Question

您可以使用以下代码使用Quartz绘制弧：

CGContextMoveToPoint(context2, x, y);
CGContextAddArcToPoint(context2, x1, y1, x2, y2, r);

在这些函数中，(x,y)是起点，r是圆弧半径，但(x1,y1)和(x2,y2)是什么？

Answer 1

AddArcToPoint的工作原理如下：

ArcToPoint Diagram

其中P1是路径当前所在的点，r是给予函数的radius，红线是addArcToPoint将添加到当前路径的行。它不会继续到x2, y2的第二点;它会在弧线结束时停止。

我有关于此here的博文。

Answer 2

http://developer.apple.com/library/ios/documentation/GraphicsImaging/Reference/CGContext/Reference/reference.html#//apple_ref/c/func/CGContextAddArcToPoint

x1：用户空间坐标中的x值，表示第一条切线的终点。第一条切线从当前点绘制到（x1，y1）。

y1：用户空间坐标中y值，表示第一条切线的终点。第一条切线从当前点绘制到（x1，y1）。

x2：用户空间坐标中的x值，表示第二条切线的终点。第二条切线从（x1，y1）到（x2，y2）。

y2：用户空间坐标中y值，表示第二条切线的终点。第二条切线从（x1，y1）到（x2，y2）。

Answer 3

这是我刚刚为解决这个问题而构建的代码，从圆心的角度来看，带有声明和示例值：

CGPoint arcCenter = CGPointMake(30,20);
float arcLengthRad = M_PI_4; // Whatever, the full span of the arc in radians
float radius = 10;
float arcCenterRad = M_PI_2; // the angle of the center of the arc, in radians

float arcP1hyp = 1/cos(arcLengthRad/2) * radius;
float arcP1x = arcCenter.x + cosf(arcCenterRad)*arcP1hyp;
float arcP1y = arcCenter.y + sinf(arcCenterRad)*arcP1hyp;
float arcP2tx = arcCenter.x + cosf(arcCenterRad+(arcLengthRad/2))*radius;
float arcP2ty = arcCenter.y + sinf(arcCenterRad+(arcLengthRad/2))*radius;
float arcP2x = (arcP1x - arcP2tx)*-1 + arcP2tx;
float arcP2y = (arcP1y - arcP2ty)*-1 + arcP2ty;
CGContextAddArcToPoint(context, 
                       arcP1x, 
                       arcP1y,
                       arcP2x, 
                       arcP2y,
                       radius);

所以上面的代码应该在圆的顶部产生一个小的45度角弧。

<强>编辑：在回复收到的评论时，上面列出的超简洁代码如下所示，带有注释并包含在方法中（加上对arcP2计算的微小调整）

/*
EOTContext:addArcWithCenter:arcLength:radius:arcMiddlePointAngle:

Use this method for building a circle with breaks at certain points, 
for example to use other CGContext methods to draw notches in the 
circle, or protruding points like gear teeth.

This method builds up the values to use in CGContextAddArcToPoint(), 
which are the x and y coordinates of two points.  First  added to 
the current point in context, form two lines that are the tangents of 
the entry and exit angles of the arc.

This method's arguments define the length of the arc in radians, and 
the position of start and end using the angle centerpoint of the arc.  
This is useful when drawing a certain defined amount of gear teeth, 
rotating around the circle.

It is beyond this method's scope to maintain or calculate the 
centerpoint relative to an arbitrary current point in the context, because this 
is primarily used for drawing a gear/notch circle.
*/
-(void)EOTContext:(CGContext*)context 
addArcWithCenter:(CGPoint)arcCenter 
arcLength:(CGFloat)arcLengthRad
radius:(CGFloat)radius
arcMiddlePointAngle:(CGFloat)arcCenterRad {



    /*
    Calculate the hypotenuse of the larger, outer circle where the 
    points of the tangent lines would rest upon (imagine wrapping 
    the drawn circle in a bounding regular convex polygon of tangent 
    lines, then wrap that polygon in an outer circle)
    */
    float arcP1hyp = 1/cos(arcLengthRad/2) * radius;

    // Build first tangent point
    CGPoint arcP1 = (CGPoint){
        arcCenter.x + cosf(arcCenterRad)*arcP1hyp,
        arcCenter.y + sinf(arcCenterRad)*arcP1hyp
    };

    // Build the final endpoint of the arc
    CGPoint arcP2final = (CGPoint){
        arcCenter.x + cosf(arcCenterRad+(arcLengthRad/2))*radius,
        arcCenter.y + sinf(arcCenterRad+(arcLengthRad/2))*radius
    };

    // Build second tangent point using the first tangent point and the final point of the arc.  
    // This point is resting on the bounding outer circle like arcP1 is.
    // This would also work using the final point itself, using the simple assignment of arcP2 = arcP2final;  
    //   or of course simply omitting arcP2 altogether.
    CGPoint arcP2 = (CGPoint){
        (arcP2final.x - arcP1.x) + arcP2final.x,
        (arcP2final.y - arcP1.y) + arcP2final.y
    };

    // The following adds an arc of a circle to the current path, using a radius and tangent points.
    CGContextAddArcToPoint(context, 
                           arcP1.x, 
                           arcP1.y,
                           arcP2.x, 
                           arcP2.y,
                           radius);
}

Answer 4

我简要描述了苹果文档。

http://developer.apple.com/library/ios/documentation/GraphicsImaging/Reference/CGContext/Reference/reference.html#//apple_ref/c/func/CGContextAddArcToPoint

x1：用户空间坐标中第一条切线终点的x值。第一条切线从当前点绘制到（x1，y1）。

y1：用户空间坐标中y值，表示第一条切线的终点。第一条切线从当前点绘制到（x1，y1）。

x2：用户空间坐标中的x值，表示第二条切线的终点。第二条切线从（x1，y1）到（x2，y2）。

y2：用户空间坐标中y值，表示第二条切线的终点。第二条切线从（x1，y1）到（x2，y2）。

使用CGContextAddArcToPoint（）绘制弧时，（x1，y1）和（x2，y2）是什么意思？

4 个答案: