如何正确补偿CGContextRef坐标系，使原点在左上角 [英] How to properly compensate CGContextRef coordinate system, so the origin is at the top left corner

问题描述

我正在尝试使用 CoreText 绘制文本.我有以下代码:

I'm trying to draw text using CoreText. I have the following code:

CGContextRef uiContext = UIGraphicsGetCurrentContext();

NSMutableDictionary<NSAttributedStringKey,id> *attributes = [NSMutableDictionary<NSAttributedStringKey,id> new];
[attributes setObject:UIColor.redColor forKey:NSForegroundColorAttributeName];
[attributes setObject:[UIFont systemFontOfSize:40] forKey:NSFontAttributeName];
NSAttributedString *attrString = [[NSAttributedString alloc] initWithString:@"Str" attributes:attributes];

CGMutablePathRef path = CGPathCreateMutable();
CGPathAddRect(path, nil, CGRectMake(0, 50, attrString.size.width, attrString.size.height));
CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)attrString);
CTFrameRef frame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, attrString.length), path, nil);

CTFrameDraw(frame, uiContext);

使用上面的代码，文本被正确放置，但在 y 方向上是镜像的

With the above code, text is placed properly, but is mirrored across y-direction

所以，我尝试应用翻译和缩放转换，这对文本有帮助，但现在它被放置在左下角，这不是所需的行为.

So, I tried to apply translate and scale transformation, which helped with text, but now it's placed at to bottom left corner, which isn't the desired behavior.

CGContextSetTextMatrix(uiContext, CGAffineTransformIdentity);
CGContextTranslateCTM(uiContext, 0.0, self.bounds.size.height);
CGContextScaleCTM(uiContext, 1.0, -1.0);

有什么想法我在这里遗漏了吗?谢谢.

Is there any ideas what am I missing here? Thanks.

推荐答案

这行 y 原点的 50 值从何而来?

Where does this 50 value for the y origin come from on this line?

CGPathAddRect(path, nil, CGRectMake(0, 50, attrString.size.width, attrString.size.height));

如果您从左下角的 y 坐标开始，该 50 值将从该位置开始绘制 y 原点 50 的路径.

If you're starting from the lower left y coordinate, that 50 value will start the path drawing y origin 50 up from that.

您应该能够通过使用容器视图的高度(您要将文本绘制到其中)减去 attrString.size.height 来调整 y 原点以从顶部偏移.

You should be able to adjust the y origin to be offset from the top by using the container view's height (that you'd like to draw the text into) minus the attrString.size.height.

这是一个示例，说明如果您正在绘制自定义视图的子类'drawRect:rect:

Here's an example of what it might look like if you are doing the drawing a custom view's subclass' drawRect:rect:

CGPathAddRect(path, nil, CGRectMake(0, rect.size.height - attrString.size.height, attrString.size.width, attrString.size.height));

结果如下:

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