c++macosqtcore-graphics

How to convert a CGImageRef to a QPixmap in Qt 6?


In Qt 5, converting a CGImageRef to a QPixmap was easy using their QtMacExtras module and the QtMac::fromCGImageRef function.

In Qt 6, QtMacExtras has been removed. Most of the functions there have explicit replacements, except for QtMac::fromCGImageRef which was removed "due to lack of known clients of the API".

Well it turns out I was a client of that API, just not a known one. Nonetheless, now I need to convert a CGImageRef to a QPixmap in Qt 6.

I looked at the source code for QtMac::fromCGImageRef in Qt 5.15.2, which lead me to a Qt internal function named qt_mac_toQImage. However, it makes use of a ton of internal functions and classes that I don't have access to in Qt 5, much less Qt 6, so that doesn't help.

How can I go about doing this conversion?


Solution

  • It turns out that a lot of the private Qt stuff used in the Qt 5.15 implementation of qt_mac_toQImage applied to situations other than attempting to do CoreGraphics rendering to a QImage. Once I pulled all of those parts out and simplified, the method for converting a CGImageRef to a QImage or QPixmap is fairly straight forward:

    CGBitmapInfo CGBitmapInfoForQImage(const QImage &image)
    {
        CGBitmapInfo bitmapInfo = kCGImageAlphaNone;
        
        switch (image.format()) {
            case QImage::Format_ARGB32:
                bitmapInfo = kCGImageAlphaFirst | kCGBitmapByteOrder32Host;
                break;
            case QImage::Format_RGB32:
                bitmapInfo = kCGImageAlphaNoneSkipFirst | kCGBitmapByteOrder32Host;
                break;
            case QImage::Format_RGBA8888_Premultiplied:
                bitmapInfo = kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big;
                break;
            case QImage::Format_RGBA8888:
                bitmapInfo = kCGImageAlphaLast | kCGBitmapByteOrder32Big;
                break;
            case QImage::Format_RGBX8888:
                bitmapInfo = kCGImageAlphaNoneSkipLast | kCGBitmapByteOrder32Big;
                break;
            case QImage::Format_ARGB32_Premultiplied:
                bitmapInfo = kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Host;
                break;
            default:
                break;
        }
        
        return bitmapInfo;
    }
    
    QImage CGImageToQImage(CGImageRef cgImage)
    {
        const size_t width = CGImageGetWidth(cgImage);
        const size_t height = CGImageGetHeight(cgImage);
        QImage image(width, height, QImage::Format_ARGB32_Premultiplied);
        image.fill(Qt::transparent);
        
        CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceSRGB);
        CGContextRef context = CGBitmapContextCreate((void *)image.bits(), image.width(), image.height(), 8,
                                                     image.bytesPerLine(), colorSpace, CGBitmapInfoForQImage(image));
    
        // Scale the context so that painting happens in device-independent pixels
        const qreal devicePixelRatio = image.devicePixelRatio();
        CGContextScaleCTM(context, devicePixelRatio, devicePixelRatio);
        
        CGRect rect = CGRectMake(0, 0, width, height);
        CGContextDrawImage(context, rect, cgImage);
        
        CFRelease(colorSpace);
        CFRelease(context);
        
        return image;
    }
    

    Once you've converted a CGImageRef to a QImage, it's simple converting that to a QPixmap using QPixmap::convertFromImage.