ios - 当 "over-zoomed"超出叠加图 block 集时，计算要在 MapRect 中显示的图 block

Question

我正在开发一个应用程序，该应用程序使用 MKOverlay View 将我自己的自定义 map 分层放置在 Google 基本 map 之上。我一直在使用 Apple 出色的 TileMap 示例代码(来自 WWDC 2010)作为指南。

我的问题 - 当“过度缩放”到比我生成的图 block 集更深的细节级别时，代码不显示任何内容，因为在计算的 Z 级别没有可用的图 block 。

我想要的行为 - 当“过度缩放”时，应用程序应该继续放大最深层次的图 block 。叠加层变得模糊是一种很好的用户体验 - 叠加层消失是一种非常糟糕的体验。

这是返回要绘制的图 block 的代码 - 我需要弄清楚如何修改它以在不破坏为覆盖图 block 计算的框架缩放的情况下限制 Z 深度。有什么想法吗？？？

---

- (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;
}

仅供引用，这是有人询问的 zoomScaleToZoomLevel 辅助函数:

// 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;
}

Answer 1

假设叠加层是云覆盖 - 或者在我们的例子中是蜂窝信号覆盖。放大时它可能“看起来不太好”，但叠加层仍在向用户传达基本信息。

我通过添加 OverZoom 模式来增强 Apple 的 TileMap 示例代码来解决这个问题。

这是 TileOverlay.m 中的新 tilesInMapRect 函数:

- (NSArray *)tilesInMapRect:(MKMapRect)rect zoomScale:(MKZoomScale)scale
{
    NSInteger z = zoomScaleToZoomLevel(scale);

    // OverZoom Mode - Detect when we are zoomed beyond the tile set.
    NSInteger overZoom = 1;
    NSInteger zoomCap = MAX_ZOOM;  // A constant set to the max tile set depth.

    if (z > zoomCap) {
        // overZoom progression: 1, 2, 4, 8, etc...
        overZoom = pow(2, (z - zoomCap));
        z = zoomCap;
    }

    // When we are zoomed in beyond the tile set, use the tiles
    // from the maximum z-depth, but render them larger.
    NSInteger adjustedTileSize = overZoom * TILE_SIZE;

    // Number of tiles wide or high (but not wide * high)
    NSInteger tilesAtZ = pow(2, z);

    NSInteger minX = floor((MKMapRectGetMinX(rect) * scale) / adjustedTileSize);
    NSInteger maxX = floor((MKMapRectGetMaxX(rect) * scale) / adjustedTileSize);
    NSInteger minY = floor((MKMapRectGetMinY(rect) * scale) / adjustedTileSize);
    NSInteger maxY = floor((MKMapRectGetMaxY(rect) * scale) / adjustedTileSize);
    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 * adjustedTileSize) / scale,
                                                (double)(y * adjustedTileSize) / scale,
                                                adjustedTileSize / scale,
                                                adjustedTileSize / 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;
}

这是 TileOverlayView.m 中的新 drawMapRect:

- (void)drawMapRect:(MKMapRect)mapRect
          zoomScale:(MKZoomScale)zoomScale
          inContext:(CGContextRef)context
{
    // OverZoom Mode - Detect when we are zoomed beyond the tile set.
    NSInteger z = zoomScaleToZoomLevel(zoomScale);
    NSInteger overZoom = 1;
    NSInteger zoomCap = MAX_ZOOM;

    if (z > zoomCap) {
        // overZoom progression: 1, 2, 4, 8, etc...
        overZoom = pow(2, (z - zoomCap));
    }

    TileOverlay *tileOverlay = (TileOverlay *)self.overlay;

    // Get the list of tile images from the model object for this mapRect.  The
    // list may be 1 or more images (but not 0 because canDrawMapRect would have
    // returned NO in that case).

    NSArray *tilesInRect = [tileOverlay tilesInMapRect:mapRect zoomScale:zoomScale];
    CGContextSetAlpha(context, tileAlpha);

    for (ImageTile *tile in tilesInRect) {
        // For each image tile, draw it in its corresponding MKMapRect frame
        CGRect rect = [self rectForMapRect:tile.frame];
        UIImage *image = [[UIImage alloc] initWithContentsOfFile:tile.imagePath];
        CGContextSaveGState(context);
        CGContextTranslateCTM(context, CGRectGetMinX(rect), CGRectGetMinY(rect));

        // OverZoom mode - 1 when using tiles as is, 2, 4, 8 etc when overzoomed.
        CGContextScaleCTM(context, overZoom/zoomScale, overZoom/zoomScale);
        CGContextTranslateCTM(context, 0, image.size.height);
        CGContextScaleCTM(context, 1, -1);
        CGContextDrawImage(context, CGRectMake(0, 0, image.size.width, image.size.height), [image CGImage]);
        CGContextRestoreGState(context);

        // Added release here because "Analyze" was reporting a potential leak. Bug in Apple's sample code?
        [image release];
    }
}

现在似乎工作得很好。

顺便说一句 - 我认为 TileMap 示例代码缺少 [image release] 并且正在泄漏内存。请注意我在上面的代码中添加它的位置。

希望对遇到同样问题的其他人有所帮助。

干杯，

• 克里斯