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Editorial Team
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Asked: 2026-06-07T09:44:13+00:00

Im trying to draw line by stroking an image to have brush like effect

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Im trying to draw line by stroking an image to have brush like effect via CoreGraphics framework.

I followed this post and this post and tried to draw from the touchesMoved method and the code sample’ll be of the form

UITouch *touch  = [touches anyObject];
CGPoint currentPoint    = [touch locationInView:self.view];
UIImage *texture = [UIImage imageNamed:@"texture.png"];
CGPoint vector = CGPointMake(currentPoint.x - lastPoint.x, currentPoint.y - lastPoint.y);
NSLog(@" vector %@", NSStringFromCGPoint(vector));
CGFloat distance = hypotf(vector.x, vector.y);
vector.x /= distance;
vector.y /= distance;
for (CGFloat i = 0; i < distance; i += 1.0f) {
    CGPoint p = CGPointMake(lastPoint.x + i * vector.x, lastPoint.y + i * vector.y);
    [texture drawAtPoint:p blendMode:kCGBlendModeNormal alpha:1.0f];
    NSLog(@"p %@", NSStringFromCGPoint(p));
}
previousPoint = currentPoint;

By doing so, I got error msgs,

Jul  9 11:51:27 mac1.local Smooth Line View[1035] <Error>: CGContextSaveGState: invalid context 0x0
Jul  9 11:51:27 mac1.local Smooth Line View[1035] <Error>: CGContextSetBlendMode: invalid context 0x0
Jul  9 11:51:27 mac1.local Smooth Line View[1035] <Error>: CGContextSetAlpha: invalid context 0x0
Jul  9 11:51:27 mac1.local Smooth Line View[1035] <Error>: CGContextTranslateCTM: invalid context 0x0
Jul  9 11:51:27 mac1.local Smooth Line View[1035] <Error>: CGContextScaleCTM: invalid context 0x0
Jul  9 11:51:27 mac1.local Smooth Line View[1035] <Error>: CGContextDrawImage: invalid context 0x0
Jul  9 11:51:27 mac1.local Smooth Line View[1035] <Error>: CGContextRestoreGState: invalid context 0x0

So, I put on the code related for context and the modified code is

    UITouch *touch  = [touches anyObject];
    currentPoint    = [touch locationInView:self];

    UIGraphicsBeginImageContext(self.frame.size);    
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetBlendMode(context, kCGBlendModeNormal);
    CGContextSetAlpha(context, 1.0f);
    CGContextSaveGState(context);

    UIImage *texture = [UIImage imageNamed:@"textureImg.png"];
    [texture drawAtPoint:currentPoint blendMode:kCGBlendModeNormal alpha:1.0f];
    CGContextSetLineCap(context, kCGLineCapRound);
    CGContextSetLineWidth(context, 25.0);
    CGContextSetRGBStrokeColor(context, texture.CGColor);
    CGContextBeginPath(context);
    CGContextMoveToPoint(context, previousPoint.x, previousPoint.y);
    CGContextAddLineToPoint(context, currentPoint.x, currentPoint.y);
    CGContextDrawPath(context, kCGPathFillStroke);
    UIGraphicsEndImageContext();

But when I run and try to draw on touch move, nothing is drawn. I cant get what went wrong. Can someone point me my mistake? Im still spending much time in sorting this out, but couldn’t do it yet now. Any timely help is much appreciated.
Thanks.

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1 Answer

  1. Editorial Team

    I see, your touchesMoved should look like this.

    BOOL touchSwiped = YES;
UITouch *touch = [touches anyObject];   
currentPoint = [touch locationInView:self.view];
currentPoint.y -= 685;     //<
//currentPoint.x = ???;  <------ adjust the x and y points of the currentPoint if it is not on the exact point of drawing.

UIGraphicsBeginImageContext(touchDraw.frame.size);
[touchDraw.image drawInRect:CGRectMake(0, 0, touchDraw.frame.size.width, touchDraw.frame.size.height)];
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
UIImage * textureColor = [UIImage imageNamed:@"brushImg.png"];

CGPoint vector = CGPointMake(currentPoint.x - previousPoint.x, currentPoint.y - previousPoint.y);
CGFloat distance = hypotf(vector.x, vector.y);
vector.x /= distance;
vector.y /= distance;
for (CGFloat i = 0; i < distance; i += 1.0f) {
    CGPoint p = CGPointMake(previousPoint.x + i * vector.x, previousPoint.y + i * vector.y);
    [textureColor drawAtPoint:p blendMode:blendMode alpha:0.5f];
}
CGContextBeginPath(UIGraphicsGetCurrentContext());
CGContextStrokePath(UIGraphicsGetCurrentContext());
touchDraw.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

previousPoint = currentPoint;

touchMoved++;

if (touchMoved == 1) {
    touchMoved = 0;
}
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