CGBitmapContextGetData not correct

architectpianist · 12-26-2011, 03:58 PM

Hi,
I'm trying to get the pixel color of a color wheel at a touch point. I set the color wheel image in drawRect, along with the bitmap context and data. I need to set the color wheel image (vs using an image on file) because I use an overlay to make the color wheel darker to get black. Then in the touches____ methods, I find the pixel in the bitmap data. The code is here:

Code:

- (void)drawRect:(CGRect)rect
{
	CGContextRef context = UIGraphicsGetCurrentContext();
	CGImageRef wheel = [UIImage imageNamed:@"ColorWheel"].CGImage;
	CGContextTranslateCTM(context, 0.0, rect.size.height);
	CGContextScaleCTM(context, 1.0, -1.0);
	CGContextDrawImage(context, CGRectMake(rect.size.width / 2 - 140.0, rect.size.height / 2 - 140.0, 280.0, 280.0), wheel);

	UIGraphicsBeginImageContext(rect.size);
	currentColorWheel = UIGraphicsGetImageFromCurrentImageContext().CGImage;
	UIGraphicsEndImageContext();
	
	//Thanks to http://www.markj.net/iphone-uiimage-pixel-color/
	CGColorSpaceRef colorSpace;
	int bitmapByteCount, bitmapBytesPerRow;
	size_t width = CGImageGetWidth(currentColorWheel),
	height = CGImageGetHeight(currentColorWheel);
	bitmapBytesPerRow = width * 4;
	bitmapByteCount = bitmapBytesPerRow * height;
	colorSpace = CGColorSpaceCreateDeviceRGB();
	if (colorSpace == NULL)
	{
		NSLog(@"Color space could not be created.");
		return;
	}
	currentBitmapContext = CGBitmapContextCreate(NULL, width, height,  8, bitmapBytesPerRow, colorSpace, kCGImageAlphaPremultipliedFirst);
	if (currentBitmapContext == NULL)
		NSLog(@"Bitmap context could not be created.");
	
	CGContextDrawImage(currentBitmapContext, CGRectMake(0.0, 0.0, width, height), currentColorWheel);
	imageData = CGBitmapContextGetData(currentBitmapContext);
	if (imageData == NULL)
	{
		NSLog(@"Image data could not be created.");
	}
	
	CGColorSpaceRelease(colorSpace);
}

//...

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
	CGPoint touchPoint = [[touches anyObject] locationInView:self];
	if (DistanceBetweenPoints(CGPointMake(self.frame.size.width / 2, self.frame.size.height / 2), touchPoint) <= 138.0)	//Check if the point is within the radius
	{
		if (currentBitmapContext != NULL && imageData != NULL)
		{
			size_t width = CGImageGetWidth(currentColorWheel);
			int index = 4 * (width * round(touchPoint.y) + round(touchPoint.x));
			NSLog(@"Index: %i size: %lu", index, sizeof(imageData) / sizeof(unsigned char));
			UIColor *color = [UIColor colorWithRed:imageData[index] green:imageData[index + 1] blue:imageData[index + 2] alpha:imageData[index + 3]];
			NSLog(@"Color: %@", color);
		}
	}	
}

Strangely, the log about the index and size in touchesEnded shows that imageData has a size of only 4 unsigned chars! As a result, the color is 0, 0, 0, 0. Any ideas on why this is happening and how I can get a correct color?

Thanks!

Duncan C · 12-26-2011, 07:43 PM

Quote:

Originally Posted by architectpianist

Hi,
I'm trying to get the pixel color of a color wheel at a touch point. I set the color wheel image in drawRect, along with the bitmap context and data. I need to set the color wheel image (vs using an image on file) because I use an overlay to make the color wheel darker to get black. Then in the touches____ methods, I find the pixel in the bitmap data. The code is here:

Code:

- (void)drawRect:(CGRect)rect
{
	CGContextRef context = UIGraphicsGetCurrentContext();
	CGImageRef wheel = [UIImage imageNamed:@"ColorWheel"].CGImage;
	CGContextTranslateCTM(context, 0.0, rect.size.height);
	CGContextScaleCTM(context, 1.0, -1.0);
	CGContextDrawImage(context, CGRectMake(rect.size.width / 2 - 140.0, rect.size.height / 2 - 140.0, 280.0, 280.0), wheel);

	UIGraphicsBeginImageContext(rect.size);
	currentColorWheel = UIGraphicsGetImageFromCurrentImageContext().CGImage;
	UIGraphicsEndImageContext();
	
