iOS Issue with erase or transparent stroke using CoreGraphics framework

Hi Friends,
In one of my apps, am trying to erase/transparent stroke a part of UIImage which am drawing using CoreGraphics framework (CGContextRef etc..). Well in the process am able to clear the drawing in one shot by calling "removeAllObjects" message. but I could not able to figure it out, how to erase a part of the drawing image. Gosh!! i sat the whole day but still no results, nw its killing me . Please guys help me out from here. In short, my requirement is something like an eraser which can erase a part of of my drawing image. Appreciate ur help!!

robbieduncan said:

Can you post your code? removeAllObjects makes no sense in this context. If you are trying to "paint" with transparent then you are probably in the wrong drawing mode: it's painting transparent over the top of the current image in the buffer which does not alter it. You need to set the correct blend mode with CGContextSetBlendMode. If I remember correctly then kCGBlendModeClear allows you to use a clear colour to erase back to transparent.

Click to expand...

well i do nt ve the source code now, i can post it tomorrow.But i had tried with CGContextSetBlendMode as well, may b am going on a direction, i just could not figure out with that. removeAllObjects is been called as am using a NSMutableArray to keep the drawing objects in that buffer, which am clearing by calling the removeAllObjects message. Here i am not trying to "paint" with transparent, but i dont know where & how to use CGContextSetBlendMode. My app has three tab bar items, one to save the drawing image, 2nd one is to clear the image & 3rd one is to clear/erase a portion of the drawing image (something like eraser in MSPaint) which i just posted here as my query.
Please help me on this as am still not very good @ CoreGraphics/2D physics.
I ll post the code for your reference tomorrow.

robbieduncan said:

Before we go any further you will need to sort out your UI: you should not be using a tab bar as a toolbar. Follow the HIG and use the correct controls for the purpose.

Click to expand...

Oops, my mistake, i wrongly posted here tab bar, its actually a toolbar with 3 items to save image,clear image & erase some portion of image.

robbieduncan said:

OK, good. Then lets wait for the code and see where we go...

Click to expand...

Ok here is my code:

DrawView.h:

#import <UIKit/UIKit.h>


@interface DrawView : UIView {

UIImage *myPic;
NSMutableArray *myDrawing;
}

-(void)drawPicUIImage *)thisPic;
-(void)erasePic;
-(void)eraseStart;
-(void)eraseEnd;
-(void)cancelDrawing;

@end

DrawView.m:

#import "DrawView.h"


@implementation DrawView

-(id)init{


return self;
}

-(void)drawPicUIImage *)thisPic {

myPic = thisPic;
[myPic retain];
[self setNeedsDisplay];
}

- (void)drawRectCGRect)rect {

float newHeight;
float newWidth;

if (!myDrawing) {
myDrawing = [[NSMutableArray alloc] initWithCapacity:0];
}

[[UIColor redColor] set];
CGContextRef ctx = UIGraphicsGetCurrentContext();

if (myPic != NULL) {
float ratio = myPic.size.height/460;
if (myPic.size.width/320 > ratio) {
ratio = myPic.size.width/320;
}

newHeight = myPic.size.height/ratio;
newWidth = myPic.size.width/ratio;

[myPic drawInRect:CGRectMake(0,0,newWidth,newHeight)];
}
if ([myDrawing count] > 0) {
CGContextSetLineWidth(ctx, 5);

for (int i = 0 ; i < [myDrawing count] ; i++) {
NSArray *thisArray = [myDrawing objectAtIndex:i];

if ([thisArray count] > 2) {
float thisX = [[thisArray objectAtIndex:0] floatValue];
float thisY = [[thisArray objectAtIndex:1] floatValue];

CGContextBeginPath(ctx);
CGContextMoveToPoint(ctx, thisX, thisY);

for (int j = 2; j < [thisArray count] ; j+=2) {
thisX = [[thisArray objectAtIndex:j] floatValue];
thisY = [[thisArray objectAtIndex:j+1] floatValue];

CGContextAddLineToPoint(ctx, thisX,thisY);
}
CGContextStrokePath(ctx);
}
}
}
}

- (void) touchesBeganNSSet *)touches withEventUIEvent *)event {

[myDrawing addObject:[[NSMutableArray alloc] initWithCapacity:4]];

CGPoint curPoint = [[touches anyObject] locationInView:self];
[[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.x]];
[[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.y]];
}

- (void) touchesMovedNSSet *)touches withEventUIEvent *)event {

CGPoint curPoint = [[touches anyObject] locationInView:self];
[[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.x]];
[[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.y]];

[self setNeedsDisplay];
}

- (void) touchesEndedNSSet *)touches withEventUIEvent *)event {

CGPoint curPoint = [[touches anyObject] locationInView:self];
[[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.x]];
[[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.y]];

[self setNeedsDisplay];

}

-(void)eraseStart
{

}

-(void)eraseEnd
{

}

-(void)erasePic
{

}

-(void)cancelDrawing {

[myDrawing removeAllObjects];
[self setNeedsDisplay];

}

- (void)dealloc {
[super dealloc];
[myPic release];
[myDrawing release];
}


@end

Please let me know if anything else you want.