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TECHNOTE 1020
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FEBRUARY 1996
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by Alan Mimms
revised by Michael Marinkovich
marink@applelink.apple.com
Apple Developer Technical
Support (DTS)
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 two
common problems.
CONTENTS
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.
Further Reference
For more information, see Inside Macintosh: Imaging with
QuickDraw, "Cursor Utilities."
Change History
This Technote was originally written in June, 1989.
Since June, 1989, a warning has been added about purgeable 'clut' resources.
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[last modified by fch on 1-Feb-96]
Using Color Cursors