The document for NSBitmapImageRep says the following: "Alpha Premultiplication 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, specifically CGImageCreate with kCGImageAlphaLast. Note that premultiplying does not affect the output quality. Given source bitmap pixel s, destination pixel d, and alpha value a, a blend is basically d' = a * s + (1 - a) * d All premultiplication does is precalculate a * s."
First question -- what is d' in that equation? I read images from files and create histograms from them using pixel data from the NSBitmapImageRep. Things seem to work ok except for images that have an alpha channel (that is, bitsPerPixel = 32 from the NSBitmapImageRep, and an alpha channel is present). The histograms in that case have gaps in them (certain levels seem to be under-represented). Some specific images with this problem were downloaded through Image Capture and rotated in Image Capture. When an image is rotated in Image Capture, apparently the pixel data is rotated and the Exif Orientation tag is changed to reflect the rotation. (This is different from what I thought Apple said in previous documentation. I thought just the Exif Orientation tag was changed, and it was up to the image display program to rotate the image). Anyway, after rotation, the order of the data is ARGB, whereas before the rotation the order was RGBA. Also, before the rotation, the alpha channel appears to be just an unused channel -- i.e. it's not really an alpha channel. Two additional questions: are the pixel data premultiplied with the alpha channel? Also, for a histogram, should the data be premultiplied with alpha? That is, should the histogram reflect the alpha? Thanks for any help. Jim Merkel_______________________________________________ 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
