Question

I’m using this well known sample code from Apple to convert camera buffer still images into UIImages.

-(UIImage*) getUIImageFromBuffer:(CMSampleBufferRef) imageSampleBuffer{

// Get a CMSampleBuffer's Core Video image buffer for the media data
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(imageSampleBuffer); 

if (imageBuffer==NULL) {
    NSLog(@"No buffer");
}

// Lock the base address of the pixel buffer 
if((CVPixelBufferLockBaseAddress(imageBuffer, 0))==kCVReturnSuccess){
    NSLog(@"Buffer locked successfully");
} 

void *baseAddress = CVPixelBufferGetBaseAddress(imageBuffer); 

// Get the number of bytes per row for the pixel buffer
size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer); 
NSLog(@"bytes per row %zu",bytesPerRow );
// Get the pixel buffer width and height
size_t width = CVPixelBufferGetWidth(imageBuffer); 
NSLog(@"width %zu",width);

size_t height = CVPixelBufferGetHeight(imageBuffer); 
NSLog(@"height %zu",height);

// Create a device-dependent RGB color space
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); 

// Create a bitmap graphics context with the sample buffer data
CGContextRef context = CGBitmapContextCreate(baseAddress, width, height, 8, 
bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst); 

// Create a Quartz image from the pixel data in the bitmap graphics context
CGImageRef quartzImage = CGBitmapContextCreateImage(context); 

// Free up the context and color space
CGContextRelease(context); 
CGColorSpaceRelease(colorSpace);

// Create an image object from the Quartz image
UIImage *image= [UIImage imageWithCGImage:quartzImage scale:SCALE_IMAGE_RATIO orientation:UIImageOrientationRight];

// Release the Quartz image
CGImageRelease(quartzImage);

// Unlock the pixel buffer
CVPixelBufferUnlockBaseAddress(imageBuffer,0);


return (image );}

The problem is that usually the image that you obtain is 90° rotated. Using the method +imageWithCGImage:scale:orientation I’m able to rotate it, but before getting into this method I was trying to rotate and scale the image using the CTM function, before passing it to a UIImage. the problem was that CTM transformation didn’t affect the image.
I’m asking myself why… is that because I’m locking the buffer? or because the context is created with the image inside, so the changes will affect only the further mod?
Thank you

Answer 1

The answer is that it affects only further modifications, and it has nothing to deal with the buffer locking.
As you can read from this answer mod to the context are applied by time Vertical flip of CGContext