Mac Forum / Programming / Mac Programming / December 2005

I am porting a specialized video plugin to OS/X.  This is a
multithreaded plugin that runs under Safari and Firefox, using the
Netscape API.

To render the video, I have a YUv image that I blit to a graphics port
using the following code snippet:

CodecFlags    returnedCodecFlags;
CGrafPtr        prevGrafPort;
Rect saved_rect, check;
RgnHandle saved_clip;

saved_clip = NewRgn();

// save the current port, port bounds, and clipping rectangle
GetPort (&prevGrafPort);
GetPortBounds(m_port, &saved_rect);
GetClip(saved_clip);

// Make sure (0,0) is at the left-top of the browser window.
SetPort (m_port);
LockPortBits(m_port);  // doc says not to use this with QT, but has no
obvious effect
SetOrigin (0,0);
ClipRect(&m_clipping_rect);

OSErr err = DecompressSequenceFrameWhen(m_sequence, (Ptr) image,
            sizeof(PlanarPixmapInfoYUV420), 0, &returnedCodecFlags, nil);

GetPortBounds(m_port, &check);
if (check.left != 0 || check.top != 0)
    Log("Warning: origin moved to %dx%d during draw operation\n",
                    check.left, check.top);

QDFlushPortBuffer(m_port, NULL);

// Restore previous settings
SetOrigin( saved_rect.left, saved_rect.top );
SetClip( saved_clip );

UnlockPortBits(m_port);
SetPort (prevGrafPort);

The problem is, there doesn't seem to be any way to lock the port from
being simultaneously drawn in another thread, and hence the origin
moves around during my drawing operation (most noticably on a dual-CPU
Mac).

Although we wrote our code independently, in researching this problem I
found that the open-source MPEG player VLC has the same issue (it even
has a comment in the code asking about a lock).

So -- my questions:

1. Is it possible to somehow lock a port so that between a call to
SetOrigin and another call (in this case DecompresSequenceFrameWhen),
the origin won't change?

2. Alternately -- is it possible to somehow get my own port which is a
child of the existing port handed in by the browser?  How would I go
about this?

Thanks very much for your help,
--Howdy Pierce
 howdy@cardinalpeak.com
 www.cardinalpeak.com