Creating ad displaying a UIImage from raw BGRA data - objective-c

Question

I'm collecting image data from the camera using this code:
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
{
// Called when a frame arrives
// Should be in BGRA format
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
CVPixelBufferLockBaseAddress(imageBuffer, 0);
size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
size_t width = CVPixelBufferGetWidth(imageBuffer);
size_t height = CVPixelBufferGetHeight(imageBuffer);
unsigned char *raw = (unsigned char *)CVPixelBufferGetBaseAddress(imageBuffer);
// Copy memory into allocated buffer
unsigned char *buffer = malloc(sizeof(unsigned char) * bytesPerRow * height);
memcpy(buffer, raw, bytesPerRow * height);
[self processVideoData:buffer width:width height:height bytesPerRow:bytesPerRow];
CVPixelBufferUnlockBaseAddress(imageBuffer, 0);
}
The processVideoData: method looks like this:
- (void)processVideoData:(unsigned char *)data width:(size_t)width height:(size_t)height bytesPerRow:(size_t)bytesPerRow
{
dispatch_sync(dispatch_get_main_queue(), ^{
CGDataProviderRef dataProvider = CGDataProviderCreateWithData(NULL, data, bytesPerRow * height, NULL);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGImageRef image = CGImageCreate(width, height, 8, 32, bytesPerRow, colorSpace, kCGBitmapByteOrderDefault | kCGImageAlphaLast, dataProvider, NULL, NO, kCGRenderingIntentDefault);
// Set layer contents???
UIImage *objcImage = [UIImage imageWithCGImage:image];
self.imageView.image = objcImage;
free(data);
CGDataProviderRelease(dataProvider);
CGColorSpaceRelease(colorSpace);
CGImageRelease(image);
});
}
No complaints, no leaks but nothing shows up in the image view it just stays blank (yes I have checked the outlet connection). Previously I had the bitmapInfo set just to kCGBitmapByteOrderDefault which was causing a crash when setting the image property of the image view however the image view would go dark which was promising just before the crash.
I summarised that the crash was due to the image being in BGRA not BGR so I set the bitmapInfo to kCGBitmapByteOrderDefault | kCGImageAlphaLast and that solved the crash but no image.
I realise that the image will look weird as the CGImageRef is expecting an RGB image and I'm passing it BGR but that should only result in a weird looking image due to channel swapping. I have also logged out the data that I'm getting and it seems to be in order something like: b:65 g:51 r:42 a:255 and the alpha channel is always 255 as expected.
I'm sorry if it's obvious but I can't work out what is going wrong.

Answer 1

You can use this flag combination to achieve BGRA format:
kCGBitmapByteOrder32Little | kCGImageAlphaSkipFirst
You should prefer to use this solution, it will be more performant way in comparison to OpenCV conversion.
Here is more common way to convert sourcePixelFormat to bitmapInfo:
sourcePixelFormat = CVPixelBufferGetPixelFormatType(pixelBuffer);
bitmapInfo = #{
#(kCVPixelFormatType_32ARGB) : #(kCGBitmapByteOrder32Big | kCGImageAlphaNoneSkipFirst),
#(kCVPixelFormatType_32BGRA) : #(kCGBitmapByteOrder32Little | kCGImageAlphaNoneSkipFirst),
}[#(sourcePixelFormat)].unsignedIntegerValue;

Answer 2

It turns out the data was just in the wrong format and I wasn't feeding it into the CGImageCreate function correctly.
The data comes out in BGRA format so I fed this data into an IplImage structure (I'm using OpenCV v 2.4.9) like so:
// Pack IplImage with data
IplImage *img = cvCreateImage(cvSize((int)width, (int)height), 8, 4);
img->imageData = (char *)data;
I then converted it to RGB like so:
IplImage *converted = cvCreateImage(cvSize((int)width, (int)height), 8, 3);
cvCvtColor(img, converted, CV_BGRA2RGB);
I then fed the data from the converted IplImage into a CGImageCreate function and it works nicely.