Drawing Pixels in a BitmapContext is very slow. Need Help.

[Raphael]

Hello!

I'm pretty new to the iPhone developing subject.
My idea is to draw pixels on the screen. Actually I got it working but it is very slow. In the simulator it is ok, but on the iPhone I got around three frames per second or so.

Right now I'm just changing three bytes in the buffer from my BitmapContext every time the drawRect method runs. But I guess it is drawing a complete new image.

Do you guys have some ideas on how to optimize this code?
Is there a way of avoiding creating a complete new image and drawing it into the graphics context?

PHP Code:

//
//  DrawingView.m
//

#import "DrawingView.h"


@implementation DrawingView


@synthesize firstTouch;
@synthesize lastTouch;

- (id)initWithCoder:(NSCoder*)coder
{
     if ( ( self = [super initWithCoder:coder] ) )
     {
          firstRun = TRUE;
          [self setupBitmapContext];
     }
     
     return self;
}


- (void)drawRect:(CGRect)rect
{
     CGContextRef myContext = UIGraphicsGetCurrentContext();

     if (bitmapData != NULL)
     {
          int posY = MAX(0, round( pixelsHigh - lastTouch.y - 1.0));
          int posX = round( lastTouch.x );
          int offset = 4 * ( ( pixelsWide * posY ) + posX );
          bitmapData[offset+1] = 255;
          bitmapData[offset+2] = 255;
          bitmapData[offset+3] = 255;
     }
     
     bitmapImage = CGBitmapContextCreateImage (bitmapContext);
     CGContextDrawImage(myContext, bitmapRect, bitmapImage);
}


-(void)setupBitmapContext
{
     pixelsWide = 320;
     pixelsHigh = 400;
     bitmapBytesPerRow   = (pixelsWide * 4);
     bitmapByteCount     = (pixelsWide * pixelsHigh * 4);
     colorSpace = CGColorSpaceCreateDeviceRGB();
     if (colorSpace == NULL)     NSLog(@"Error allocating color space");
     
     bitmapData = malloc( bitmapByteCount );
     if (bitmapData == NULL)
     {
          NSLog(@"Memory not allocated!");
          CGColorSpaceRelease( colorSpace );
     }
     
     bitmapContext = CGBitmapContextCreate (bitmapData,
                                         pixelsWide,
                                                  pixelsHigh,
                                                  8,
                                                  bitmapBytesPerRow,
                                                  colorSpace,
                                                  kCGImageAlphaPremultipliedFirst);
     if (bitmapContext == NULL)
     {
          free (bitmapData);
          NSLog(@"Context not created!");
     }
     
     bitmapRect = CGRectMake(0.0, 0.0, pixelsWide, pixelsHigh-3);
}


- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
     UITouch *touch = [touches anyObject];
     firstTouch = [touch locationInView:self];
     lastTouch = [touch locationInView:self];
     [self setNeedsDisplay];
}


- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
     UITouch *touch = [touches anyObject];
     lastTouch = [touch locationInView:self];

     [self setNeedsDisplay];
}


- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
     UITouch *touch = [touches anyObject];
     lastTouch = [touch locationInView:self];

     [self setNeedsDisplay];
}




- (void)dealloc
{
     [super dealloc];
}

@end 


You can also download this class here: http://www.rwichmann.com/downloads/DrawingView.m


Thank you for every tipp!

Cheers,
Raphael