Masking a CALayer with another CALayer

Question

I\'m trying to make a donut shape with CALayers. One CALayer will be a large circle, the other one will be a smaller circle positioned in its center, masking it.

The la

Answer 1

It is the alpha value of the masking layers content that is used as a mask. (If you would add the mask as a sublayer instead of using it as a mask. Everything that is covered by the sublayer would be visible when used as a mask. Everything that is not covered by the sublayer would be hidden when used as a mask.)

Since your small circle is fully transparent , everything is masked away (is hidden). If you set the backgroundColor of it to any, fully opaque color (only the alpha value is used for the mask) then it will let those pixels through.

Note that this is the reverse of what you want. This will leave you with only "the hole of the donut" visible. There is no built in way to do a reverse mask Instead you would have to draw the content of the mask some other way like using a CAShapeLayer or using drawInContext:.