Undo state of bitmap (CGContext) - objective-c

Question

I'm making a multiplayer game which involves drawing lines. Now i'm trying to implement online multiplayer into the game. However I've had some doing struggle doing this. The thing is that I will need to reverse the state of the draw lines in case a packet from the server comes late to the client. I've searched here on stack overflow but haven't found any real answer how to "undo" a bitmap context. The biggest problem is that the drawing needs to be done very fast since the game updates every 20th millisecond. However I figured out and tried some different approaches to this:
Save the state of the whole context and then redraw it. This is probably the slowest method.
Only save a part of the context (100x100) in a another bitmap hidden by looping through each pixel, then looping through each pixel from that bitmap to the main bitmap that is shown on the screen.
Save each point of the drawn path in a CGMutablePath ref, then when reverting the context, draw this path with a transparent color (0,0,0,0).
Saving the position in the bitmap of each pixel that gets drawn in a separate array and then setting that pixel alpha to 0 (in the drawn bitmap) when I need to undo.
The last approach should be the fastest of them all. However, I'm not sure how I can get the position of each drawn pixel unless i do it completely manual by. Right now I uses this code to drawn lines.
CGContextSetStrokeColorWithColor(cacheContext, [color CGColor]);
CGContextSetLineCap(cacheContext, kCGLineCapRound);
CGContextSetLineWidth(cacheContext, 6+thickness);
CGContextBeginPath(cacheContext);
CGContextMoveToPoint(cacheContext, point1.x, point1.y);
CGContextAddLineToPoint(cacheContext, point2.x, point2.y);
CGContextStrokePath(cacheContext);
CGRect dirtyPoint1 = CGRectMake(point1.x-10, point1.y-10, 20, 20);
CGRect dirtyPoint2 = CGRectMake(point2.x-10, point2.y-10, 20, 20);
[self setNeedsDisplayInRect:CGRectUnion(dirtyPoint1, dirtyPoint2)];
Here is how the CGBitmapcontext is setup
- (BOOL) initContext:(CGSize)size {
scaleFactor = [[UIScreen mainScreen] scale];
// scaleFactor = 1;
//non-retina
// scalefactor = 2; retina
int bitmapBytesPerRow;
// Declare the number of bytes per row. Each pixel in the bitmap in this
// example is represented by 4 bytes; 8 bits each of red, green, blue, and
// alpha.
bitmapBytesPerRow = (size.width * 4*scaleFactor);
bitmapByteCount = (bitmapBytesPerRow * (size.height*scaleFactor));
// Allocate memory for image data. This is the destination in memory
// where any drawing to the bitmap context will be rendered.
cacheBitmap = malloc( bitmapByteCount );
if (cacheBitmap == NULL){
return NO;
}
CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little;
colorSpace = CGColorSpaceCreateDeviceRGB();
cacheContext = CGBitmapContextCreate (cacheBitmap, size.width*scaleFactor, size.height *scaleFactor, 8, bitmapBytesPerRow, colorSpace, bitmapInfo);
CGContextScaleCTM(cacheContext, scaleFactor, scaleFactor);
CGColorSpaceRelease(colorSpace);
CGContextSetRGBFillColor(cacheContext, 0, 0, 0, 0.0);
CGContextFillRect(cacheContext, (CGRect){CGPointZero, CGSizeMake(size.height*scaleFactor, size.width*scaleFactor)});
return YES;
}
Is there anyway other better way to undo the bitmap? If not, how can I get the positions of each pixels that gets draw with core graphics? Is this even possible?

Answer 1

Your 4th approach will either duplicate the whole canvas bitmap (should you consider a flat NxM matrix representation) or result a performance mess in case of map-based structure or something like that.
Actually, I believe 2nd way does the trick. I have had implemented that way of undo few times during past years, including a DirectX-based drawing app with some 25-30fps rendering pipeline.
However, your #2 description has a strange mention of some "loop" you want to perform across the area. You do not need a loop, what you need is a proper API method for copying a portion of the bitmap/graphics context, it might be CGContextDrawImage used to preserve your canvas portion and same method to undo/redo the drawing.