Trim transparent pixels around UIImage

UIImage+Trim.m

- (UIImage *) imageByTrimmingTransparentPixels {
	int rows = self.size.height;
	int cols = self.size.width;
	int bytesPerRow = cols*sizeof(uint8_t);
	
	if ( rows < 2 || cols < 2 ) {
		return self;
	}
	
	//allocate array to hold alpha channel
	uint8_t *bitmapData = calloc(rows*cols, sizeof(uint8_t));
	
	//create alpha-only bitmap context
	CGContextRef contextRef = CGBitmapContextCreate(bitmapData, cols, rows, 8, bytesPerRow, NULL, kCGImageAlphaOnly);
	
	//draw our image on that context
	CGImageRef cgImage = self.CGImage;
	CGRect rect = CGRectMake(0, 0, cols, rows);
	CGContextDrawImage(contextRef, rect, cgImage);

	//summ all non-transparent pixels in every row and every column
	uint16_t *rowSum = calloc(rows, sizeof(uint16_t));
	uint16_t *colSum = calloc(cols, sizeof(uint16_t));
	
	//enumerate through all pixels
	for ( int row = 0; row < rows; row++) {
		for ( int col = 0; col < cols; col++)
		{
			if ( bitmapData[row*bytesPerRow + col] ) { //found non-transparent pixel
				rowSum[row]++;
				colSum[col]++;
			}
		}
	}
	
	//initialize crop insets and enumerate cols/rows arrays until we find non-empty columns or row
	UIEdgeInsets crop = UIEdgeInsetsMake(0, 0, 0, 0);
	
	for ( int i = 0; i<rows; i++ ) { 		//top
		if ( rowSum[i] > 0 ) {
			crop.top = i; break;
		}
	}
	
	for ( int i = rows; i >= 0; i-- ) {		//bottom
		if ( rowSum[i] > 0 ) {
			crop.bottom = MAX(0, rows-i-1); break;
		}
	}
	
	for ( int i = 0; i<cols; i++ ) {		//left
		if ( colSum[i] > 0 ) {
			crop.left = i; break;
		}
	}
	
	for ( int i = cols; i >= 0; i-- ) {		//right
		if ( colSum[i] > 0 ) {
			crop.right = MAX(0, cols-i-1); break;
		}
	}
	
	free(bitmapData);
	free(colSum);
	free(rowSum);
	
	if ( crop.top == 0 && crop.bottom == 0 && crop.left == 0 && crop.right == 0 ) {
		//no cropping needed
		return self;
	}
	else {
		//calculate new crop bounds
		rect.origin.x += crop.left;
		rect.origin.y += crop.top;
		rect.size.width -= crop.left + crop.right;
		rect.size.height -= crop.top + crop.bottom;
		
		//crop it
		CGImageRef newImage = CGImageCreateWithImageInRect(cgImage, rect);
		
		//convert back to UIImage
		return [UIImage imageWithCGImage:newImage];
	}
}

I've created this file and set UIImage as base class! Everything setup without error.

I even imported .h file where I want to implement imageByTrimmingTransparentPixels method.

But the problem is how to use this method? I have a UIImageView object as imageView.

How do I implement this method in imageView object?

Please help me.

@Rashesh-Bosamiya if you implemented this as a category, you would call it like:

// Trimming the image from 'imageView'
UIImage *trimmedImage = [imageView.image imageByTrimmingTransparentPixels];

// Setting the image back to 'imageView'
imageView.image = trimmedImage;

Not sure exactly your use case, but that should get you going.

1. you need to release "contextRef".
2. and, the width and height should multipy self.scale to get the real pixel with and height.
3. you need to release "newImage".