Question

I am using https://github.com/PaulSolt/UIImage-Conversion

However, it seems to have a problem on the alpha channel. Problem reconstituting UIImage from RGBA pixel byte data

I have simplified the problem so I present it as a tidier question.

I download Paul’s project, run it. It displays an image in the centre of the screen — everything so far is correct.

But his demo is not testing for Alpha.

So I attempt to composite my button image…

… on top.

The result is:

here is the code — I have just modified AppDelegate_iPad.m:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    

    // Override point for customization after application launch.
    {
        UIImage *image = [UIImage imageNamed:@"Icon4.png"];
        int width = image.size.width;
        int height = image.size.height;

        // Create a bitmap
        unsigned char *bitmap = [ImageHelper convertUIImageToBitmapRGBA8:image];

        // Create a UIImage using the bitmap
        UIImage *imageCopy = [ImageHelper convertBitmapRGBA8ToUIImage:bitmap withWidth:width withHeight:height];

        // Cleanup
        if(bitmap) {
            free(bitmap);   
            bitmap = NULL;
        }

        // Display the image copy on the GUI
        UIImageView *imageView = [[UIImageView alloc] initWithImage:imageCopy];
        CGPoint center = CGPointMake([UIScreen mainScreen].bounds.size.width / 2.0, 
                                     [UIScreen mainScreen].bounds.size.height / 2.0);
        [imageView setCenter:center];
        [window addSubview:imageView];
        [imageView release];
    }


    if( 1 )
    {
        UIImage *image = [UIImage imageNamed:@"OuterButton_Dull.png"];
        int width = image.size.width;
        int height = image.size.height;

        // Create a bitmap
        unsigned char *bitmap = [ImageHelper convertUIImageToBitmapRGBA8:image];

        // Create a UIImage using the bitmap
        UIImage *imageCopy = [ImageHelper convertBitmapRGBA8ToUIImage:bitmap withWidth:width withHeight:height];

        // Cleanup
        if(bitmap) {
            free(bitmap);   
            bitmap = NULL;
        }

        // Display the image copy on the GUI
        UIImageView *imageView = [[UIImageView alloc] initWithImage:imageCopy]; // <-- X
        imageView.backgroundColor = [UIColor clearColor];
        CGPoint center = CGPointMake([UIScreen mainScreen].bounds.size.width / 2.0, 
                                     [UIScreen mainScreen].bounds.size.height / 2.0);
        [imageView setCenter:center];
        [window addSubview:imageView];
        [imageView release];

    }


    [window makeKeyAndVisible];

    return YES;
}

If I switch the line marked…

// <-- X 

to say…

... initWithImage:image]; // instead of imageCopy

… it displays as it should:

Is anyone up having a crack at this? It looks a really nice library to use, but I can’t figure out where the problem is.

Answer 1

I just got a reply from the author:

At the bottom of his blog post, http://paulsolt.com/2010/09/ios-converting-uiimage-to-rgba8-bitmaps-and-back/ Scott Davies presents a fix:

There’s a simple one-line fix for your code so that alpha channels are
preserved when you go from the byte buffer to the UIImage. In
convertBitmapRGBA8ToUIImage, change this:

CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;

to this:

CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault |
kCGImageAlphaPremultipliedLast;