sprite-kit - 如何使 Sprite 遵循贝塞尔曲线

Question

我对objective-c和sprite kit还很陌生，但我已经做了一段时间的游戏开发。我目前正在开发一个 2d 游戏，我的敌舰在屏幕上从右向左移动。我一直在关注我游戏不同部分的教程，然后在必要时添加它。我找到了一个教程，其中游戏中的敌人遵循贝塞尔曲线，我已经设法在我的游戏中实现了这一点，但是由于我是贝塞尔曲线的新手，我并不完全理解它们，并且算法使我的 Sprite 从上到下移动但我需要他们从左到右。

我已经包含了我用来将贝塞尔曲线添加到我的 Sprite 的代码片段，关于如何让它们从右到左而不是从上到下移动的任何建议。

CGMutablePathRef cgpath = CGPathCreateMutable();

        float xStart = [self getRandomNumberBetween:0+asteroid.size.width to:self.frame.size.width-asteroid.size.width];
        float xEnd = [self getRandomNumberBetween:0+asteroid.size.width to:self.frame.size.width-asteroid.size.width];
        float cp1X =[self getRandomNumberBetween:0+asteroid.size.width to:self.frame.size.width-asteroid.size.width];
        float cp1y = [self getRandomNumberBetween:0+asteroid.size.width to:self.frame.size.width-asteroid.size.height];
        float cp2x = [self getRandomNumberBetween:0+asteroid.size.width to:self.frame.size.width-asteroid.size.width];
        float cp2Y = [self getRandomNumberBetween:0 to:cp1y];

        CGPoint s = CGPointMake(xStart, 1024.0);
        CGPoint e = CGPointMake(xEnd, -100);
        CGPoint cp1 = CGPointMake(cp1X, cp1y);
        CGPoint cp2 = CGPointMake(cp2x, cp2Y);
        CGPathMoveToPoint(cgpath, NULL, s.x, s.y);
        CGPathAddCurveToPoint(cgpath, NULL, cp1.x, cp1.y, cp2.x, cp2.y, e.x, e.y);

        SKAction *enemyCurve = [SKAction followPath:cgpath asOffset:NO orientToPath:YES duration:5];

  CGPoint location = CGPointMake(-self.frame.size.width-asteroid.size.width, randY);

        SKAction *moveAction = [SKAction moveTo:location duration:randDuration];
        SKAction *doneAction = [SKAction runBlock:(dispatch_block_t)^(){

            asteroid.hidden = YES;
        }];

        SKAction *moveAsteroidActionWithDone = [SKAction sequence:@[enemyCurve,moveAction, doneAction]];

感谢您的任何帮助和建议。

Answer 1

贝塞尔曲线用于生成两点之间的平滑曲线。
要从左向右移动路径，请选择左侧的起点并选择右侧的终点。这两个控制点决定了从左到右的路径形状。改变 startpoint和 endpoint下面的代码将控制贝塞尔曲线的起点和终点。改变 control points改变曲线的形状。您可以通过附带的 GIF 看到这一点。

CGMutablePathRef cgpath = CGPathCreateMutable();

CGPoint startingPoint = CGPointMake(50, 100);

CGPoint controlPoint1 = CGPointMake(160, 250);
CGPoint controlPoint2 = CGPointMake(200, 140);

CGPoint endingPoint = CGPointMake(303, 100);


CGPathMoveToPoint(cgpath, NULL, startingPoint.x, startingPoint.y);
CGPathAddCurveToPoint(cgpath, NULL, controlPoint1.x, controlPoint1.y,
                      controlPoint2.x, controlPoint2.y,
                      endingPoint.x, endingPoint.y);


SKAction *enemyCurve = [SKAction followPath:cgpath asOffset:NO orientToPath:YES duration:5];

[enemy runAction:[SKAction sequence:@[[SKAction waitForDuration:1],enemyCurve]]];

P0和 P3是起点和终点， P1和 P2是控制点。

查看此页面以更多地使用贝塞尔曲线。
http://www.jasondavies.com/animated-bezier/