Rendering UIImage/CGImage into CGPDFContext results in… blankness!

问题

I'm trying to take an image that I have in a image object and render into a Core Graphics PDF context-- happens to be on an iPhone but this question surely applies equally to desktop Quartz. This UIImage is a simple color-on-white image at about 600x800 resolution. If I (say) turn it into a PNG file, that file looks exactly as expected-- so the data is OK.

Here's what I'm doing to generate the PDF:

NSMutableData * outputData = [[NSMutableData alloc] init];
CGDataConsumerRef dataConsumer = CGDataConsumerCreateWithCFData((CFMutableDataRef)outputData);

CFMutableDictionaryRef attrDictionary = NULL;    
attrDictionary = CFDictionaryCreateMutable(NULL, 0,
                                         &kCFTypeDictionaryKeyCallBacks,
                                         &kCFTypeDictionaryValueCallBacks);
CFDictionarySetValue(attrDictionary, kCGPDFContextTitle, @"My Awesome Document");
CGContextRef pdfContext = CGPDFContextCreate(dataConsumer, NULL, attrDictionary); 
CFRelease(dataConsumer);
CFRelease(attrDictionary);
CGImageRef pageImage = [myUIImage CGImage];
CGPDFContextBeginPage(pdfContext, NULL);
CGContextDrawImage(pdfContext, CGRectMake(0, 0, [myUIImage size].width, [myUIImage size].height), pageImage);
CGPDFContextEndPage(pdfContext);
CGPDFContextClose(pdfContext);
CGContextRelease(pdfContext);

The resulting PDF, which ends up in outputData, seems like a valid PDF file (opens correctly, document title is present in metadata), but it consists of precisely one blank page.

What am I doing wrong?

Thanks.

UPDATE: Ha! This was my fault. My test code for just generating the PNG file went through a different path to get the data. The PDF path indeed was receiving an empty image.

回答1:

I tested your code and it seems to work.
Are you sure your UIImage is valid and not nil when drawing it into the context?
My test method loads a .png from the main bundle and writes the final pdf to a file in the applications document folder:

- (IBAction)outputPDF:(id)sender
{
    NSMutableData* outputData = [[NSMutableData alloc] init];
    CGDataConsumerRef dataConsumer = CGDataConsumerCreateWithCFData((CFMutableDataRef)outputData);
    CFMutableDictionaryRef attrDictionary = NULL;    
    attrDictionary = CFDictionaryCreateMutable(NULL, 0, &kCFTypeDictionaryKeyCallBacks, &kCFTypeDictionaryValueCallBacks);
    CFDictionarySetValue(attrDictionary, kCGPDFContextTitle, @"My Awesome Document");
    CGContextRef pdfContext = CGPDFContextCreate(dataConsumer, NULL, attrDictionary); 
    CFRelease(dataConsumer);
    CFRelease(attrDictionary);
    UIImage* myUIImage = [UIImage imageNamed:@"tmp.png"];
    CGImageRef pageImage = [myUIImage CGImage];
    CGPDFContextBeginPage(pdfContext, NULL);
    CGContextDrawImage(pdfContext, CGRectMake(0, 0, [myUIImage size].width, [myUIImage size].height), pageImage);
    CGPDFContextEndPage(pdfContext);
    CGPDFContextClose(pdfContext);
    CGContextRelease(pdfContext);   
    NSArray* paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString* documentsDirectory = [paths objectAtIndex:0];
    NSString* appFile = [documentsDirectory stringByAppendingPathComponent:@"tmp.pdf"];
    [outputData writeToFile:appFile atomically:YES];
}

回答2:

This issue seems to have been addressed on the iphone dev sdk forums: PDF creation tutorial

All credit goes to danielb21: here is a reprint of their CreatePDFFile method:

// Our method to create a PDF file natively on the iPhone
// This method takes two parameters, a CGRect for size and
// a const char, which will be the name of our pdf file
void CreatePDFFile (CGRect pageRect, const char *filename) {

    // This code block sets up our PDF Context so that we can draw to it
    CGContextRef pdfContext;
    CFStringRef path;
    CFURLRef url;
    CFMutableDictionaryRef myDictionary = NULL;
    // Create a CFString from the filename we provide to this method when we call it
    path = CFStringCreateWithCString (NULL, filename,
                                      kCFStringEncodingUTF8);
    // Create a CFURL using the CFString we just defined
    url = CFURLCreateWithFileSystemPath (NULL, path,
                                         kCFURLPOSIXPathStyle, 0);
    CFRelease (path);
    // This dictionary contains extra options mostly for 'signing' the PDF
    myDictionary = CFDictionaryCreateMutable(NULL, 0,
                                             &kCFTypeDictionaryKeyCallBacks,
                                             &kCFTypeDictionaryValueCallBacks);
    CFDictionarySetValue(myDictionary, kCGPDFContextTitle, CFSTR("My PDF File"));
    CFDictionarySetValue(myDictionary, kCGPDFContextCreator, CFSTR("My Name"));
    // Create our PDF Context with the CFURL, the CGRect we provide, and the above defined dictionary
    pdfContext = CGPDFContextCreateWithURL (url, &pageRect, myDictionary);
    // Cleanup our mess
    CFRelease(myDictionary);
    CFRelease(url);
    // Done creating our PDF Context, now it's time to draw to it

    // Starts our first page
    CGContextBeginPage (pdfContext, &pageRect);

    // Draws a black rectangle around the page inset by 50 on all sides
    CGContextStrokeRect(pdfContext, CGRectMake(50, 50, pageRect.size.width - 100, pageRect.size.height - 100));

    // This code block will create an image that we then draw to the page
    const char *picture = "Picture";
    CGImageRef image;
    CGDataProviderRef provider;
    CFStringRef picturePath;
    CFURLRef pictureURL;

    picturePath = CFStringCreateWithCString (NULL, picture,
                                      kCFStringEncodingUTF8);
    pictureURL = CFBundleCopyResourceURL(CFBundleGetMainBundle(), picturePath, CFSTR("png"), NULL);
    CFRelease(picturePath);
    provider = CGDataProviderCreateWithURL (pictureURL);
    CFRelease (pictureURL);
    image = CGImageCreateWithPNGDataProvider (provider, NULL, true, kCGRenderingIntentDefault);
    CGDataProviderRelease (provider);
    CGContextDrawImage (pdfContext, CGRectMake(200, 200, 207, 385),image);
    CGImageRelease (image);
    // End image code

    // Adding some text on top of the image we just added
    CGContextSelectFont (pdfContext, "Helvetica", 16, kCGEncodingMacRoman);
    CGContextSetTextDrawingMode (pdfContext, kCGTextFill);
    CGContextSetRGBFillColor (pdfContext, 0, 0, 0, 1);
    const char *text = "Hello World!";
    CGContextShowTextAtPoint (pdfContext, 260, 390, text, strlen(text));
    // End text

    // We are done drawing to this page, let's end it
    // We could add as many pages as we wanted using CGContextBeginPage/CGContextEndPage
    CGContextEndPage (pdfContext);

    // We are done with our context now, so we release it
    CGContextRelease (pdfContext);
}

来源：https://stackoverflow.com/questions/2582235/rendering-uiimage-cgimage-into-cgpdfcontext-results-in-blankness