Re: Decompressing JPEG data into specific format, getting raw pixels
Re: Decompressing JPEG data into specific format, getting raw pixels
- Subject: Re: Decompressing JPEG data into specific format, getting raw pixels
- From: "Sean McBride" <email@hidden>
- Date: Thu, 17 Aug 2017 18:24:59 -0400
- Organization: Rogue Research Inc.
On Wed, 16 Aug 2017 18:39:56 -0500, Ken Thomases said:
>On Aug 16, 2017, at 6:29 PM, Sean McBride <email@hidden> wrote:
>>
>>
>> Right, but then I'm back where I started. :( Because there's
>(apparently) no way to ensure that the NSBitmapImageRep I get is in a
>particular format, and I absolutely need 8 bit per pixel & greyscale.
>
>It's quite old now, but the 10.6 AppKit release notes had a lot of good
>information about working with images in Cocoa viz. Core Graphics. From
><https://developer.apple.com/library/content/releasenotes/AppKit/RN-
>AppKitOlderNotes/#X10_6Notes>, search for "NSImage, CGImage, and
>CoreGraphics impedance matching" and read from there.
>
>They also have advice about what you're trying to do. The advice is to
>*not* try to interpret an arbitrary image rep's bitmap data. It's to
>make a bitmap with the format you desired and draw the source image into
>it. Then, you have bitmap data in the format you want, for sure. For
>that, search for "NSBitmapImageRep: CoreGraphics impedance matching and
>performance notes".
Thanks, that's quite helpful! I know I've read it before, but forgot to
recheck those release notes, they sure are great, I'm glad Apple writes them!
I also found 'QA1509: Getting the pixel data from a CGImage object' which is
even older, but was useful too.
I also found vImageBuffer_InitWithCGImage() which has been great!
So my typical data is 1280x1024. With NSBitmapImageRep I can't get it into an
NSData in 30 ms, whereas with CGImageCreateWithJPEGDataProvider() +
vImageBuffer_InitWithCGImage() it takes only 9 ms and produces byte-for-byte
identical results interestingly enough. I suspect bitmapData makes a copy
somewhere.
Cheers,
--
____________________________________________________________
Sean McBride, B. Eng email@hidden
Rogue Research www.rogue-research.com
Mac Software Developer Montréal, Québec, Canada
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