Question

If I set shouldRasterize = YES on a CALayer, do I have to set it on each of the sublayers as well if I wanted the whole hierarchy to be flattened for better animation performance?

I’m asking because when I set shouldRasterize = YES on my root layer and enable “Color Blended Layers” in Instruments, all the sublayers are still there and marked as blended. It’s not flattening anything.

Answer 1

Setting shouldRasterize does not do quite what you are thinking it does. In order to composite the look of the parent view, rasterized or not, it has to check subviews to see if they are opaque or transparent. When child objects are opaque, they do not need to be blended. When they are transparent, the view needs to be blended with whatever is behind them (or higher in the hierarchy).

So, shouldRasterize will not affect the green/red you see using Instruments. In order to have everything green, you’ll need to not use transparency and have all your child objects be opaque. Sometimes its unavoidable to still have red areas depending on your design. The instrument is just there to help you optimize ones that could be opaque and reduce the amount of blending the GPU has to do.

Edit:
To explain further, suppose you have a UILabel and its sitting on top of a photo. You only want to see the text and not its background color, so you set its backgroundColor to clear, and the opaque property to NO. In instruments, this will now appear red. The GPU has to blend this transparency over the image behind it, performing two draw operations instead of one.

If we had set opaque to YES and gave it a solid background color, the view would now show up green in instruments because it didn’t have to blend that view with any other view.

So, whether the layer is rasterized or not, it still has to composite its child views so shouldRasterize really has no effect either way on what you see in Instruments.