Technotes
Pictures and Clip Regions
| Technote QD 11 | January 1986 |
Revised by: March 1988
Written by: Jennifer Jernigan January 1986
This note describes a problem that affects creation of QuickDraw pictures.
When a GrafPort is created, the fields in the GrafPort are
given default values; one of these is the clip region, which is set to the
rectangle (-32767, -32767, 32767, 32767). If you create a picture, then call
DrawPicture with a destination rectangle that is not the same size as
the picFrame without ever changing the default clip region, nothing
will be drawn.
When the picture frame is compared with the destination rectangle and the
picture is scaled, the clip region is scaled too. In the process of scaling,
the clip region you end up overflows and becomes empty, and your picture
doesn't get drawn. If you call ClipRect(thePort^.portRect) before you
record the picture, the picture will be drawn correctly. The clipping on the
destination port when playing back the picture is irrelevant: once a picture is
incorrectly recorded, it is too late.
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