crop and get rest of the Image in 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).