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- To: shen@schemer.Jpl.Nasa.Gov (Sheldon Shen)
- Subject: Re: _DrawPicture
- From: email@example.com
- Date: Fri, 3 Jan 1992 11:31:55 -0500
- Cc: info-mcl
>On page 88 and 89 of Inside Macintosh V, an example is given
>to spool a picture from disk. In that example, QuickDraw
>repeatedly calls GetPICTData, which in turn calls
>I assume that dataPtr is a pointer to the buffer and
>longCount is the number of bytes transferred from the disk
>to the buffer. Therefore, if I increase longCount, I/O can
>be reduced and drawing a picture can be expedited. This
>does not seem to work. Worse, when I set longCount to 1024,
>the system simply crashes.
>(There are two files, "mac-file-io.lisp" and
>"picture-files.lisp", at cambridge.apple.com. These files
>implement the example on page 88 and 89 in MCL. The
>longCount used there is 10.)
>How to speed up _DrawPicture by providing a larger buffer?
Nice idea, but I'm afrad it won't work. The QuickDraw code is
the one asking for the bytes. It asks for a certain number of
bytes to be read into a certain area of memory. Reading more
will be highly unexpected, and will very possibly overwrite
memory that is holding important information (hence the crash).
The relevant code (from "picture-files.lisp") is:
(defpascal *get-pict-data* (:ptr dataPtr :word byteCount)
(FSRead *pict-input-pb* byteCount dataPtr 0 nil))
And inside of OPEN-PICT-INPUT-FILE:
(rset newGrafProcs :QDProcs.getPicProc *get-pict-data*)
The getPicProc field of the QDProcs record is what QuickDraw calls to
get bytes from a picture. It takes two arguments, dataPtr & byteCount:
where to put the bytes and how many to read.
Clear as mud, right?
Bill St. Clair