AVFoundation: add text to the CMSampleBufferRef vi

I'm building an app using AVFoundation.

Just before I call [assetWriterInput appendSampleBuffer:sampleBuffer] in - (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection-method.

I manipulate the pixels in the sample buffer (using a pixelbuffer to apply an effect).

But the client wants me to put in a text (timestamp & framecounter) as well on the frames, but I haven't found a way to do this yet.

I tried to convert the samplebuffer to an Image, apply text on the image, and convert the image back to a samplebuffer, but then

CMSampleBufferDataIsReady(sampleBuffer)

fails.

Here are my UIImage category methods:

 +  (UIImage *) imageFromSampleBuffer:(CMSampleBufferRef) sampleBuffer
    {
    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);

    CVPixelBufferLockBaseAddress(imageBuffer,0);

    uint8_t *baseAddress = (uint8_t *)CVPixelBufferGetBaseAddress(imageBuffer);
    size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
    size_t width = CVPixelBufferGetWidth(imageBuffer);
    size_t height = CVPixelBufferGetHeight(imageBuffer);


    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef newContext = CGBitmapContextCreate(baseAddress, width, height, 8, bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst);

    CGImageRef newImage = CGBitmapContextCreateImage(newContext);

    CGContextRelease(newContext);
    CGColorSpaceRelease(colorSpace);

    UIImage *newUIImage = [UIImage imageWithCGImage:newImage];

    CFRelease(newImage);

    return newUIImage;
    }

And

 - (CMSampleBufferRef) cmSampleBuffer
    {
        CGImageRef image = self.CGImage;

        NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
                                 [NSNumber numberWithBool:YES], kCVPixelBufferCGImageCompatibilityKey,
                                 [NSNumber numberWithBool:YES], kCVPixelBufferCGBitmapContextCompatibilityKey,
                                 nil];
        CVPixelBufferRef pxbuffer = NULL;

        CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault,
                                              self.size.width,
                                              self.size.height,
                                              kCVPixelFormatType_32ARGB,
                                              (__bridge CFDictionaryRef) options,
                                              &pxbuffer);
        NSParameterAssert(status == kCVReturnSuccess && pxbuffer != NULL);

        CVPixelBufferLockBaseAddress(pxbuffer, 0);
        void *pxdata = CVPixelBufferGetBaseAddress(pxbuffer);
        NSParameterAssert(pxdata != NULL);

        CGColorSpaceRef rgbColorSpace = CGColorSpaceCreateDeviceRGB();
        CGContextRef context = CGBitmapContextCreate(pxdata, self.size.width,
                                                     self.size.height, 8, 4*self.size.width, rgbColorSpace,
                                                     kCGImageAlphaNoneSkipFirst);
        NSParameterAssert(context);
        CGContextConcatCTM(context, CGAffineTransformMakeRotation(0));
        CGContextDrawImage(context, CGRectMake(0, 0, CGImageGetWidth(image),
                                               CGImageGetHeight(image)), image);
        CGColorSpaceRelease(rgbColorSpace);
        CGContextRelease(context);
        CVPixelBufferUnlockBaseAddress(pxbuffer, 0);
        CMVideoFormatDescriptionRef videoInfo = NULL;
        CMSampleBufferRef sampleBuffer = NULL;
        CMSampleBufferCreateForImageBuffer(kCFAllocatorDefault,
                                           pxbuffer, true, NULL, NULL, videoInfo, NULL, &sampleBuffer);
        return sampleBuffer;
    }

Any ideas?

EDIT:

I changed my code with Tony's answer. (Thank you!) This code works:

CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);

    CVPixelBufferLockBaseAddress( pixelBuffer, 0 );

    EAGLContext *eaglContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
    CIContext *ciContext = [CIContext contextWithEAGLContext:eaglContext options:@{kCIContextWorkingColorSpace : [NSNull null]} ];

    UIFont *font = [UIFont fontWithName:@"Helvetica" size:40];
    NSDictionary *attributes = @{NSFontAttributeName: font,
                                 NSForegroundColorAttributeName: [UIColor lightTextColor]};

    UIImage *img = [UIImage imageFromText:@"01 - 13/02/2014 15:18:21:654" withAttributes:attributes];
    CIImage *filteredImage = [[CIImage alloc] initWithCGImage:img.CGImage];

    [ciContext render:filteredImage toCVPixelBuffer:pixelBuffer bounds:[filteredImage extent] colorSpace:CGColorSpaceCreateDeviceRGB()];

    CVPixelBufferUnlockBaseAddress(pixelBuffer, 0);

You should refer the CIFunHouse sample from apple, and you may use this api to draw directly to the buffer

-(void)render:(CIImage *)image toCVPixelBuffer:(CVPixelBufferRef)buffer bounds:(CGRect)r colorSpace:(CGColorSpaceRef)cs

You can download it here WWDC2013

Create the context

_eaglContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
_ciContext = [CIContext contextWithEAGLContext:_eaglContext options:@{kCIContextWorkingColorSpace : [NSNull null]} ];

Now render the image

CVPixelBufferRef renderedOutputPixelBuffer = NULL;
OSStatus err = CVPixelBufferPoolCreatePixelBuffer(nil, self.pixelBufferAdaptor.pixelBufferPool, &renderedOutputPixelBuffer);
[_ciContext render:filteredImage toCVPixelBuffer:renderedOutputPixelBuffer bounds:[filteredImage extent]