Re: premultiplying alpha
On 5 May 2010, at 06:05, Ken Ferry wrote: On Tue, May 4, 2010 at 9:30 AM, David Duncan david.dun...@apple.com wrote: On May 4, 2010, at 8:57 AM, Thomas Davie wrote: After much searching about, I discovered (contrary to what the docs say), that NSBitmapImageReps do not always premultiply their r/g/b by their alpha. I think you're misreading something. NSAlphaNonpremultipliedBitmapFormat exists. :-) Indeed, but nearly the first thing the documentation says is: If a coverage (alpha) plane exists, a bitmap’s color components are premultiplied with it. If you modify the contents of the bitmap, you are therefore responsible for premultiplying the data. For this reason, though, if you want to manipulate the actual data, an NSBitmapImageRep object is not recommended for storage. If you need to work with data that is not premultiplied, you should use Quartz, specificallyCGImageCreate with kCGImageAlphaLast. This is repeated variously through the documentation, with no mention of the alphaNonPremultiplied format except in the constants section. I've filed a documentation bug report :) Bob___ 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
Re: premultiplying alpha
On May 4, 2010, at 8:57 AM, Thomas Davie wrote: After much searching about, I discovered (contrary to what the docs say), that NSBitmapImageReps do not always premultiply their r/g/b by their alpha. Because of this, I've dived in, and attempted to write my own premultiplication, but something is going very wrong. Some images are coming out a quarter of the width they went in, others coming out completely empty... Before diving too deeply into you code, why do you want to do this manually? Why not just create a second NSBitmapImageRep with the geometry that you desire, then draw the first into the second? This should do the premultiplication for you as well as cover any format that NSBitmapImageRep supports, rather than the ones that you manually code (as well as being more efficient since we've spent the effort to performance optimize those conversions). -- 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/archive%40mail-archive.com This email sent to arch...@mail-archive.com
Re: premultiplying alpha
On Tue, May 4, 2010 at 9:30 AM, David Duncan david.dun...@apple.com wrote: On May 4, 2010, at 8:57 AM, Thomas Davie wrote: After much searching about, I discovered (contrary to what the docs say), that NSBitmapImageReps do not always premultiply their r/g/b by their alpha. I think you're misreading something. NSAlphaNonpremultipliedBitmapFormat exists. :-) Other than that, what David said. You should never be calling -bitmapData on an NSBitmapImageRep that you did not create explicitly yourself in a known pixel format. The 10.6 AppKit release noteshttp://developer.apple.com/mac/library/releasenotes/cocoa/appkit.htmlhave a long discussion of such things, concerning performance and correctness. -Ken Because of this, I've dived in, and attempted to write my own premultiplication, but something is going very wrong. Some images are coming out a quarter of the width they went in, others coming out completely empty... Before diving too deeply into you code, why do you want to do this manually? Why not just create a second NSBitmapImageRep with the geometry that you desire, then draw the first into the second? This should do the premultiplication for you as well as cover any format that NSBitmapImageRep supports, rather than the ones that you manually code (as well as being more efficient since we've spent the effort to performance optimize those conversions). -- 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/kenferry%40gmail.com This email sent to kenfe...@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
