Technical Q&A QD16

CopyBits and Background Printing

Q: When I use CopyBits to move a cGrafPort's portPixMap to another cGrafPort (my printing port), it works like a charm when background printing is turned on, but when CopyBits gets called with background printing turned off, the image that prints isn't the image at all. Why is this happening?

A: You should be aware that since you're copying the pixels directly from the screen, the baseAddr pointer for the screen's pixMap may be 32-bit addressed. In fact, with 32-Bit QuickDraw, this is the case. This in itself isn't a problem, since CopyBits knows enough to access the baseAddr of the port's pixMap in 32-bit mode, as follows:

mode = true32b;             // Make sure we're in 32-bit addressing mode. 
                            // Access pixels directly; make no other system calls. 
SwapMMUMode(&mode);         // Restore the current mode.

That's how you'd normally handle things if you were accessing the pixels directly yourself. Unfortunately, the LaserWriter driver doesn't know enough to do the SwapMMUMode and instead ends up copying garbage (from a 32-bit pointer stripped to a 24-bit pointer).

So why does background printing work? Because when you print in the background, everything is rolled into a PICT, which the driver saves off for PrintMonitor. Since the driver is using the standard QuickDraw picture bottlenecks to do this, and CopyBits knows to swap the MMU mode before copying the data into the picture, everything works great. Later, at PrintMonitor time, the picture is played back. Since the data is no longer 32-bit addressed, the LaserWriter driver doesn't have to call SwapMMUMode to do the right thing; it can just play the picture back.

The solution we propose for you is something similar. At print time (before your PrOpenPage call), call OpenPicture, copy the data from the screen with CopyBits, call ClosePicture, and then call DrawPicture within your PrOpenPage/PrClosePage loop. That should do the trick.

Note that copying bits directly from the screen is not something we recommend. Unless you have no alternative, you should always copy from the original source of the data instead.