CODEWITHSUNDEEP
objective-cmacoscocoaquartz-graphicsnsview
Sam
Sam

Reputation: 2707

CGContextShowTextAtPoint renders upside down

I am trying to draw some text via Quartz onto an NSView via CGContextShowTextAtPoint(). This worked well until I overrode (BOOL)isFlipped to return YES in my NSView subclass in order to position the origin in the upper-left for drawing. The text draws in the expected area but the letters are all inverted. I also tried the (theoretically, at least) equivalent of flipping my CGContext and translating by the context's height.

e.x.

// drawRect:
CGContextScaleCTM(theContext, 1, -1);
CGContextTranslateCTM(theContext, 0, -dirtyRect.size.height);

This yields the same result.

Many suggestions to similar problems have pointed to modifying the text matrix. I've set the text matrix to the identity matrix, performed an additional inversion on it, and done both, respectively. All these solutions have lead to even stranger rendering of the text (often just a fragment shows up.)

Another suggestion I saw was to simply steer clear of this function in favor of other means of drawing text (e.x. NSString's drawing methods.) However, this is being done amongst mostly C++ / C and I'd like to stay at those levels if possible.

Any suggestions are much appreciated and I'd be happy to post more code if needed.

Thanks, Sam

Upvotes: 1

Views: 1463

Answers (3)

stigi
stigi

Reputation: 6711

This question has been answered here.

Basically it's because the coordinate system on iOS core graphics is fliped (x:0, y:0 in the top left) opposed to the one on the Mac (where x:0, y:0 is bottom left). The solution for this is setting the text transform matrix like this:

CGContextSetTextMatrix(context, CGAffineTransformMake(1.0,0.0, 0.0, -1.0, 0.0, 0.0));

Upvotes: 2

Sam
Sam

Reputation: 2707

Turns out the answer was to modify the text matrix. The weird "fragments" that were showing up instead of the text was because the font size (set via CGContextSelectFont()) was too small when the "default" text matrix was replaced. The initial matrix had, for some reason, a large scale transform so smaller text sizes looked fine when the matrix was unmodified; when replaced with a inverse scale (1, -1) or an identity matrix, however, they would become unreadably small.

Upvotes: 0

Rob Keniger
Rob Keniger

Reputation: 46020

You need to use the view's bounds rather than the dirtyRect and perform the translation before the scale:

CGContextTranslateCTM(theContext, 0, -NSHeight(self.bounds));
CGContextScaleCTM(theContext, 1, -1);

Upvotes: 1

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