benbritten.com

Hey All!

Sorry this has taken so long. I had it sitting in my to-do pile forever. And I had figured out the problem, just not fixed it.

Thanks to Morgan at bluecrash.com for sending me an email to kick me in the ass and spend the ten minutes to fix it.

Anyhow,

If you are wondering what the problem was it was that I had forgotten to lock the CVPixelBaseAddress before trying to access the CV frame pixel data. (when grabbing frames off the QTCapture frame buffer)

On Leopard this worked fine (for some reason) but was wrong. Here is the proper way to go about it:

 
// Lock the base addy for the pixels
// or it will return nil
CVPixelBufferLockBaseAddress(videoFrame,0);
unsigned char * srcBytes = CVPixelBufferGetBaseAddress(videoFrame);
.
.
.
// dont forget to unlock it when you are done
CVPixelBufferUnlockBaseAddress(videoFrame,0);
 

And that seemed to be the problem on snow leopard. I know, lame and easy. So again my apologies for not just getting it done sooner.

Also, I do plan to release the code for xTouch, but it needs a refactor and a clean first, so that will have to wait. You can get the binary (works on leopard and snow leopard) here:

xTouch.zip

Cheers!
-B

I am going to post this here in hopes that others who have had this issue can find it easily. I spent many many hours today fiddling with custom image units (ie CIFilters) and this one stumped me for quite some time.

OK, here is the thing. If you follow apple's (generally very good) tutorial on creating custom filters, you g through this rough timeline:

1) total confusion.. what the hell is this kernal thing?
2) general understanding .. ahh haa!
3) coding .. this is so EASY! it is great!
4) validating .. WTF?
5) fiddling and finding all the syntax errors by hand in your kernal
6) validating: PASS! yay!
7) loading into QC for testing.. this is soo exciting!!!
8) Cannot Render: Bounds: Infinite (must crop to render).. WTF?!?!
9) spiral into deep deep dark place because after many hours of scouring the docs you still cannot figure out how in the name of Zues's BUTTHOLE do you get a nice filter like ALL THE REST that render without cropping!!

so, here is the solution:

After many hours of looking for clues (and spending an inordinate amount of time trying to figure out how the ROI had to do with the output extent (hint, it doesnt) i stumbled across the fact that you can pass extra info to the kernel via three special kernel keys:
kCIApplyOptionExtent, kCIApplyOptionDefinition, and kCIApplyOptionUserInfo.

the one that we care about for today is kCIApplyOptionExtent. and you use it like so:

in your - (CIImage *)outputImage method:

NSArray * outputExtent = [NSArray arrayWithObjects:
    [NSNumber numberWithInt:0],
    [NSNumber numberWithInt:0],
    [NSNumber numberWithFloat:[inputImage extent].size.width],
    [NSNumber numberWithFloat:[inputImage extent].size.height],nil];

return [self apply:_BBSubtractionCompositeFilterKernel, src, bg,
   kCIApplyOptionExtent, outputExtent, nil];

and that is it. that will set the output extent to be the same as the inputImage extent. You can, of course set it to whatever you want, but i imagine the general case is that you want to get out something just as big as you sent in.

I hope this helps someone!