Question

I have a grid of images, like a 2D tile set. This method is supposed to give me the image from the tile set when I just give the the tile index.

Can anyone see where my math is screwed up? Currently this function just gives me n and n.(random decimal) for the x and y.

- (CGImageRef)tileFromTileset:(int)gid {
    CGImageRef tile;
    CGPoint point;
    CGFloat fGid = (CGFloat)gid;
    point.x = fmodf(fGid, tileSource.size.width);
    point.y = (fGid - (fmod(fGid, tileSource.size.width)) / (tileSource.size.width) + 1);
    NSLog(@"%@", NSStringFromCGPoint(point));
    tile = CGImageCreateWithImageInRect([tileSource CGImage], CGRectMake(point.x, point.y, kTileWidth, kTileHeight));
    return tile;
}

Answer 1

I think you have a simple error of units. At the moment, you’re dividing a number representing an index by a number representing a coordinate; the unit that you end up with is therefore “index per coordinate”.* What you want, though, is just coordinate units.

The assignment to point.x should therefore be:

point.x = fmodf(fGid * kTileWidth, tileSource.size.width);

That is, the tile index (units: “index”), multiplied by the width of one tile (units: “coordinates per index”), gives you its location if this were a single row of tiles (units: “coordinates”). Taking the remainder after dividing by the actual width of a row of tiles gives you the correct x coordinate.

For point.y, you need the quotient of the same calculation:

point.y = trunc(fGid * kTileWidth / tileSouce.size.width) * kTileHeight;

The index, multiplied by the width of a tile, divided by the width of a row, tells you how many rows are completed at that index; multiply by the height of a tile to get the coordinate.

This might be even clearer with one more constant, kNumTilesInRow:

point.x = (gid % kNumTilesInRow) * kTileWidth;
point.y = (gid / kNumTilesInRow) * kTileHeight;

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*You keep getting n back because, for abs(n) < abs(m), n % m always equals n.