CATiledLayer gets a incorrect frame & contextCTM.a value (zoomScale)

Question

following WWDC2010 Session 104 I just created a CATiledLayer within a UIScrollView for a 10000 x 8078 px image.

I am troubled with the frame & zoomScale when the view is first shows at its top level

1. like in session 104 I defined levelsOfDetail as 4
2. in my drawRect I call CGFloat lScale = CGContextGetCTM(lContext).a;

Strangely at runtime lScale is assigned 0.124907158, not 0.125 as expected.

As well the rect passed to drawRect has floatpoint values, such as

• rect.origin.x = 4099.04443
• rect.origin.y = 6144
• rect.size.width = 2049.52222
• rect.size.height = 2048

Most irritating for me is that the frame of the CATiledLayer shows an origin 0 x 26.93 even though I created the tiledImageView using initWithFrame using a 0x0 origin.

Any ideas where I can find the calculation that is responsible for this?

---

EDIT: I found the source for the wierd frame position, which is the ScrollView bounds being incorrectly set at 450 px. that should be 420 px. The layoutSubview routine in the UIScrollView has a center view routine that inadvertently adjusted the y coordinate.

Answer 1

I found a cure to this.

The WWDC 2010 session 104 sample has two UIScrollView methods that control the view behaviour.

The first is configureForImageSize that I mentioned in my previously posted answer where the zoomscales are set.

Due to the explained float-point difference the imageView-bounds get generated with the decimals as highlighted before (see wx/hx or wy/hy).

To get clean this up I used the override of layoutSubviews where I calculate the view frame:

- (void) layoutSubviews {
    [super layoutSubviews];

    CGSize lBoundsSize = self.bounds.size;
    CGRect lFrameToCenter = imageView.frame;

    // cure to the decimals problem
    lFrameToCenter.size.width  = roundf(lFrameToCenter.size.width);
    lFrameToCenter.size.height = roundf(lFrameToCenter.size.height);

    if (lFrameToCenter.size.width < lBoundsSize.width)
        lFrameToCenter.origin.x = (lBoundsSize.width - lFrameToCenter.size.width) / 2;
    else
        lFrameToCenter.origin.x = 0;

    if (lFrameToCenter.size.height < lBoundsSize.height)
        lFrameToCenter.origin.y = (lBoundsSize.height - lFrameToCenter.size.height) / 2;
    else
        lFrameToCenter.origin.y = 0;

    imageView.frame = lFrameToCenter;

    if ([imageView isKindOfClass:[tileView class]]) {
        imageView.contentScaleFactor = 1.0;
    }
}

Note that I round the frame size before doing anything else with it!