On 4 Dec 2013, at 9:33 pm, Graham Cox <graham....@bigpond.com> wrote:
> But that leaves those annoying cases when you have the whole view to redraw.
> I wondered if it would be worth dividing up the view into rects and rendering
> each one on a separate thread. The problem seems to me to be that they’d all
> be drawing into the same CGContext, and I wonder how well that could work -
> e.g. one thread could set a clip ready for its next drawing operation and
> another could then change that clip so they’d all be tripping over each
> other, even though they were all drawing into a different part of the
> context. If access to the context were synchronised, then that would end up
> serialising all the drawing so there wouldn’t be any gain.
>
> Has anyone trod this path? It would be useful to know whether there’s
> anything that can be done along these lines, because rendering 10,000 or more
> objects is just taking too darn long!
OK, after some thought and a bit of experimentation, I think I’ve got a handle
on this. I just thought I’d share with the list in case anyone is remotely
interested, or has anything to suggest/add/criticise…
Obviously, you cannot share a graphics context across multiple threads for the
reasons I mentioned - a single context cannot be thread safe, by design. But
that doesn’t mean you can’t have a separate context per thread, all drawing
into the same backing store. That was my breakthrough insight I guess, for what
it’s worth.
So proceeding on that basis, I wrote a test view that uses a NSOperationQueue
to dispatch chunks of drawing work by dividing up the view into tiles, creating
a context for each tile (but all drawing into the same window backing store).
As long as you wait for the tiles to all finish, everything works just
tickety-boo.
Here’s the code, which is the -drawRect: method of a view. The view creates an
NSOperationQueue called mDrawingQueue in its -initWithFrame: method.
#define TILE_SIZE NSMakeSize( 100, 100 )
#define DRAW_THREADED 1
- (void) drawRect:(NSRect) dirtyRect
{
[[NSColor whiteColor] set];
NSRectFill( dirtyRect );
// get current context:
CGContextRef ctx = [[NSGraphicsContext currentContext] graphicsPort];
// divide into tiles
NSSize tileSize = TILE_SIZE;
NSRect br = self.bounds;
NSUInteger tx, ty;
tx = ceil(NSWidth( br ) / tileSize.width);
ty = ceil(NSHeight( br ) / tileSize.height);
NSRect tileRect;
tileRect.size = tileSize;
for( NSInteger i = 0; i < ty; ++i )
{
for( NSInteger j = 0; j < tx; ++j )
{
tileRect.origin.x = br.origin.x + tileSize.width * j;
tileRect.origin.y = br.origin.y + tileSize.height * i;
if([self needsToDrawRect:tileRect])
{
#if DRAW_THREADED
NSBlockOperation* op = [NSBlockOperation
blockOperationWithBlock:^
{
NSGraphicsContext* context =
[NSGraphicsContext graphicsContextWithWindow:[self window]];
CGContextRef ncx = [context
graphicsPort];
// align and clip this new context to
the view
CGAffineTransform ctm =
CGContextGetCTM( ctx );
CGContextConcatCTM( ncx, ctm );
CGContextClipToRect( ncx, br );
// also clip to the tile (not strictly
necessary)
CGContextClipToRect( ncx, tileRect );
[self drawTile:tileRect inContext:ncx];
}];
[mDrawingQueue addOperation:op];
#else
[self drawTile:tileRect inContext:ctx];
#endif
}
}
}
#if DRAW_THREADED
// need to wait until tiles all drawn before flushing
[mDrawingQueue waitUntilAllOperationsAreFinished];
#endif
}
- (void) drawTile:(NSRect) tile inContext:(CGContextRef) ctx
{
// draw something in the tile. Not very challenging at the moment
NSRect tr = NSInsetRect( tile, 10, 10 );
CGContextSetFillColorWithColor( ctx, [[NSColor redColor] CGColor]);
CGContextFillRect( ctx, tr );
CGContextSetStrokeColorWithColor(ctx, CGColorGetConstantColor(
kCGColorBlack ));
CGContextStrokeRect( ctx, tile );
}
_______________________________________________
Cocoa-dev mailing list (Cocoa-dev@lists.apple.com)
Please do not post admin requests or moderator comments to the list.
Contact the moderators at cocoa-dev-admins(at)lists.apple.com
Help/Unsubscribe/Update your Subscription:
https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com
This email sent to arch...@mail-archive.com
