Hi, I am a long time reader, first time poster on StackOverflow, and must say it has been a great source of knowledge for me. I am trying to get to know the AVFoundation framework. What I want to do is save what the camera sees and then detect when something changes. Here is the part where I save the image to a UIImage : if (shouldSetBackgroundImage) { CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); // Create a bitmap graphics context with the sample buffer data CGContextRef context = CGBitmapContextCreate(rowBase, bufferWidth, bufferHeight, 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]; [self setBackgroundImage:image]; NSLog(@"reference image actually set"); // Release the Quartz image CGImageRelease(quartzImage); //Signal that the image has been saved shouldSetBackgroundImage = NO; } and here is the part where I check if there is any change in the image seen by the camera : else { CGImageRef cgImage = [backgroundImage CGImage]; CGDataProviderRef provider = CGImageGetDataProvider(cgImage); CFDataRef bitmapData = CGDataProviderCopyData(provider); char* data = CFDataGetBytePtr(bitmapData); if (data != NULL) { int64_t numDiffer = 0, pixelCount = 0; NSMutableArray * pointsMutable = [NSMutableArray array]; for( int row = 0; row < bufferHeight; row += 8 ) { for( int column = 0; column < bufferWidth; column += 8 ) { //we get one pixel from each source (buffer and saved image) unsigned char *pixel = rowBase + (row * bytesPerRow) + (column * BYTES_PER_PIXEL); unsigned char *referencePixel = data + (row * bytesPerRow) + (column * BYTES_PER_PIXEL); pixelCount++; if ( !match(pixel, referencePixel, matchThreshold) ) { numDiffer++; [pointsMutable addObject:[NSValue valueWithCGPoint:CGPointMake(SCREEN_WIDTH - (column/ (float) bufferHeight)* SCREEN_WIDTH - 4.0, (row/ (float) bufferWidth)* SCREEN_HEIGHT- 4.0)]]; } } } numberOfPixelsThatDiffer = numDiffer; points = [pointsMutable copy]; } For some reason, this doesn't work, meaning that the iPhone detects almost everything as being different from the saved image, even though I set a very low threshold for detection in the match function... Do you have any idea of what I am doing wrong?