Inside Macintosh: Imaging with QuickDraw
You must call the LockPixels function before drawing to or copying from an offscreen graphics world. The LockPixels function prevents the base address for an offscreen pixel image from being moved while you draw into it or copy from it.
If the base address for an offscreen pixel image hasn't been purged by the Memory Manager or if its base address is not purgeable, LockPixels returns TRUE as its function result, and your application can draw into or copy from the offscreen pixel image. However, if the base address for an offscreen pixel image has been purged, LockPixels returns FALSE to indicate that you cannot draw into it or copy from it. (At that point, your application should either call the UpdateGWorld function to reallocate the offscreen pixel image and then reconstruct it, or draw directly into an onscreen graphics port.)
After setting the offscreen graphics world to the current graphics port, MyPaintRectsThruGWorld in Listing 6-1 uses the GetGWorldPixMap function to get a handle to an offscreen pixel map. Passing this handle to the LockPixels function, MyPaintRectsThruGWorld locks the memory for the offscreen pixel image in preparation for drawing into its pixel map.
On a system running only basic QuickDraw, the GetGWorldPixMap function returns the handle to a 1-bit pixel map that your application can supply as a parameter to LockPixels and the other routines related to offscreen graphics worlds that are described in this chapter. On a basic QuickDraw system, however, your application should not supply this handle to Color QuickDraw routines.
The MyPaintRectsThruGWorld routine initializes the offscreen pixel image to all white by calling the EraseRect procedure, which is described in the chapter "Basic QuickDraw." The MyPaintRectsThruGWorld routine then calls another application-defined routine, MyPaintAndFillColorRects , to draw color rectangles into the pixel map for the offscreen graphics world.
You cannot dereference the GWorldPtr data structure to get to the pixel map. The baseAddr field of the PixMap record for an offscreen graphics world contains a handle instead of a pointer, which is what the baseAddr field for an onscreen pixel map contains. You must use the GetPixBaseAddr function (described on GetPixBaseAddr ) to obtain a pointer to the PixMap record for an offscreen graphics world.