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Tips & Tidbits

MacTutor has a new column called “Tips and Tidbits”. This column is your opportunity to spread the word about little bits of information that you find out about. These tidbits can be programming related or they can be user tips that are particularly useful to programmers.

Tip of the Month

If you are working with offscreen GWorlds and you have set the current GWorld to something via SetGWorld, you need to use LockPixels on the GWorld’s PixMap before using GetCPixel or SetCPixel or anything else that directly accesses the PixMap’s baseAddr. It turns out that the baseAddr of a PixMap is actually a Handle, and LockPixels locks the handle and places its dereferenced value into baseAddr. UnlockPixels unlocks the handle and places the Handle back into baseAddr. So:

GWorldPtr       myOffscreenGWorld, saveWorld;
PixMapHandle    myOffscreenPixMap;
GDHandle        saveGDH;
RGBColor        myCPix;

BuildMyGWorld( &myOffscreenGWorld);
GetGWorld( &saveWorld, &saveGDH);   // Isn't it nice to save the world?
SetGWorld( myOffscreenGWorld, NULL);
myOffscreenPixMap = GetGWorldPixMap( myOffscreenGWorld);

if (!LockPixels( myOffscreenPixMap)) DoError(); //the PixMap has been 
purged
ModifyTheColorPixel( &myCPix);
SetCPixel( 10, 10, &myCPix);
UnlockPixels( myOffscreenPixMap);

SetGWorld( saveWorld, saveGDH);

- Troy Anderson

New 'STR ' resource

System 7 brings with it an interesting new 'STR ' resource. In earlier systems, 'STR ' resource -16096 contained the computer’s user name (also known as the Chooser name, since you edited the name via the Chooser).

System 7 adds a new resource, designed to go along with the user name. 'STR ' resource -16413 contains the computer’s name. As a default, the computer’s name is user name’s Macintosh. For example, my computer’s name is Dave Mark’s Macintosh.

How about a cdev that allows me to edit my user and computer names under System 7???

- Dave Mark, Arlington, VA

Reading Partial resources

Trying to read a resource that’s too big to fit in memory? You should look into ReadPartialResource() and WritePartialResource(). They are described in Inside Macintosh, Volume VI, Chapter 13, pages 21-23. These routines really come in handy when working with mondo-sized 'snd ' resources.

- Dave Mark, Arlington, VA