Paul, thanks for such an exhaustive answer! Here are answers to your questions: 1. My case is the simple one -- I'm rendering all text layers at once. 2. Yes and no. As I said, I am implementing the cross-fade effect, when one text fades out and another text fades in. The texts can be of the same color (simplest case) or of different color (different shades of gray), and their alphas change during the fading animation. The problem is that the semi-transparent areas of text, when overlapping, produce brighter pixels, the annoying "flash" effect, I'm trying to get rid of.
My further research shows that kCGBlendModeLighten does not seem to apply to the alpha channel. Namely, colors are indeed composited using the lighten rule, but then the resulting color is alpha blended with the background as usual, so that two overlapping semitransparent pixels still produce a more opaque pixel, which is undesirable Am I missing the clue how to make kCGBlendModeLighten apply to the alpha channel as well? I have come to the following workaround: I fill the text layers with opaque black background. In this case the kCGBlendModeLighten works perfectly, as expected. Unfortunately, I am not fully satisfied with this solution, because I need the background of the text layers to be half-transparent as well. This returns me to the question of how to make kCGBlendModeLighten to the alpha channel. You said I could "average the color components but max the alphas." How can I do this? Thanks! On Sun, Sep 27, 2009 at 3:45 AM, Paul M <l...@no-tek.com> wrote: > This is a multi-pass operation - you wont find a single > blend mode that will do this in any simple manner. > > First question: Are you doing this all in one hit, or sequentially, > ie, rendering the first layer of text then adding subsequent > layers later? > Second question: is the text all the same colour and have the > same alpha component? > > If you're rendering all the text layers at once, you need to first > combine the text layers, then composite over the background. In the > simple case you describe, I imagine you'll need to average the colour > components but max the alphas. This should give you the results you > asked for. > > If you need to comosite different layers at different times, it > becomes a lot more complex. > If your text is not all the same colour then you need access to all > the previous layers so that they may be combined as above and the > composite redone. > If your text is all the same colour then you can write the alpha from > the first layer into the composite so that this may be extracted and > used later in the 2nd and subsequent composites. The process for > these later composites gets pretty gnarley, and you're really better > of keeping a reference to the preceeding layers if that's at all > possible. > > > paulm > > > On 27/09/2009, at 12:17 AM, Oleg Krupnov wrote: > >> Thanks! >> >> It seemed exactly what I need but I've found that it doesn't work as >> expected. The interpolated semi-transparent parts of the rendered text >> still appears brighter than it should. >> >> Namely, consider drawing white text on black background, and then >> drawing the same white text one more time over the first one. >> >> Suppose that when a letter is rasterized, one of the resulting >> interpolated pixels is semitransparent white, with alpha = 50%. After >> alpha blending with the black background, the pixel becomes a 50% gray >> pixel. >> >> Now when the second text is drawn on top, there will be also the same >> semitransparent white pixel with alpha = 50% at the same location. >> What result I'd like to achieve would be to have the same 50% gray >> pixel at this location. However, with kCGBlendModeLighten mode, what I >> think what's happening is that the system picks pure white as the >> lighter of two colors: 50% gray of the background and pure white >> (without taking into account the alpha of the new pixel), and then >> applies the usual alpha blending rule, which results in 75% gray. >> >> Is there a workaround? Or am I doing something wrong? Thanks! >> >> On Tue, Sep 22, 2009 at 7:23 PM, David Duncan <david.dun...@apple.com> >> wrote: >>> >>> On Sep 22, 2009, at 6:22 AM, Oleg Krupnov wrote: >>> >>>> Hi, >>>> >>>> I'd like to draw in a graphics context in such a compositing mode: >>>> >>>> R = MAX(S, D) >>>> >>>> i.e. out of two colors (source and destination), the maximum color >>>> (channel-wise) was chosen. This is basically equivalent to ORing the >>>> colors. >>> >>> >>> This is the Lighten blend mode (kCGBlendModeLighten). There doesn't >>> appear >>> to be a corresponding NS composting mode, and I'm not sure why, but you >>> can >>> easily get a CGContext from an NSGraphicsContext by asking it for its >>> graphicsPort, so this shouldn't be hard to integrate. >>> -- >>> David Duncan >>> Apple DTS Animation and Printing >>> >>> >> _______________________________________________ >> >> Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) >> >> Please do not post admin requests or moderator comments to the list. >> Contact the moderators at cocoa-dev-admins(at)lists.apple.com >> >> Help/Unsubscribe/Update your Subscription: >> http://lists.apple.com/mailman/options/cocoa-dev/list%40no-tek.com >> >> This email sent to l...@no-tek.com >> > > _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. 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