Modifying the bitmap data yourself shouldn't be too difficult, for an NSBitmapImageRep you'd just grab the data with the -bitmapData method. But have you checked out CIFilter? I'm not entirely sure I understood what your goal is but there's likely a filter in place already that will do it for you:
https://developer.apple.com/mac/library/documentation/GraphicsImaging/Reference/CoreImageFilterReference/Reference/reference.html https://developer.apple.com/mac/library/documentation/GraphicsImaging/Reference/QuartzCoreFramework/Classes/CIFilter_Class/Reference/Reference.html On Mon, Sep 21, 2009 at 9:32 PM, Roland King <r...@rols.org> wrote: > I'm trying to construct an image which is colored according to the alpha > value of a different image. I've been hunting around the Quartz Core docs > and I can't figure out a good way to do this. The motivation is to do > something similar to what apple does on the iphone tabbar items which are > drawn white or blue using the alpha of the image you supply and it's a > pretty good effect I want to reuse. > > So for instance if I my drawing color is D and the point on my underlying > image is {r,g,b,a}, I want to draw a point of color S * a. > > Assume that I have the image I want to process and I can get that into a > bitmap - because I do and I have. > > I've looked at all the colour blending modes but unless I've missed > something, all of them deal, as you'd expect, in colour, not alpha, so I > can't find one of those I think I can use which totally ignores the colour > information. > > I thought about using a CFImageMask which has points which are white where > the underlyer is alpha=1 and black where it's 0 and all points between but > creating that from the original image is the same problem.. > > I thought about wandering through the bytes of data in the bitmap (assuming > I can even figure out how to access them which currently I cannot) and > changing them but that seems pretty hokey even if I deal correctly with the > byte orderings the packing and .. getting to the data in the first place. > > I'm sure I've entirely missed some piece of Quartz or CF here which does > exactly what I want, or a blending mode I can use cleverly for this. Any > pointers would be very useful. > > Roland > _______________________________________________ > > 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/bravobug%40gmail.com > > This email sent to bravo...@gmail.com > _______________________________________________ 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/archive%40mail-archive.com This email sent to arch...@mail-archive.com
