crop and get rest of the Image in Objective-C - objective-c

Question

I want to crop the image, and want to get the other portion of the image. Like the image as per below
and
Here I want to create a transparent area of the selected portion of the image and make a new image.
I have also tried to get all the pixel and set alpha to 0 of select portion, but it didn't work.
Does anyone have any other solutions?
Here is the code I have used:
CGSize size = [UIImage imageNamed:fileName].size;
CGImageRef inImage = [UIImage imageNamed:fileName].CGImage;
CFDataRef ref = CGDataProviderCopyData(CGImageGetDataProvider(inImage));
UInt8 * buf = (UInt8 *) CFDataGetBytePtr(ref);
int length = CFDataGetLength(ref);
float value2 = (1 + value-0.5);
NSLog(#"length = %d",length);
int row = 0,col = 0;
for(int i=0; i<length; i+=4)
{
int r = i;
int g = i+1;
int b = i+2;
int a = i+3;
col++;
if ((col % (int)size.width)==0 ) {
row++;
col=0;
}
int red = buf[r];
int green = buf[g];
int blue = buf[b];
int alpha = buf[a];
if (col > 25 && col < 75 && row > 25 && row < 75) {
alpha = 0;
}
buf[r] = SAFECOLOR(red);
buf[g] = SAFECOLOR(green);
buf[b] = SAFECOLOR(blue);
buf[a] = SAFECOLOR(alpha);
}
NSLog(#"CGImageGetAlphaInfo %d",CGImageGetAlphaInfo(inImage));
NSLog(#"CGImageGetColorSpace %#",CGImageGetColorSpace(inImage));
CGContextRef ctx = CGBitmapContextCreate(buf,
CGImageGetWidth(inImage),
CGImageGetHeight(inImage),
CGImageGetBitsPerComponent(inImage),
CGImageGetBytesPerRow(inImage),
kCGColorSpaceGenericRGB,
kCGImageAlphaPremultipliedLast);
CGImageRef img = CGBitmapContextCreateImage(ctx);
imgView.image = [UIImage imageWithCGImage:img];

Answer 1

Try doing it by painting transparent rectangle in kCGBlendModeCopy (which replaces destination pixels completely rather than blending with them)
UIImage* dst = [UIImage imageNamed:fileName];
UIGraphicsBeginImageContext(dst.size);
[dst drawInRect:CGRectMake(0,0,dst.size.width,dst.size.height)];
[[UIBezierPath bezierPathWithRect:CGRectMake(25,25,50,50)]
fillWithBlendMode:kCGBlendModeCopy alpha:0];
UIImage* result = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
The problem with your solution is probably in that in a bitmap with premultiplied alpha the R,G,B components are expected to be multiplied by A component, and that means if you set A to zero you must set R,G,B to zero too (as any value multiplied by A=0 will be zero).