图片的简单处理

需求：很多时候我们需要对拍照后的图片进行处理展示或者上传，但是原图的数据是很大，或者我们要展示的区域很小，这时我们就需要对图片进行简单的处理

做法：1.原图originalImage进行上传
2.缩略图thumImage进行展示

       //image是通过系统相机代理方法回调获取拍照的原图
        UIImage *originalImage = image;
        
        //计算比例和长宽
        CGFloat rate = 200.0/266.0;
        
        CGFloat cw = originalImage.size.width;
        
        CGFloat h = originalImage.size.height;
        
        CGFloat ch = originalImage.size.width*rate;

        //originalImagePath-原图的本地存放路径
        NSString *originalImagePath = photo.originalImagePath;

        //thumImagePath-缩略图的本地存放路径
        NSString *thumImagePath = photo.thumImagePath;
    
        //按比例截取中间部分
        UIImage *thumImage = [originalImage getSubImage:CGRectMake(0, (h-ch)/2.0, cw, ch)];
    
        //等比例压缩
        thumImage = [thumImage scaleToSize:CGSizeMake(266.0, 200.0)];
        
        /*
         * UIImageJPEGRepresentation与UIImagePNGRepresentation的区别：
         * 1.UIImageJPEGRepresentation处理的图片比UIImagePNGRepresentation小（这是很重要的，上传图片我们考虑的就是体积大小和图片质量，如果上传的图片过大肯定是不好的）
         * 2.UIImageJPEGRepresentation有一个缩放系数，可以将图片缩放到能接受的质量，一般在0.3-0.7之间，从而也减小了了图片的大小，而UIImagePNGRepresentation没有这个系数
         */
        //写入文件
        NSData *imagedata = UIImageJPEGRepresentation(originalImage,0.6);
        
        [imagedata writeToFile:originalImagePath atomically:YES];
        
        NSData *thumImagedata = UIImageJPEGRepresentation(thumImage,1);
        
        [thumImagedata writeToFile:thumImagePath atomically:YES];

上面两个是用于展示剪切后的缩略图的方法：
1.- (UIImage)getSubImage:(CGRect)rect //截取部分图像
2.- (UIImage)scaleToSize:(CGSize)size //等比例缩放

将其抽成UIImage的分类，具体实现：

//处理图片方向（比如横拍，反拍等最后都转成正常方向展示）
- (UIImage *)fixOrientation {
    
    // No-op if the orientation is already correct
    if (self.imageOrientation == UIImageOrientationUp)
        return self;
    
    // We need to calculate the proper transformation to make the image upright.
    // We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored.
    CGAffineTransform transform = CGAffineTransformIdentity;
    
    switch (self.imageOrientation) {
        case UIImageOrientationDown:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, self.size.width, self.size.height);
            transform = CGAffineTransformRotate(transform, M_PI);
            break;
            
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
            transform = CGAffineTransformTranslate(transform, self.size.width, 0);
            transform = CGAffineTransformRotate(transform, M_PI_2);
            break;
            
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, 0, self.size.height);
            transform = CGAffineTransformRotate(transform, -M_PI_2);
            break;
        default:
            break;
    }
    
    switch (self.imageOrientation) {
        case UIImageOrientationUpMirrored:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, self.size.width, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
            
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, self.size.height, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
        default:
            break;
    }
    
    // Now we draw the underlying CGImage into a new context, applying the transform
    // calculated above.
    CGContextRef ctx = CGBitmapContextCreate(NULL, self.size.width, self.size.height,
                                             CGImageGetBitsPerComponent(self.CGImage), 0,
                                             CGImageGetColorSpace(self.CGImage),
                                             CGImageGetBitmapInfo(self.CGImage));
    CGContextConcatCTM(ctx, transform);
    switch (self.imageOrientation) {
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            // Grr...
            CGContextDrawImage(ctx, CGRectMake(0,0,self.size.height,self.size.width), self.CGImage);
            break;
            
        default:
            CGContextDrawImage(ctx, CGRectMake(0,0,self.size.width,self.size.height), self.CGImage);
            break;
    }
    
    // And now we just create a new UIImage from the drawing context
    CGImageRef cgimg = CGBitmapContextCreateImage(ctx);
    UIImage *img = [UIImage imageWithCGImage:cgimg];
    CGContextRelease(ctx);
    CGImageRelease(cgimg);
    return img;
}

//截取部分图像
- (UIImage*)getSubImage:(CGRect)rect
{
    UIImage *image = [self fixOrientation];
    CGImageRef subImageRef = CGImageCreateWithImageInRect(image.CGImage, rect);
    
    CGRect smallBounds = CGRectMake(0, 0, CGImageGetWidth(subImageRef), CGImageGetHeight(subImageRef));
    
    UIGraphicsBeginImageContext(smallBounds.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextDrawImage(context, smallBounds, subImageRef);
    UIImage* smallImage = [UIImage imageWithCGImage:subImageRef];
    UIGraphicsEndImageContext();
    CGImageRelease(subImageRef);
    
    return smallImage;
}

//等比例缩放
- (UIImage*)scaleToSize:(CGSize)size
{
    CGFloat width = CGImageGetWidth(self.CGImage);
    CGFloat height = CGImageGetHeight(self.CGImage);
    
    float verticalRadio = size.height*1.0/height;
    float horizontalRadio = size.width*1.0/width;
    
    float radio = 1;
    if(verticalRadio>1 && horizontalRadio>1)
    {
        radio = verticalRadio > horizontalRadio ? horizontalRadio : verticalRadio;
    }
    else
    {
        radio = verticalRadio < horizontalRadio ? verticalRadio : horizontalRadio;
    }
    
    width = width*radio;
    height = height*radio;
    
    int xPos = (size.width - width)/2;
    int yPos = (size.height-height)/2;
    
    // 创建一个bitmap的context
    // 并把它设置成为当前正在使用的context
    UIGraphicsBeginImageContext(size);
    
    // 绘制改变大小的图片
    [self drawInRect:CGRectMake(xPos, yPos, width, height)];
    
    // 从当前context中创建一个改变大小后的图片
    UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();
    
    // 使当前的context出堆栈
    UIGraphicsEndImageContext();
    
    // 返回新的改变大小后的图片
    return scaledImage;
}

这两个方法的结合使用应该注意的是rect和size中宽高比要保持一致，这样展示的图片才不会有问题。应该只有宽高比一致，那么- (UIImage*)scaleToSize:(CGSize)size方法中的xPos，yPos就都为0，缩放后的图片才会布满整个size，否则的话就会有留白。

如果我们要展示原图的缩略图（不需要剪切），我们也可以交换两个方法的使用，同样的要保持rect和size中宽高比要保持一致。

本文非原创，来源网络资源与现实项目需求的整合。