	//Thanks to http://www.markj.net/iphone-uiimage-pixel-color/
	CGColorSpaceRef colorSpace;
	int bitmapByteCount, bitmapBytesPerRow;
	size_t width = CGImageGetWidth(currentColorWheel),
	height = CGImageGetHeight(currentColorWheel);
	bitmapBytesPerRow = width * 4;
	bitmapByteCount = bitmapBytesPerRow * height;
	colorSpace = CGColorSpaceCreateDeviceRGB();
	if (colorSpace == NULL)
	{
		NSLog(@"Color space could not be created.");
		return;
	}
	currentBitmapContext = CGBitmapContextCreate(NULL, width, height,  8, bitmapBytesPerRow, colorSpace, kCGImageAlphaPremultipliedFirst);
	if (currentBitmapContext == NULL)
		NSLog(@"Bitmap context could not be created.");
	
	CGContextDrawImage(currentBitmapContext, CGRectMake(0.0, 0.0, width, height), currentColorWheel);
	imageData = CGBitmapContextGetData(currentBitmapContext);
	if (imageData == NULL)
	{
		NSLog(@"Image data could not be created.");
	}
	
	CGColorSpaceRelease(colorSpace);
}

//...

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
	CGPoint touchPoint = [[touches anyObject] locationInView:self];
	if (DistanceBetweenPoints(CGPointMake(self.frame.size.width / 2, self.frame.size.height / 2), touchPoint) <= 138.0)	//Check if the point is within the radius
	{
		if (currentBitmapContext != NULL && imageData != NULL)
		{
			size_t width = CGImageGetWidth(currentColorWheel);
			int index = 4 * (width * round(touchPoint.y) + round(touchPoint.x));
			NSLog(@"Index: %i size: %lu", index, sizeof(imageData) / sizeof(unsigned char));
			UIColor *color = [UIColor colorWithRed:imageData[index] green:imageData[index + 1] blue:imageData[index + 2] alpha:imageData[index + 3]];
			NSLog(@"Color: %@", color);
		}
	}	
}

Strangely, the log about the index and size in touchesEnded shows that imageData has a size of only 4 unsigned chars! As a result, the color is 0, 0, 0, 0. Any ideas on why this is happening and how I can get a correct color?

Thanks!

If you pass a pointer type to sizeof(), it tells you the size of the pointer, not the size of the data structure that the pointer points to. That's not your problem.

I haven't looked over your code in enough detail to tell what IS wrong with it. I find it easier to troubleshoot code by stepping through it in the debugger. That would be my suggestion.

architectpianist · 12-26-2011, 08:01 PM

Quote:

If you pass a pointer type to sizeof(), it tells you the size of the pointer, not the size of the data structure that the pointer points to. That's not your problem.

I assume you're talking about this line?

Code:

NSLog(@"Index: %i size: %lu", index, sizeof(imageData) / sizeof(unsigned char));

So if this NSLog is representing the imageData array wrongly, how do I log the array size correctly?
Also, I tried stepping through it with the debugger. Nothing jumps out, since these are Core Graphics opaque types. But since nothing is being created in the touches___ methods, I think the cause is probably in drawRect.
Additionally, the imageData shows up in the debugger as
imgData = (unsigned char *) 0xdc5c020
*imgData = (unsigned char) 0 '\000'

Thanks for your reply!

architectpianist · 12-26-2011, 11:09 PM

By repetitive logging I found that the size of the bitmapData is in fact in the 300,000s, which is the range it should be in for a 280 x 280 image with 4 bytes per pixel. Then I added this at the end of the drawRect method, just to check:

Code:

CGContextClearRect(context, rect);
CGContextDrawImage(context, rect, currentColorWheel);

Nothing shows up on the screen when I do this! So it comes down to currentColorWheel being empty, but not NULL.
I've changed the creation of currentColorWheel from

Code:

UIGraphicsBeginImageContext(rect.size);
currentColorWheel = UIGraphicsGetImageFromCurrentImageContext().CGImage;
UIGraphicsEndImageContext();

to

Code:

UIGraphicsBeginImageContextWithOptions(rect.size, NO, 0.0);
[self.layer renderInContext:UIGraphicsGetCurrentContext()];
currentColorWheel = UIGraphicsGetImageFromCurrentImageContext().CGImage;
UIGraphicsEndImageContext();

but it doesn't seem to have done anything. How can I get the color wheel to show up in the currentColorWheel CGImage?
Thanks!