海神

代码：

    // create a 2 bit CGImage containing a gradient that will be used for masking the 
    // main view content to create the 'fade' of the reflection.  The CGImageCreateWithMask
    // function will stretch the bitmap image as required, so we can create a 1 pixel wide
    // gradient
    CGImageRef gradientMaskImage=AEViewCreateGradientImage(1,height);

    // Create an image by masking the bitmap of the mainView content with the gradient view
    // then release the  pre-masked content bitmap and the gradient bitmap
    CGImageRef reflectionImage=CGImageCreateWithMask(mainViewContentBitmapContext,gradientMaskImage);
    CGImageRelease(mainViewContentBitmapContext);
    CGImageRelease(gradientMaskImage);

    CGImageRef AEViewCreateGradientImage (int pixelsWide,
                                          int pixelsHigh)
    {
        CGImageRef theCGImage = NULL;
        CGContextRef gradientBitmapContext = NULL;
        CGColorSpaceRef colorSpace;
        CGGradientRef grayScaleGradient;
        CGPoint gradientStartPoint, gradientEndPoint;

        // Our gradient is always black-white and the mask
        // must be in the gray colorspace
        colorSpace = CGColorSpaceCreateDeviceGray();

        // create the bitmap context
        gradientBitmapContext = CGBitmapContextCreate (NULL, pixelsWide, pixelsHigh,
                                                       8, 0, colorSpace, kCGImageAlphaNone);

        if (gradientBitmapContext != NULL) {
            // define the start and end grayscale values (with the alpha, even though
            // our bitmap context doesn't support alpha the gradient requires it)
            CGFloat colors[] = {0.0, 1.0,1.0, 1.0,};

            // create the CGGradient and then release the gray color space
            grayScaleGradient = CGGradientCreateWithColorComponents(colorSpace, colors, NULL, 2);

            // create the start and end points for the gradient vector (straight down)
            gradientStartPoint = CGPointZero;
            gradientEndPoint = CGPointMake(0,pixelsHigh);

            // draw the gradient into the gray bitmap context
            CGContextDrawLinearGradient (gradientBitmapContext, grayScaleGradient, gradientStartPoint, gradientEndPoint, kCGGradientDrawsAfterEndLocation);

            // clean up the gradient
            CGGradientRelease(grayScaleGradient);

            // convert the context into a CGImageRef and release the
            // context
            theCGImage=CGBitmapContextCreateImage(gradientBitmapContext);
            CGContextRelease(gradientBitmapContext);

        }

        // clean up the colorspace
        CGColorSpaceRelease(colorSpace);

        // return the imageref containing the gradient
        return theCGImage;
    }