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Re: How to get pixel information from CIImage of QT frame
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Re: How to get pixel information from CIImage of QT frame


On Jan 15, 2008, at 5:51 PM, JArod Wen wrote:

I am now on a video analysis program. We have get the QT movie and show it in a OpenGLView using Core Video(mainly based on the source code of civideoDemo). Now we need to do pixel analysis on the frame of the movie. I am sure that there is no problem on the video->frame since we have done some composite filter based on the CIImage frame we get from the movie.

It is a straight forward idea that we convert each frame into a Core Image object using

[CIImage imageWithCVImageBuffer:currentFrame]

Here currentFrame is defined by:

CVImageBufferRef	currentFrame;

and then our idea is to get the pixel information using NSBitmapImageRep.

One solution of using [[NSBitmapImageRep alloc] initWithCIImage] failed when the program run into a Exec_Bad_Access error.

I tried another way: draw the CIImage on to a CIContext and then render it to bitmap data:

NSData* pixelData = nil;
[ciContextOffScreen render:inputCIImage toBitmap:pixelData rowBytes:rowBytes bounds:[inputCIImage extent] format:kCIFormatRGBAf colorSpace:nil];
if(!pixelData)
[ciContext render:inputCIImage toBitmap:pixelData rowBytes:rowBytes bounds:[inputCIImage extent] format:kCIFormatRGBAf colorSpace:nil];
return [[NSBitmapImageRep alloc] initWithData:pixelData];

But the problem is finally there is no data in pixelData. So finally the returned NSBitmapImageRep is empty.


Searching on 'CVImageBufferRef NSImage' turned up a decent amount of hits including the following link from the QTKit Capture Guide:

<http://developer.apple.com/documentation/QuickTime/Conceptual/QTKitCaptureProgrammingGuide/CreatingStopMotionApplication/chapter_5_section_7.html >

In the addFrame: block of sample code, they build an image with the frame buffer data using an NSCIImageRep:

NSCIImageRep *imageRep = [NSCIImageRep imageRepWithCIImage:[CIImage imageWithCVImageBuffer:imageBuffer]];
NSImage *image = [[[NSImage alloc] initWithSize:[imageRep size]] autorelease];

Note that there's also other lines around that (to deal with stuff like calling from a thread other than main), so definitely read the entire example and see what may apply in your case. No idea on why you were getting the EXEC_BAD_ACCESS; could be that you were also calling things from other threads? Could be that it's not possible to create an NSBitmapImageRep from such a data provider. After all, a CVImageBufferRef is really an abstract type. Since you mentioned OpenGL, your CVImageBufferRef is probably really a CVOpenGLBufferRef
You may also want to move this thread to the Quicktime developers list.
___________________________________________________________
Ricky A. Sharp mailto:email@hidden
Instant Interactive(tm) http://www.instantinteractive.com

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 >How to get pixel information from CIImage of QT frame (From: JArod Wen <email@hidden>)























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