ios図形描画関数異常
7132 ワード
iosの描画関数を使用すると、座標が常に上下に逆転していることがわかります.理由は次のとおりです.
Side Effects of Drawing with Different Coordinate Systems
Some rendering oddities are brought to light when you draw an object with with reference to the default coordinate system of one drawing technology and then render it in a graphics context of the other. You may want to adjust your code to account for these side effects.
Arcs and Rotations
If you draw a path with functions such as CGContextAddArc and CGPathAddArc and assume a LLO coordinate system, then you need to flip the CTM to render the arc correctly in a UIKit view. However, if you use the same function to create an arc with points located in a ULO coordinate system and then render the path in a UIKit view, you’ll notice that the arc is an altered version of its original. The terminating endpoint of the arc now points in the opposite direction of what that endpoint would do were the arc created using the UIBezierPath class. For example, a downward-pointing arrow now points upward (as shown in Figure 1-5), and the direction in which the arc “bends” is also different. You must change the direction of Core Graphics-drawn arcs to account for the ULO-based coordinate system; this direction is controlled by the startAngle and endAngle parameters of those functions.
Figure 1-5 Arc rendering in Core Graphics versus UIKit
You can observe the same kind of mirroring effect if you rotate an object (for example, by calling CGContextRotateCTM). If you rotate an object using Core Graphics calls that make reference to a ULO coordinate system, the direction of the object when rendered in UIKit is reversed. You must account for the different directions of rotation in your code; with CGContextRotateCTM, you do this by inverting the sign of the angle parameter (so, for example, a negative value becomes a positive value).
上記のシナリオは開発者が自分で方向に注意することを要求し,もう1つのソリューションはy軸flip変換(fromhttp://stackoverflow.com/questions/16974258/cgcontextaddarc-counterclockwise-instead-of-clockwise)
Side Effects of Drawing with Different Coordinate Systems
Some rendering oddities are brought to light when you draw an object with with reference to the default coordinate system of one drawing technology and then render it in a graphics context of the other. You may want to adjust your code to account for these side effects.
Arcs and Rotations
If you draw a path with functions such as CGContextAddArc and CGPathAddArc and assume a LLO coordinate system, then you need to flip the CTM to render the arc correctly in a UIKit view. However, if you use the same function to create an arc with points located in a ULO coordinate system and then render the path in a UIKit view, you’ll notice that the arc is an altered version of its original. The terminating endpoint of the arc now points in the opposite direction of what that endpoint would do were the arc created using the UIBezierPath class. For example, a downward-pointing arrow now points upward (as shown in Figure 1-5), and the direction in which the arc “bends” is also different. You must change the direction of Core Graphics-drawn arcs to account for the ULO-based coordinate system; this direction is controlled by the startAngle and endAngle parameters of those functions.
Figure 1-5 Arc rendering in Core Graphics versus UIKit
You can observe the same kind of mirroring effect if you rotate an object (for example, by calling CGContextRotateCTM). If you rotate an object using Core Graphics calls that make reference to a ULO coordinate system, the direction of the object when rendered in UIKit is reversed. You must account for the different directions of rotation in your code; with CGContextRotateCTM, you do this by inverting the sign of the angle parameter (so, for example, a negative value becomes a positive value).
上記のシナリオは開発者が自分で方向に注意することを要求し,もう1つのソリューションはy軸flip変換(fromhttp://stackoverflow.com/questions/16974258/cgcontextaddarc-counterclockwise-instead-of-clockwise)
CGContextSaveGState(ctx); CGContextTranslateCTM(ctx, 0.0, self.bounds.size.height); CGContextScaleCTM(ctx, 1.0, -1.0); // Draw... CGContextRestoreGState(ctx);