Comment dessiner un transparent de l'avc (ou en tout cas clairement partie d'une image) sur l'iPhone

J'ai fini à l'aide de Bresenham ligne de l'algorithme (en revenant à la les jours d'antan, lorsque j'ai eu à écrire mes propres routines graphiques)...

- (void) contextEraseLine:(CGContextRef) ctx from:(CGPoint)startPoint to:(CGPoint) endPoint withThickness:(int)thickness {
int x, cx, deltax, xstep,
y, cy, deltay, ystep,
error, st, dupe;
int x0, y0, x1, y1;
x0 = startPoint.x;
y0 = startPoint.y;
x1 = endPoint.x;
y1 = endPoint.y;
//find largest delta for pixel steps
st = (abs(y1 - y0) > abs(x1 - x0));
//if deltay > deltax then swap x,y
if (st) {
(x0 ^= y0); (y0 ^= x0); (x0 ^= y0); //swap(x0, y0);
(x1 ^= y1); (y1 ^= x1); (x1 ^= y1); //swap(x1, y1);
}
deltax = abs(x1 - x0);
deltay = abs(y1 - y0);
error  = (deltax / 2);
y = y0;
if (x0 > x1) { xstep = -1; }
else         { xstep =  1; }
if (y0 > y1) { ystep = -1; }
else         { ystep =  1; }
for ((x = x0); (x != (x1 + xstep)); (x += xstep))
{
(cx = x); (cy = y); //copy of x, copy of y
//if x,y swapped above, swap them back now
if (st) { (cx ^= cy); (cy ^= cx); (cx ^= cy); }
(dupe = 0); //initialize no dupe
if(!dupe) { //if not a dupe, write it out
//NSLog(@"(%2d, %2d)", cx, cy);
CGContextClearRect(ctx, CGRectMake(cx, cy, thickness, thickness));
}
(error -= deltay); //converge toward end of line
if (error < 0) { //not done yet
(y += ystep);
(error += deltax);
}
}
}

Ouf! C'est un long chemin à parcourir pour créer une (un peu), maladroit gomme ligne.

Pour l'utiliser, faire quelque chose comme:

- (void)eraseStart {
//erase lines
UIGraphicsBeginImageContext(drawingBoard.size);
ctx = UIGraphicsGetCurrentContext();
CGContextDrawImage(ctx,CGRectMake(0,0,drawingBoard.size.width, drawingBoard.size.height),[drawingBoard CGImage]); 
}
- (void)eraseEnd {
drawingBoard = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
[drawingView removeFromSuperview];
[drawingView release];
drawingView = [[UIImageView alloc] initWithImage:drawingBoard];
drawingView.frame = CGRectMake(intEtchX, intEtchY, intEtchWidth, intEtchHeight);
[self.view addSubview:drawingView];
}

Cela suppose que vous avez déjà créé un drawingView (UIImageView) et drawingBoard (UIImage).

Alors, pour effacer une ligne, il suffit de faire quelque chose comme:

CGContextRef ctx = UIGraphicsGetCurrentContext();
[self eraseStart];
[self contextEraseLine:ctx from:CGPointMake (x1, y1) to:CGPointMake (x2, y2) withThickness:10];
[self eraseEnd];

(remplacer x1, y1, x2 et y2 par les valeurs appropriées)...

Si vous souhaitez effacer l'Image avec le tactile c'est mon code .... le unerase partie n'est pas recommandé, c'est un travail, mais un peu lent.

func addNewPathToImage(){
print("Start erasing or not erasing")
UIGraphicsBeginImageContextWithOptions(TopImageUIImageView.bounds.size, false, UIScreen.main.scale) //or 0 for the scale
//My image is aspect fit so I need to get rect
let aspect = TopImageUIImageView.image!.size.width / TopImageUIImageView.image!.size.height
let viewRatio = TopImageUIImageView.bounds.size.width / TopImageUIImageView.bounds.size.height
if aspect < viewRatio {
let scale = TopImageUIImageView.bounds.size.height / TopImageUIImageView.image!.size.height
let width = scale * TopImageUIImageView.image!.size.width
let topLeftX = (TopImageUIImageView.bounds.size.width - width) * 0.5
rect = CGRect(x: topLeftX, y: 0, width: width, height: TopImageUIImageView.bounds.size.height)
}
else {
let scale = TopImageUIImageView.bounds.size.width / TopImageUIImageView.image!.size.width
let height = scale * TopImageUIImageView.image!.size.height
let topLeftY = (TopImageUIImageView.bounds.size.height - height) * 0.5
rect = CGRect(x: 0.0, y: topLeftY, width: TopImageUIImageView.bounds.size.width, height: height)
}
////
context = UIGraphicsGetCurrentContext()
TopImageUIImageView.image?.draw(in: rect)
context?.setLineCap(.round)
context?.setLineWidth(brushSize)
if isErasing == true {
context?.setShadow(offset: CGSize(width: 0, height: 0), blur: blurNumber)
context?.setStrokeColor(UIColor.white.cgColor)
context?.setBlendMode(.clear)
}else{
print("test the unerase image .... ?")
context?.setStrokeColor(UIColor.init(patternImage: topImage.af_imageAspectScaled(toFit: CGSize(width: TopImageUIImageView.bounds.size.width, height: TopImageUIImageView.bounds.size.height))).cgColor)
}
//I am using these because I am using touch to define what to erase 
context?.move(to: CGPoint(x: lastTouch.x, y: lastTouch.y))
context?.addLine(to: CGPoint(x: currentTouch.x, y: currentTouch.y))
context?.strokePath()
context?.closePath() //when add this line or the "context?.beginPath" get ERROR CGContextClosePath: no current point.
print("close the path")
//get your image 
let image = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
TopImageUIImageView.image = image
}

UIColor *clearColor = [UIColor clearColor];