AVFoundation buffer comparison to a saved image
- by user577552
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?
