Code to demonstrate that the backside image option for the CIPageCurlTransition filter doesn't work.

main.m

//  main.m
//  pagecurlbacksidetest
//
//  Created by Kevin Meaney on 30/03/2014.

@import Foundation;
@import QuartzCore;

size_t const kWidth = 600;
size_t const kHeight = 400;
size_t const kBitsPerComponent = 8;
size_t const kBytesPerPixel = 4;

CGRect kBoundsRect = { 0.0, 0.0, kWidth, kHeight };
CGRect innerRect = { kWidth / 4.0, kHeight / 4.0, kWidth / 2.0, kHeight / 2.0 };

CGPoint trianglePoint1 = { 100, 350 };
CGPoint trianglePoint2 = { 300, 50 };
CGPoint trianglePoint3 = { 500, 350 };

CGBitmapInfo const bitmapInfo = (CGBitmapInfo)kCGImageAlphaPremultipliedFirst;


CGContextRef CreateMyContext()
{
    CGColorSpaceRef colorSpace;
    colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
    
    CGContextRef context = CGBitmapContextCreate(NULL, kWidth, kHeight,
                                                 kBitsPerComponent,
                                                 kWidth * kBytesPerPixel,
                                                 colorSpace,
                                                 bitmapInfo);
    CGColorSpaceRelease(colorSpace);
    return context;
}

CGImageRef DrawFillOvalAndCreateImage(CGContextRef context)
{
    // Draw red oval on a blue background.
    CGColorRef redColor = CGColorCreateGenericRGB(1.0, 0.0, 0.0, 1.0);
    CGColorRef blueColor = CGColorCreateGenericRGB(0.0, 0.0, 1.0, 1.0);
    
    CGContextSetFillColorWithColor(context, blueColor);
    CGContextFillRect(context, kBoundsRect);
    CGContextSetFillColorWithColor(context, redColor);
    CGContextFillEllipseInRect(context, innerRect);
    CGColorRelease(redColor);
    CGColorRelease(blueColor);
    CGImageRef image = CGBitmapContextCreateImage(context);
    return image;
}

CGImageRef DrawFillRectAndCreateImage(CGContextRef context)
{
    // Draw green oval on a red background.
    CGColorRef redColor = CGColorCreateGenericRGB(1.0, 0.0, 0.0, 1.0);
    CGColorRef greenColor = CGColorCreateGenericRGB(0.0, 1.0, 0.0, 1.0);

    CGContextSetFillColorWithColor(context, redColor);
    CGContextFillRect(context, kBoundsRect);
    CGContextSetFillColorWithColor(context, greenColor);
    CGContextFillRect(context, innerRect);

    CGColorRelease(redColor);
    CGColorRelease(greenColor);
    CGImageRef image = CGBitmapContextCreateImage(context);
    return image;
}

CGImageRef DrawFillTriangleAndCreateImage(CGContextRef context)
{
    // Draw blue triangle on a green bacgkround.
    CGColorRef blueColor = CGColorCreateGenericRGB(1.0, 0.0, 0.0, 1.0);
    CGColorRef greenColor = CGColorCreateGenericRGB(0.0, 1.0, 0.0, 1.0);

    CGContextSetFillColorWithColor(context, greenColor);
    CGContextFillRect(context, kBoundsRect);
    CGContextSetFillColorWithColor(context, blueColor);
    CGContextBeginPath(context);
    CGContextMoveToPoint(context, trianglePoint1.x, trianglePoint1.y);
    CGContextAddLineToPoint(context, trianglePoint2.x, trianglePoint2.y);
    CGContextAddLineToPoint(context, trianglePoint3.x, trianglePoint3.y);
    CGContextClosePath(context);
    CGContextFillPath(context);
    
    CGColorRelease(blueColor);
    CGColorRelease(greenColor);
    CGImageRef image = CGBitmapContextCreateImage(context);
    return image;
}

void SaveCGImageToAPNGFile(CGImageRef theImage, NSString *path)
{
    NSString *expandedPath = [path stringByExpandingTildeInPath];
    NSURL *fileURL = [[NSURL alloc] initFileURLWithPath:expandedPath];
    CGImageDestinationRef exporter = CGImageDestinationCreateWithURL(
                                                 (__bridge CFURLRef)fileURL,
                                                 kUTTypePNG, 1, NULL);
    CGImageDestinationAddImage(exporter, theImage, nil);
    CGImageDestinationFinalize(exporter);
    CFRelease(exporter);
}

void SaveCGBitmapContextToAPNGFile(CGContextRef context, NSString *path)
{
    CGImageRef image = CGBitmapContextCreateImage(context);
    SaveCGImageToAPNGFile(image, path);
    CGImageRelease(image);
}

int main(int argc, const char * argv[])
{
    if (argc < 2)
    {
        printf("Please provide a path to where you want to save public.png image.");
        return 1;
    }
    @autoreleasepool
    {
        // Create the context and the images.
        CGContextRef myContext = CreateMyContext();
        CGImageRef sourceImage = DrawFillOvalAndCreateImage(myContext);
        CGImageRef destinationImage = DrawFillRectAndCreateImage(myContext);
        CGImageRef backsideImage = DrawFillTriangleAndCreateImage(myContext);
        
