Q:
Why are my Core Graphics calls drawing upside down?
A:
Unlike the QuickDraw coordinate space, the Core Graphics
coordinate space is defined with the positive y-axis pointing
up from the lower left corner of the context. This becomes
especially disconcerting when you use CreateCGContextForPort
to do some CG drawing in a QuickDraw CGrafPort.
// Create the CGContext from a given QD port
CGContextRef context;
Rect portRect;
OSStatus err = CreateCGContextForPort( qdPort, &context );
if ( noErr == err )
{
// Adjust for any SetOrigin calls on qdPort
SyncCGContextOriginWithPort( context, qdPort );
// Move the CG origin to the upper left of the port
GetPortBounds( qdPort, &portRect );
CGContextTranslateCTM( context, 0,
(float)(portRect.bottom - portRect.top) );
// Flip the y axis so that positive Y points down
// Note that this will cause text drawn with Core Graphics
// to draw upside down
CGContextScaleCTM( context, 1.0, -1.0 );
// The CG coordinate space now matches the QD coordinate space
// ...
// Do your CG drawing here
// ...
// Release the context now that we are done with it
CGContextRelease( context );
}
// Back to normal QuickDraw drawing
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Listing 1. Setting up the Core Graphics Context
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Luckily, Core Graphics is flexible enough that it allows
several solutions. First, you should use SyncCGContextOriginWithPort
to update the CG context's origin to account for any SetOrigin calls on the QD port.
However, SyncCGContextOriginWithPort does not flip the y-axis for you or move the
origin from the bottom left to the top left. For
that you have two choices: adjust your drawing yourself to
account for the flipped-y and origin difference, or call CGContextTranslateCTM
to move the origin and CGContextScaleCTM to flip
the y axis for all remaining CG calls in that context.
Listing 1 shows how to set up your CGContext so that you
can use Core Graphics calls in a QuickDraw port using the
QuickDraw coordinate system.
[Apr 11 2001]
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