在 iOS 中将 pdf 转换为 png 会导致外观质量大大降低

Question

我正在将 pdf 文件转换为 png 图像，因此我可以将文本添加到 png 然后再转换回 pdf。我正在使用下面的代码转换为 png。这导致外观质量显着下降。有什么办法可以防止或尽量减少质量损失？

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *writeableDBPath = [documentsDirectory stringByAppendingPathComponent:@"sample.pdf"];

CFStringRef path;
CFURLRef url;

path = CFStringCreateWithCString (NULL, [writeableDBPath UTF8String], kCFStringEncodingUTF8);
url = CFURLCreateWithFileSystemPath (NULL, path, kCFURLPOSIXPathStyle, 0);

CGPDFDocumentRef myDocument;

myDocument = CGPDFDocumentCreateWithURL(url);

UIGraphicsBeginImageContext(CGSizeMake(612,792));
CGContextRef currentContext = UIGraphicsGetCurrentContext();

CGContextTranslateCTM(currentContext, 10, 792); //596,842 //640x960, 
CGContextScaleCTM(currentContext, 1.0, -1.0); // make sure the page is the right way up

CGPDFPageRef page = CGPDFDocumentGetPage (myDocument, 1); // first page of PDF is page 1 (not zero)
CGContextDrawPDFPage (currentContext, page);  // draws the page in the graphics context

UIImage* image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

NSString* imagePath = [documentsDirectory stringByAppendingPathComponent: @"test.png"];
[UIImagePNGRepresentation(image) writeToFile: imagePath atomically:YES];

Answer 1

您可以尝试增加要渲染到的图像上下文的大小：

UIGraphicsBeginImageContext(CGSizeMake(1224,1584));
...
CGContextTranslateCTM(currentContext, 10, 1584);

但你真的应该像这样使用PDF context：

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *writeableDBPath = [documentsDirectory stringByAppendingPathComponent:@"sample.pdf"];

CFStringRef path;
CFURLRef url;

path = CFStringCreateWithCString (NULL, [writeableDBPath UTF8String], kCFStringEncodingUTF8);
url = CFURLCreateWithFileSystemPath (NULL, path, kCFURLPOSIXPathStyle, 0);

CGPDFDocumentRef myDocument;
myDocument = CGPDFDocumentCreateWithURL(url);

// Create URL for PDF file
NSString *filename = @"sample-out.pdf";
NSURL *outputURL = [NSURL fileURLWithPathComponents:[NSArray arrayWithObjects:documentsDirectory, filename, nil]];

// Create PDF context
CGContextRef pdfContext = CGPDFContextCreateWithURL((CFURLRef)outputURL, NULL, NULL);
CGPDFContextBeginPage(pdfContext, NULL);
UIGraphicsPushContext(pdfContext);

// Flip coordinate system
CGRect bounds = CGContextGetClipBoundingBox(pdfContext);
CGContextScaleCTM(pdfContext, 1.0, -1.0);
CGContextTranslateCTM(pdfContext, 0.0, -bounds.size.height);

CGContextDrawPDFPage (UIGraphicsGetCurrentContext(), CGPDFDocumentGetPage (myDocument, 1));
// Drawing commands
[@"Hello World!" drawAtPoint:CGPointMake(0, 0) withFont:[UIFont boldSystemFontOfSize:72.0f]];
// Clean up
UIGraphicsPopContext();
CGPDFContextEndPage(pdfContext);
CGPDFContextClose(pdfContext);