        // Set up the radial gradient and crop filters for the inputShadingImage
        CGFloat diam = 300.0;
        CGFloat radius = diam * 0.5;
        CIFilter *radialGradientFilter = [CIFilter filterWithName:@"CIRadialGradient"];
        [radialGradientFilter setDefaults];
        CIVector *centerVector = [CIVector vectorWithX:radius Y:radius];
        [radialGradientFilter setValue:centerVector forKey:@"inputCenter"];
        
        // inner circle is 4% of the size of the outer one.
        [radialGradientFilter setValue:@(radius * 0.04) forKey:@"inputRadius0"];
        [radialGradientFilter setValue:@(radius) forKey:@"inputRadius1"];
        CIColor *color0 = [CIColor colorWithRed:1.0 green:1.0 blue:1.0 alpha:0.3];
        [radialGradientFilter setValue:color0 forKey:@"inputColor0"];
        CIColor *color1 = [CIColor colorWithRed:0.0 green:0.0 blue:0.0 alpha:0.7];
        [radialGradientFilter setValue:color1 forKey:@"inputColor1"];
        
        CIFilter *cropFilter = [CIFilter filterWithName:@"CICrop"];
        [cropFilter setDefaults];
        CIVector *extentVector;
        extentVector = [CIVector vectorWithX:0.0 Y:0.0 Z:diam W:diam];
        [cropFilter setValue:extentVector forKey:@"inputRectangle"];
        [cropFilter setValue:[radialGradientFilter valueForKeyPath:@"outputImage"]
                      forKey:@"inputImage"];

        // OK, the radial gradient fill and crop filters have been setup so that
        // we have generated a shading input image for the CIPageCurlTransition
        // filter.
        CIFilter *pageCurlFilter = [CIFilter filterWithName:@"CIPageCurlTransition"];
        [pageCurlFilter setDefaults];
        [pageCurlFilter setValue:@(0.5) forKey:@"inputTime"];
        CIImage *inputImage = [CIImage imageWithCGImage:sourceImage];
        [pageCurlFilter setValue:inputImage forKey:@"inputImage"];
        CIImage *inputTargetImage = [CIImage imageWithCGImage:destinationImage];
        [pageCurlFilter setValue:inputTargetImage forKey:@"inputTargetImage"];
        CIImage *inputBacksideImage = [CIImage imageWithCGImage:backsideImage];
        [pageCurlFilter setValue:inputBacksideImage forKey:@"inputBacksideImage"];
        CIImage *inputShadingImage = [cropFilter valueForKeyPath:@"outputImage"];
        [pageCurlFilter setValue:inputShadingImage forKey:@"inputShadingImage"];
        // [pageCurlFilter setValue:inputShadingImage forKey:@"inputBacksideImage"];
        CIVector *fullExtentVector;
        fullExtentVector = [CIVector vectorWithX:0.0 Y:0.0 Z:kWidth W:kHeight];
        [pageCurlFilter setValue:fullExtentVector forKey:@"inputExtent"];
        [pageCurlFilter setValue:@(-2.7) forKey:@"inputAngle"];
        [pageCurlFilter setValue:@(120) forKey:@"inputRadius"];
        
        // So the page Filter has been setup. Now we want to create a CIContext
        // and draw the output CIImage from the pageCurlFilter using the CIContext
        // into myContext and CGContextRef context.
        
        CIContext *ciContext = [CIContext contextWithCGContext:myContext options:nil];
        [ciContext drawImage:[pageCurlFilter valueForKey:@"outputImage"]
                      inRect:kBoundsRect
                    fromRect:kBoundsRect];
        
        // Now need to save the image to disk.
        const char * filePath = argv[1];
        SaveCGBitmapContextToAPNGFile(myContext, @(filePath));
        CGContextRelease(myContext);
        CGImageRelease(sourceImage);
        CGImageRelease(destinationImage);
        CGImageRelease(backsideImage);
    }
    return 0;
}

The problem is that the inputBacksideImage value option for the CIPageCurlTransition filter doesn't appear to work.

true