frey

So, if you happen to be doing what the subject line says – drawing anything into a CGContextRef that was created using CGBitmapContextCreate – and you’re seeing nothing but black, the trick is the following: you have to memset the the pixel data to all 0xFF’s before you draw.

For example, to draw a PDF document reference by ‘document’:

// allocate pixels
unsigned char* pixels = new unsigned char[thumb_height*thumb_width*4];

// create context
CGContextRef context = CGBitmapContextCreate(pixels, thumb_width, thumb_height, 8, 4*thumb_width, CGColorSpaceCreateWithName( kCGColorSpaceGenericRGB ), kCGImageAlphaNoneSkipLast );

// clear
memset( pixels, 255, thumb_width*thumb_height*4 );

// render page 1 into the pixels array
CGPDFPageRef page = CGPDFDocumentGetPage( document, 1 );
CGContextDrawPDFPage( context, page );

(For some reason I just wasn’t able to create any kind of RGBA context with CGBitmapContextCreate() – I always had to use the skip-alpha enum. Weird.)

This entry was posted on Saturday, November 15th, 2008 at 8:23 pm and is filed under code. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.