CGContext 总结

1.画图获取context

1.1 从drawRect:函数构造context

- (void)drawRect:(CGRect)rect {    // Drawing code    CGContextRef ref = UIGraphicsGetCurrentContext();    CGContextSetFillColorWithColor(ref, [UIColor colorWithRed:1 green:0 blue:0 alpha:0.5].CGColor);    CGContextFillRect(ref, CGRectMake(0, 0, 300, 400));    CGContextSetStrokeColorWithColor(ref, [UIColor blueColor].CGColor);    CGContextAddEllipseInRect(ref, CGRectMake(0, 30, 100, 100));    CGContextMoveToPoint(ref, 50, 80);    CGContextAddLineToPoint(ref, 200, 250);    CGContextStrokePath(ref);    CGContextRelease(ref);//context需要自己释放，CGImageRef也需自己释放}

1.2 由UIGraphicsBeginImageContex或UIGraphicsBeginImageContextWithOptions获取context或CGBitmapContextCreate，不同之处在于前两者的context坐标系原点在左下角，后者在左上角。

- (CGContextRef)myCGContextRef{//    CGContextRef ref = [self makeBitmapContextWithPixelWidth:400 height:300];    UIGraphicsBeginImageContext(CGSizeMake(400, 300));    //UIGraphicsBeginImageContextWithOptions(CGSizeMake(400, 300), NO, 1.0);    CGContextRef ref = UIGraphicsGetCurrentContext();    return ref;}- (CGContextRef)makeBitmapContextWithPixelWidth:(NSInteger)width height:(NSInteger)height{    NSInteger bitPerComponent = 8;    NSInteger bytesPerRow = width * 4 * bitPerComponent;    CGContextRef bitmapContextRef = CGBitmapContextCreate(NULL, width, height, bitPerComponent, bytesPerRow, CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB), kCGImageAlphaPremultipliedLast);    //第二种方式    /*    CGImageRef imageRef = CGImageCreateWithImageInRect([originalImage CGImage], rect);    CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(imageRef);    CGColorSpaceRef colorSpaceInfo = CGImageGetColorSpace(imageRef);    CGContextRef bitmap = CGBitmapContextCreate(NULL, rect.size.width, rect.size.height, CGImageGetBitsPerComponent(imageRef), CGImageGetBytesPerRow(imageRef), colorSpaceInfo, bitmapInfo);    */    return bitmapContextRef;}

UIGraphicsBeginImageContex或UIGraphicsBeginImageContextWithOptions获取context或CGBitmapContextCreate，不同之处在于前两者的context坐标系原点在左下角，后者在左上角。drawRect中的context是UIView自动生成返回的，坐标系原点在左上角

对以下2情况,CGContext坐标系在左上角:
1) In iOS, a drawing context returned by an UIView.(如drawRect中context)
2) In iOS, a drawing context created by calling the UIGraphicsBeginImageContextWithOptions function.

context坐标系在左上角

context坐标系在左下角

2.画图并获取image

drawRect函数可直接将图画在view上，还有一种方式便是自己画图，获取context中图片，然后将图片传给imageView或其他在view上展示出来

- (UIImage *)drawImage{    CGContextRef ref = [self myCGContextRef];    //用现有的image直接draw    UIImage *originImage = [UIImage imageNamed:@"D.jpg"];    UIImage *newImage = [UIImage imageWithCGImage:originImage.CGImage scale:1.0 orientation:UIImageOrientationRight];    CGContextDrawImage(ref, CGRectMake(0, 0, 300, 400), newImage.CGImage);//    [newImage drawInRect:CGRectMake(0, 0, 400, 300)];    //自己画    CGContextSetFillColorWithColor(ref, [UIColor colorWithRed:1 green:0 blue:0 alpha:0.5].CGColor);    CGContextFillRect(ref, CGRectMake(0, 0, 300, 400));    CGContextSetStrokeColorWithColor(ref, [UIColor blueColor].CGColor);    CGContextAddEllipseInRect(ref, CGRectMake(0, 30, 100, 100));    CGContextMoveToPoint(ref, 50, 100);    CGContextAddLineToPoint(ref, 200, 250);    CGContextStrokePath(ref);    //最后获取context中画的的image，然后传给imageView展示出来    CGImageRef imageRef = CGBitmapContextCreateImage(ref);    UIImage *image = [UIImage imageWithCGImage:imageRef];    //需要自己释放    CGImageRelease(imageRef);    CGContextRelease(ref);    return image;}

需要注意的是，用现有的image直接draw的方式，系统会将image上下翻转（坐标系不翻转）。故对于context坐标系在左下角的情况，图片的方向和我们想要的方向是一致的（UIKit坐标系），对于context坐标系在左上角的情况，图片就是上下翻转的，需要我们自己处理，或者用drawInRect:代替CGContextDrawImage，前者的坐标系是左上角，且要用UIGraphicsBeginImageContex造context，CGBitmapContextCreate造的画不出来。代码一般如下：

//一般此函数是将image方向正为UIImageOrientationUp- (UIImage *)contextImage:(UIImage *)image{    UIGraphicsBeginImageContext(CGSizeMake(_image.size.width, _image.size.height));    //UIGraphicsBeginImageContextWithOptions(CGSizeMake(_image.size.width, _image.size.height), NO, 1.0);    [_image drawInRect:CGRectMake(0, 0, _image.size.width, _image.size.height)];    UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();    UIGraphicsEndImageContext();    _image = scaledImage;    return scaledImage;}

采用CGContextDrawImage，context坐标系在左下角，因image自己会上下翻转，所以画出来的图是正的

采用CGContextDrawImage，context坐标系在左上角，因image自己会上下翻转，所以画出来的图是上下翻转的

采用drawInRect:，且UIGraphicsBeginImageContex造context，画出来的是正的