Estoy seguro de que esto se podría hacer mejor, pero aquí hay algo de código que busca en el perímetro de un cuadrado alrededor del píxel central, el examen de la primera central y moviéndose hacia las esquinas. Si no se encuentra un píxel, el perímetro (radio) se expande hasta que se alcanza el límite del radio o se encuentra un píxel. La primera implementación fue un ciclo haciendo una espiral simple alrededor del punto central, pero como se observó, no se encuentra el píxel más cercano absoluto. La creación de SomeBigObjCStruct dentro del ciclo fue muy lenta: eliminarlo del ciclo lo hizo lo suficientemente bueno y el enfoque en espiral es lo que se usó. Pero aquí está esta implementación de todos modos. Cuidado, se realizaron muy pocas pruebas.
Todo se hace con sumas y restas enteras.
- (SomeBigObjCStruct *)nearestWalkablePoint:(SomeBigObjCStruct)point {
typedef struct _testPoint { // using the IYMapPoint object here is very slow
int x;
int y;
} testPoint;
// see if the point supplied is walkable
testPoint centre;
centre.x = point.x;
centre.y = point.y;
NSMutableData *map = [self getWalkingMapDataForLevelId:point.levelId];
// check point for walkable (case radius = 0)
if(testThePoint(centre.x, centre.y, map) != 0) // bullseye
return point;
// radius is the distance from the location of point. A square is checked on each iteration, radius units from point.
// The point with y=0 or x=0 distance is checked first, i.e. the centre of the side of the square. A cursor variable
// is used to move along the side of the square looking for a walkable point. This proceeds until a walkable point
// is found or the side is exhausted. Sides are checked until radius is exhausted at which point the search fails.
int radius = 1;
BOOL leftWithinMap = YES, rightWithinMap = YES, upWithinMap = YES, downWithinMap = YES;
testPoint leftCentre, upCentre, rightCentre, downCentre;
testPoint leftUp, leftDown, rightUp, rightDown;
testPoint upLeft, upRight, downLeft, downRight;
leftCentre = rightCentre = upCentre = downCentre = centre;
int foundX = -1;
int foundY = -1;
while(radius < 1000) {
// radius increases. move centres outward
if(leftWithinMap == YES) {
leftCentre.x -= 1; // move left
if(leftCentre.x < 0) {
leftWithinMap = NO;
}
}
if(rightWithinMap == YES) {
rightCentre.x += 1; // move right
if(!(rightCentre.x < kIYMapWidth)) {
rightWithinMap = NO;
}
}
if(upWithinMap == YES) {
upCentre.y -= 1; // move up
if(upCentre.y < 0) {
upWithinMap = NO;
}
}
if(downWithinMap == YES) {
downCentre.y += 1; // move down
if(!(downCentre.y < kIYMapHeight)) {
downWithinMap = NO;
}
}
// set up cursor values for checking along the sides of the square
leftUp = leftDown = leftCentre;
leftUp.y -= 1;
leftDown.y += 1;
rightUp = rightDown = rightCentre;
rightUp.y -= 1;
rightDown.y += 1;
upRight = upLeft = upCentre;
upRight.x += 1;
upLeft.x -= 1;
downRight = downLeft = downCentre;
downRight.x += 1;
downLeft.x -= 1;
// check centres
if(testThePoint(leftCentre.x, leftCentre.y, map) != 0) {
foundX = leftCentre.x;
foundY = leftCentre.y;
break;
}
if(testThePoint(rightCentre.x, rightCentre.y, map) != 0) {
foundX = rightCentre.x;
foundY = rightCentre.y;
break;
}
if(testThePoint(upCentre.x, upCentre.y, map) != 0) {
foundX = upCentre.x;
foundY = upCentre.y;
break;
}
if(testThePoint(downCentre.x, downCentre.y, map) != 0) {
foundX = downCentre.x;
foundY = downCentre.y;
break;
}
int i;
for(i = 0; i < radius; i++) {
if(leftWithinMap == YES) {
// LEFT Side - stop short of top/bottom rows because up/down horizontal cursors check that line
// if cursor position is within map
if(i < radius - 1) {
if(leftUp.y > 0) {
// check it
if(testThePoint(leftUp.x, leftUp.y, map) != 0) {
foundX = leftUp.x;
foundY = leftUp.y;
break;
}
leftUp.y -= 1; // moving up
}
if(leftDown.y < kIYMapHeight) {
// check it
if(testThePoint(leftDown.x, leftDown.y, map) != 0) {
foundX = leftDown.x;
foundY = leftDown.y;
break;
}
leftDown.y += 1; // moving down
}
}
}
if(rightWithinMap == YES) {
// RIGHT Side
if(i < radius - 1) {
if(rightUp.y > 0) {
if(testThePoint(rightUp.x, rightUp.y, map) != 0) {
foundX = rightUp.x;
foundY = rightUp.y;
break;
}
rightUp.y -= 1; // moving up
}
if(rightDown.y < kIYMapHeight) {
if(testThePoint(rightDown.x, rightDown.y, map) != 0) {
foundX = rightDown.x;
foundY = rightDown.y;
break;
}
rightDown.y += 1; // moving down
}
}
}
if(upWithinMap == YES) {
// UP Side
if(upRight.x < kIYMapWidth) {
if(testThePoint(upRight.x, upRight.y, map) != 0) {
foundX = upRight.x;
foundY = upRight.y;
break;
}
upRight.x += 1; // moving right
}
if(upLeft.x > 0) {
if(testThePoint(upLeft.x, upLeft.y, map) != 0) {
foundX = upLeft.x;
foundY = upLeft.y;
break;
}
upLeft.y -= 1; // moving left
}
}
if(downWithinMap == YES) {
// DOWN Side
if(downRight.x < kIYMapWidth) {
if(testThePoint(downRight.x, downRight.y, map) != 0) {
foundX = downRight.x;
foundY = downRight.y;
break;
}
downRight.x += 1; // moving right
}
if(downLeft.x > 0) {
if(testThePoint(upLeft.x, upLeft.y, map) != 0) {
foundX = downLeft.x;
foundY = downLeft.y;
break;
}
downLeft.y -= 1; // moving left
}
}
}
if(foundX != -1 && foundY != -1) {
break;
}
radius++;
}
// build the return object
if(foundX != -1 && foundY != -1) {
SomeBigObjCStruct *foundPoint = [SomeBigObjCStruct mapPointWithX:foundX Y:foundY levelId:point.levelId];
foundPoint.z = [self zWithLevelId:point.levelId];
return foundPoint;
}
return nil;
}
Esta es en realidad la mejor respuesta hasta el momento. Gracias. – shoosh