I am working on an app that uses MKOverlay views to layer my own custom maps on top of the Google base map. I have been using Apple's excellent TileMap sample code (from WWDC 2010) as a guide.
My problem - when "overzoomed" to a level of detail deeper than my generated tile set, the code displays nothing because there are no tiles available at the calculated Z level.
The behavior I want - when "overzoomed" the app should just keep magnifying the deepest level of tiles. It is a good user experience for the overlay to become blurrier - it is a very bad experience to have the overlay vanish.
Here is the code which returns the tiles to draw - I need to figure out how to modify this to cap the Z-depth without breaking the scaling of the frame being calculated for the overlay tile. Any thoughts???
- (NSArray *)tilesInMapRect:(MKMapRect)rect zoomScale:(MKZoomScale)scale
{
NSInteger z = zoomScaleToZoomLevel(scale);
// PROBLEM: I need to find a way to cap z at my maximum tile directory depth.
// Number of tiles wide or high (but not wide * high)
NSInteger tilesAtZ = pow(2, z);
NSInteger minX = floor((MKMapRectGetMinX(rect) * scale) / TILE_SIZE);
NSInteger maxX = floor((MKMapRectGetMaxX(rect) * scale) / TILE_SIZE);
NSInteger minY = floor((MKMapRectGetMinY(rect) * scale) / TILE_SIZE);
NSInteger maxY = floor((MKMapRectGetMaxY(rect) * scale) / TILE_SIZE);
NSMutableArray *tiles = nil;
for (NSInteger x = minX; x <= maxX; x++) {
for (NSInteger y = minY; y <= maxY; y++) {
// As in initWithTilePath, need to flip y index
// to match the gdal2tiles.py convention.
NSInteger flippedY = abs(y + 1 - tilesAtZ);
NSString *tileKey = [[NSString alloc]
initWithFormat:@"%d/%d/%d", z, x, flippedY];
if ([tilePaths containsObject:tileKey]) {
if (!tiles) {
tiles = [NSMutableArray array];
}
MKMapRect frame = MKMapRectMake((double)(x * TILE_SIZE) / scale,
(double)(y * TILE_SIZE) / scale,
TILE_SIZE / scale,
TILE_SIZE / scale);
NSString *path = [[NSString alloc] initWithFormat:@"%@/%@.png",
tileBase, tileKey];
ImageTile *tile = [[ImageTile alloc] initWithFrame:frame path:path];
[path release];
[tiles addObject:tile];
[tile release];
}
[tileKey release];
}
}
return tiles;
}
FYI, here is the zoomScaleToZoomLevel helper function that someone asked about:
// Convert an MKZoomScale to a zoom level where level 0 contains 4 256px square tiles,
// which is the convention used by gdal2tiles.py.
static NSInteger zoomScaleToZoomLevel(MKZoomScale scale) {
double numTilesAt1_0 = MKMapSizeWorld.width / TILE_SIZE;
NSInteger zoomLevelAt1_0 = log2(numTilesAt1_0); // add 1 because the convention skips a virtual level with 1 tile.
NSInteger zoomLevel = MAX(0, zoomLevelAt1_0 + floor(log2f(scale) + 0.5));
return zoomLevel;
}
