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copied!<p>If you're using a transparent sheet, you don't know in advance what the pixels below it will be. They may change. Remember that you have a single alpha channel for all three colors: if you make it transparent, you won't see any subpixel effect, but if you make it opaque, all three subelements are going to get composited with the background. If you give an edge the right color for compositing over a white background, it won't look right if the background changes to some other color.</p> <p>For example, let's say you're drawing black text on a white background, and the subelement order is RGB. A right edge may be a faint blue: high B value (full brightness on the side away from the glyph), slightly lower but still high R and G values (lower brightness on the side closer to the glyph). If you now composite that pixel over a dark gray background, you're going to make it lighter than it would have been if you had rendered black text on dark gray background directly.</p> <p>Basically, you are not facing an arbitrary limitation of CoreAnimation: it simply makes no sense to use subpixel rendering on a transparent layer that might be composited over an arbitrary background. You'd need a separate alpha per color channel, but since the pixel format of your buffer is RGBA (or ARGB or whatever it is), you can't have it.</p> <p>But this leads us to the solution. If you know that the background will remain the same (eg, the sheet displays over a window whose contents you control), then you can simply make your layer opaque, fill it with a copy of the covered region of the background window, and render subpixel-antialiased text on it. Basically, you'd be precompositing your layer with the background. If the background stays the same, this will look identical to what normal alpha compositing would do, except that you can now do subpixel text rendering; if the background changes, then you'd have to give up on doing subpixel text rendering anyway (although I guess you could keep copying the background and redrawing your opaque overlay whenever it changes).</p>

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