iOS: not getting correct result with CGBitmapContextCreate in method call

Question

I have been scratching my head over this over hours.

I am using the following method to resize 2 images. One after another:

CGImageRef imageReference = [image CGImage];
bytes = malloc(width * height * 4); 

NSUInteger bytesPerPixel = 4;
NSUInteger bytesPerRow = bytesPerPixel * width;
NSUInteger bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(bytes, width, height, bitsPerComponent,
                                bytesPerRow, colorSpaceReference,
                                kCGImageAlphaPremultipliedLast);

CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageReference); 
CGImageRelease(imageReference);
CGContextRelease(context);

It works fine, no problem - but only one image. If I call this method again, for example:

[self resizeImageWithSize:imageSize];  //this is OK.
[self resizeImageWithSize:imageSize]; //this would not come out right

where image size is determined by: image1.size and image2.size. I have tried flipping the calling sequence of the methods, the first one is always correct.

They are not too big, 400 x 300, 300 x 360. And I just would like to resize one to 200 x 200 and the other for example 150 x 150. And the are just png's.

It works, but if I call this method again, the second image is wrong. Wrong being it has messed up pixels like water stains on paper. Sometimes it is even rendered unrecognizable.

Am I missing something very obvious here? I have tried to free(bytes);, which I don't think is needed here but for the sake of trying, but still it doesn't bring anything. Am I not releasing /freeing correctly something so that the second time when the method is called, old byte data persists still? I am just guessing here. Am using ARC.

score 1 · Answer 1 · answered Feb 24 '14 at 16:21

1

I am using this method and it works like a charm hopefully this will help:

- (UIImage *)resizeImage:(UIImage *)image toSize:(CGSize)newSize
{
    UIImage *newImage = nil;

    UIGraphicsBeginImageContextWithOptions(newSize, YES, 0.0);
    [image drawInRect:CGRectMake(0.0,
                                 0.0,
                                 newSize.width,
                                 newSize.height)];
    newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return newImage;

}

answered Feb 24 '14 at 16:21

Basheer_CAD

4,908
24
36

Thanks. But I still hope to know why my method above is wrong. +1, for sharing a nice and clean method, however. – Unheilig Feb 24 '14 at 16:50
@Unheilig, try to put UIGraphicsBeginImageContext(<#CGSize size#>), before your code and UIGraphicsEndImageContext(); after your code. Tell me if it works I am interested :) – Basheer_CAD Feb 24 '14 at 17:16
Feedback: thanks a lot for tip, but it doesn't solve it. – Unheilig Feb 24 '14 at 20:06

iOS: not getting correct result with CGBitmapContextCreate in method call

1 Answers1