Draw text into CGBitmapContext - core-graphics

Question

I have an app that renders into a UIView's CGContext in drawRect. I also export those renderings using a background renderer. It uses the same rendering logic to render (in faster than real time) into a CGBitmapContext (which I subsequently transform into an mp4 file).
I have noticed that the output video has a number of weird glitches. Such as the image being rotated, weird duplications of the rendered images, random noise, and the timing is also odd.
I'm looking for ways to debug this. For the timing issue, I thought I'd render a string that tells me which frame I'm currently viewing, only to find rendering text into CGContext's not very well documented. In fact, the documentations around much of core graphics is quite unforgiving to some one of my experience.
So specifically, I'd like to know how to render text into a context. If its Core Text, must it inter-operate some how with the core graphics context? And in general, I'd appreciate any tips and advice on doing bitmap rendering and debugging the results.

Answer 1

according another question:
How to convert Text to Image in Cocoa Objective-C
we can use the CTLineDraw to draw the text in a CGBitmapContext
sample code:
NSString* string = #"terry.wang";
CGFloat fontSize = 10.0f;
// Create an attributed string with string and font information
CTFontRef font = CTFontCreateWithName(CFSTR("Helvetica Light"), fontSize, nil);
NSDictionary* attributes = [NSDictionary dictionaryWithObjectsAndKeys:
(id)font, kCTFontAttributeName,
nil];
NSAttributedString* as = [[NSAttributedString alloc] initWithString:string attributes:attributes];
CFRelease(font);
// Figure out how big an image we need
CTLineRef line = CTLineCreateWithAttributedString((CFAttributedStringRef)as);
CGFloat ascent, descent, leading;
double fWidth = CTLineGetTypographicBounds(line, &ascent, &descent, &leading);
// On iOS 4.0 and Mac OS X v10.6 you can pass null for data
size_t width = (size_t)ceilf(fWidth);
size_t height = (size_t)ceilf(ascent + descent);
void* data = malloc(width*height*4);
// Create the context and fill it with white background
CGColorSpaceRef space = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast;
CGContextRef ctx = CGBitmapContextCreate(data, width, height, 8, width*4, space, bitmapInfo);
CGColorSpaceRelease(space);
CGContextSetRGBFillColor(ctx, 1.0, 1.0, 1.0, 1.0); // white background
CGContextFillRect(ctx, CGRectMake(0.0, 0.0, width, height));
// Draw the text
CGFloat x = 0.0;
CGFloat y = descent;
CGContextSetTextPosition(ctx, x, y);
CTLineDraw(line, ctx);
CFRelease(line);