Core Graphics 学习笔记 之 Image Drawing(一)

1.两张图合成为一张图

  UIImage * img =[UIImage imageNamed:@"icon"];
    CGSize sz = img.size;
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(sz.width*2, sz.height), NO, 0);
    [img drawAtPoint:CGPointMake(0, 0)];
    [img drawAtPoint:CGPointMake(sz.width, 0)];
    UIImage *mergeImg = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return mergeImg;
}

2.取一张图的两半，然后分开绘制，合成新的图

-(UIImage *)operateComponentsOfCGImage
{
    UIImage * img =[UIImage imageNamed:@"car.jpg"];
    CGSize sz = img.size;
    CGImageRef iconLeft  = CGImageCreateWithImageInRect([img CGImage],
                                                        CGRectMake(0, 0, sz.width/2, sz.height));
    CGImageRef iconRight = CGImageCreateWithImageInRect([img CGImage],
                                                        CGRectMake(sz.width/2.0, 0, sz.width/2.0, sz.height));
    // draw each CGImage to image context
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(sz.width*1.5, sz.height), NO, 0);
    CGContextRef con = UIGraphicsGetCurrentContext();
    CGContextDrawImage(con,
                       CGRectMake(0, 0, sz.width/2.0, sz.height), iconLeft);
    CGContextDrawImage(con,
                       CGRectMake(sz.width, 0, sz.width/2.0, sz.height), iconRight);
    
    UIImage *im = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    CGImageRelease(iconLeft);
    CGImageRelease(iconRight);
    return im;
}

问题来了，绘制的图是上下颠倒的，原书上是这么说的：

This phenomenon can arise when you create a CGImage and then draw it with CGContextDrawImage,and is due to 

a mismatch in the native  coordinte systems of source and target contexts/

大致意思是，在创建一个CGImage对象并用CGContextDrawImage方法对图像进行绘制时会出现上面的情况，这是由于原生坐标系统中，源上下文和目标上下文的不一致导致的。

注意：还有一点，在高清屏下（二倍像素的设备上），绘制的大小也有问题。

如果我们的image file是二倍图（@2x.png），调用imageNamed：方法时，UIImage 会通过设置其scale的属性来补偿大小，而CGImage没有scale属性。

正确 方式一：  

-(UIImage *)operateComponentsOfCGImage
{
    UIImage * img =[UIImage imageNamed:@"car.jpg"];
    CGSize sz = img.size;
    //Derive  CGImage and use its dimensions to extract its halves
    CGImageRef imgCG = [img CGImage];
    CGSize szCG = CGSizeMake(CGImageGetWidth(imgCG), CGImageGetHeight(imgCG));
    
    CGImageRef iconLeft  = CGImageCreateWithImageInRect([img CGImage],
                                                        CGRectMake(0, 0, szCG.width/2, szCG.height));
    CGImageRef iconRight = CGImageCreateWithImageInRect([img CGImage],
                                                        CGRectMake(szCG.width/2.0, 0, szCG.width/2.0, szCG.height));
    // draw each CGImage to image context
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(sz.width*1.5, sz.height), NO, 0);
// The rest is as before,calling flip() to compensate for flipping
    CGContextRef con = UIGraphicsGetCurrentContext();

    CGContextDrawImage(con,
                      CGRectMake(0, 0, sz.width/2.0, sz.height), flip(iconLeft));
   CGContextDrawImage(con,
                      CGRectMake(sz.width, 0, sz.width/2.0, sz.height), flip(iconRight));
    
    UIImage *im = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    CGImageRelease(iconLeft);
    CGImageRelease(iconRight);
    return im;
}

//
CGImageRef flip(CGImageRef im){
    CGSize sz  = CGSizeMake(CGImageGetWidth(im), CGImageGetHeight(im));
    UIGraphicsBeginImageContextWithOptions(sz, NO, 0);
    CGContextDrawImage(UIGraphicsGetCurrentContext(), CGRectMake(0, 0, sz.width, sz.height), im);
    CGImageRef result = [UIGraphicsGetImageFromCurrentImageContext() CGImage];
    UIGraphicsEndImageContext();
    return result;

}

方式二：通过在UIImage中那个包含一个CGImage，绘制UIImage 而不是绘制CGImage。

UIImage 在通过CGImage生成UIImage的时候可以通过调用imageWithCGImage:scale:orientation:方法，来弥补scale的问题。而且通过绘制一个UIImage对象而不是CGImage对象，我们可以避免flipping的问题。

下面是解决flipping和scale问题的方法（不调用flip方法）

-(UIImage *)operateCGImage
{
    UIImage * img =[UIImage imageNamed:@"car.jpg"];
    CGSize sz = img.size;
    //Derive  CGImage and use its dimensions to extract its halves
    CGImageRef imgCG = [img CGImage];
    CGSize szCG = CGSizeMake(CGImageGetWidth(imgCG), CGImageGetHeight(imgCG));
    
    CGImageRef iconLeft  = CGImageCreateWithImageInRect([img CGImage],
                                                        CGRectMake(0, 0, szCG.width/2, szCG.height));
    CGImageRef iconRight = CGImageCreateWithImageInRect([img CGImage],
                                                        CGRectMake(szCG.width/2.0, 0, szCG.width/2.0, szCG.height));
    // draw each CGImage to image context
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(sz.width*1.5, sz.height), NO, 0);
    [[UIImage imageWithCGImage:iconLeft
                         scale:img.scale
                   orientation:UIImageOrientationUp]
    drawAtPoint:CGPointMake(0, 0)];
    
    [[UIImage imageWithCGImage:iconRight
                         scale:img.scale
                   orientation:UIImageOrientationUp]
     drawAtPoint:CGPointMake(sz.width+100, 0)];
    
    
    UIImage *im = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    CGImageRelease(iconLeft);
    CGImageRelease(iconRight);
    return im;
}

#pragma mark ----   convertViewToImage
- (UIImage *)convertViewToImage{
    if (iOS7) {
        UIGraphicsBeginImageContextWithOptions(asycImageView.bounds.size, YES, 1);
        [asycImageView drawViewHierarchyInRect:asycImageView.bounds afterScreenUpdates:YES];
        UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        return image;
    }else
    {
        UIGraphicsBeginImageContextWithOptions(self.view.frame.size, YES, 0.0);
        [self.view.layer renderInContext:UIGraphicsGetCurrentContext()];
        UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        return image;
    }
    return nil;
}

最后

以上就是舒服鞋垫为你收集整理的Core Graphics 学习笔记 之 Image Drawing(一)的全部内容，希望文章能够帮你解决Core Graphics 学习笔记 之 Image Drawing(一)所遇到的程序开发问题。

如果觉得靠谱客网站的内容还不错，欢迎将靠谱客网站推荐给程序员好友。