom edge
x = CGRectGetMinX(rect);
for (; x < CGRectGetMaxX(rect); x++) {
CGPoint point = CGPointMake(x, y);
if (CGPathContainsPoint(path, nil, point, NO)) {
intersects = YES;
break;
}
}
if (intersects) {
// The path intersects the current rect on the bottom edge
}
// left edge
x = CGRectGetMinX(rect);
y = CGRectGetMinY(rect);
for (; y < CGRectGetMaxY(rect); y++) {
CGPoint point = CGPointMake(x, y);
if (CGPathContainsPoint(path, nil, point, NO)) {
intersects = YES;
break;
}
}
if (intersects) {
// The path intersects the current rect on the left edge
}
}