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Technical Note TN1020
Color Cursing: Two Major Causes

CONTENTS

The creation and use of color cursors is discussed in the chapter on cursor utilities in Inside Macintosh: Imaging with QuickDraw.

If you're building an application that uses color cursors, you may encounter some quirks present in Color QuickDraw. This Technote provides solutions to a few common problems.

 [Dec 23 1997]






Using Color Cursors

To avoid system errors or crashes while using a color cursor, incorporate the following information in your application development.

Setting Bounds for your Cursor

If your cursor is, for example, 15 pixels tall and 9 pixels wide, you might be tempted to use these values for the bounds.bottom and bounds.right, respectively, in your cursor's pixel map. Don't. When the cursor's image needs to be expanded (that is, when you specify a two bit-per-pixel cursor and the mouse pointer is on an eight-bit screen) the SetCCursor trap rounds the width of the pixel map in such a way that you'll get only the space required for a 15 by 8 pixel map allocated for the expanded cursor data. When the cursor's image is expanded into this too-small expanded cursor data handle as a 15 by 9 pixel map, something in your heap will get munched.

To avoid this problem, always specify the pixmapHandle^^.bounds to be 16 by 16. This will cause SetCCursor to properly allocate the expanded data area. Since the amount of data drawn for a cursor is specified by the cursor's pixel values and 'clut' resource, trying to save a few bytes by making the bounds rectangle smaller than 16 by 16 won't be very helpful anyway.

Inopportune Purging of a `CLUT' resource

If you load a color cursor's color table from a 'clut' resource using GetCTable, make sure that the 'clut' is marked non-purgeable while the color cursor is in use. If you don't take this precaution, bombs will occur if your 'clut' gets purged at an inopportune time.

For more information, see Inside Macintosh: Imaging with QuickDraw, "Cursor Utilities."

Calling SetCCursor can change the GDevice

Starting with System 7.5.2, the cursor images are copied into offscreen GWorlds via CopyBits. To do this, SetCCursor saves and restores the port and device using a fairly standard set of code:

{
  GetGWorld (&savePort, &saveDevice);
  SetGWorld (offPort, NULL);
...copybits and other stuff goes here

  SetGWorld (savePort, saveDevice);

}

A documented side effect of SetGWorld is that the device parameter will be ignored when you pass in a GWorld port. So, if the current port is a GWorld port, the save-restore code in SetCCursor will always make the current device that of the GWorld.

One place where this is especially dangerous is when the current port is a disposed GWorld. DisposeGWorld sets the current GDevice to a safe device, but doesn't affect the current port. If you then call SetCCursor before a valid port has been set, then SetCCursor will cause the current GDevice to point at the disposed GDevice, and the system will crash shortly afterwards.

The easiest way to avoid this side-effect of SetCCursor is to:

  • Ensure that your port and device always match. Never allow a situation where the port and device can be mismatched. This helps cure many problems unrelated to color cursors.
  • Never dispose of the current port without setting a valid QuickDraw port first. Drawing to a disposed port or device will most likely result in a crash.

Don't Call SetCCursor at Interrupt Time

SetCCursor is documented as moving memory. In 7.5.2 and later, SetCCursor makes calls to CopyBits and other QuickDraw routines. Never, ever call SetCCursor at interrupt time! Calling SetCursor at interrupt time is still allowed, however.

References

Inside Macintosh: Imaging with QuickDraw

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Change History

01-June-1989

Originally written.

01-February-1996

The warning about purgeable 'clut' resources added.

23-December-1997

Warnings about SetCCursor changing the GDevice, and SetCCursor moving memory added.

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