NSImage Overlay/Mask Gradient like iOS
I'm looking to replicate with Cocoa/Core Graphics the process which seems to occur on iOS when setting an image for a UITabBarItem. When the tab bar item is selected, the image is overlayed with a gradient.
For example,
becomes...
I'm unsure exactly what I should be doing to achieve this effect. It seems like the image is masking the gradient. Any pointers in the right direction (or code!) would be much appreciated.
You can use a monochrome CGImage with alpha channel (like most iPhone tool-/tabbar icons) as a mask. Basically, you'd use CGContextClipToMask with your monochrome image. Then you'd draw the gradient which is then clipped to the mask image. You might also want to have a look at the code of UMEKit, which implements this effect on Mac OS X (haven't looked at how exactly they do it, there are probably several ways).
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