iOS 绘图学习笔记 - (3)绘制图像

作者: brownfeng | 来源:发表于2016-12-22 21:04 被阅读354次

UIKit Images

UIkit都是围绕UIImage类的,我们可以从file或者NSData中创建一个UIImage,同时通过UIImageView显示这个UIImage.我们也可以使用CoreGraphic直接绘制一个Image:

UIImage *SwatchWithColor(UIColor *color, CGFloat side){
    // Create image context(using the main screen scale)
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(side, side), NO, 0.0);
    // 开始绘制
    [color setFill];
    UIRectFill(CGRectMake(0, 0, side, side));
    
    // 获取图片
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

对于Image需要记住以下要点:

如果要知道一个图像的范围extent,直接使用size属性,这个属性返回的单位是点point而不是像素pixel,具体的像素范围和具体的mainscreen scale相关.如果是retina屏幕1 point = 2 pixel.
我们可以将UIImage压缩成png或者jpg - UIImagePNGRepresentation,UIImageJPEGRepresentation.这两个方法是将UIImage转化成压缩以后的NSData.
可以直接从UIImage中获取image的Quartz表示CGImage - 可以这样说UIImage是对CGImage(或者Core Image)的轻量级包装.

Building Thumbnails

通常将一个大图转化成小图赖创建缩略图,一般使用image的drawRect方法绘制到目标大小的rect中:

// 获取context!!!
UIImage *image = [UIImage imageNamed:@"myImage"];
[image drawInRect:destinationRect];
UIImage *thumbnail = UIGraphicsGetImageFromCurrentImageContext();

通常创建缩略图需要进行取舍,究竟是Filling rect,还是Fitting Rect:

UIImage *BuildingThumnail(UIImage *sourceImage, CGSize targetSize, BOOL useFitting){
    UIGraphicsBeginImageContextWithOptions(targetSize, NO, 0.0);
    CGRect targetRect = SizeMakeRect(targetSize);
    // 创建source Image 的bounding rect
    CGRect naturalRect = (CGRect){.size = sourceImage.size};
    // 计算 fitting 或者 filling dest rectangle, 使用Rect工具中的工具方法
    CGRect destinationRect = useFitting ? RectByFittingRect(naturalRect, targetRect) : RectByFillingRect(naturalRect, targetRect);
    
    // Draw the new thumbnail
    [sourceImage drawInRect:destinationRect];
    
    // Retrieve and return the new image
    UIImage *thumbnail = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return thumbnail;
}

Extracting Subimages

与获取缩略图不同,获取子图片无需压缩图片的大小,它只需要获取图片中的部分内容即可,其中的核心方法是CGImageCreateWithImageInRect()

// 所有的单位都是像素
UIImage *ExtractRectFromImage(UIImage *sourceImage, CGRect subRect){
    // Extract image
    CGImageRef imageRef = CGImageCreateWithImageInRect(sourceImage.CGImage, subRect);
    if (imageRef != NULL) {
        UIImage *output = [UIImage imageWithCGImage:imageRef];
        CGImageRelease(imageRef);
        return output;
    }
    return nil;
}

Converting an Image to Grayscale

将图片绘制成grayscale模式.其中最核心的grayscale方法是GrayscaleVersionOfImage,全部的demo code:

UIImage *GrayscaleVersionOfImage(UIImage *sourceImage){
    // Establish grayscale color space
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    if (colorSpace == NULL) {
        return nil;
    }
    // Extents are integers
    int width = sourceImage.size.width;
    int height = sourceImage.size.height;
    
    // Build context: one byte per pixel, no alpha
    CGContextRef context = CGBitmapContextCreate(NULL, width, height, 8, width, colorSpace, (CGBitmapInfo)kCGImageAlphaNone);
    if (context == NULL) {
        return nil;
    }
    // Replicate image using new color space
    CGRect rect = CGRectMake(0, 0, sourceImage.size.width, sourceImage.size.height);
    CGContextDrawImage(context, rect, sourceImage.CGImage);
    CGImageRef imageRef = CGBitmapContextCreateImage(context);
    CGContextRelease(context);
    
    // Return the grayscale image
    UIImage *output = [UIImage imageWithCGImage:imageRef];
    CFRelease(imageRef);
    return output;
}

Watermarking Images

有时候我们需要给图片加上水印.

-(UIImage *)WatermarkingAnImage{
    CGSize targetSize = CGSizeMake(200, 200);
    UIGraphicsBeginImageContextWithOptions(targetSize, NO, 0.0);
    CGContextRef context = UIGraphicsGetCurrentContext();
    // Draw the original image into the context
    CGRect targetRect = SizeMakeRect(targetSize);
    UIImage *sourceImage = [UIImage imageNamed:@"pronghorn.jpg"];
    CGRect imgRect = RectByFillingRect(SizeMakeRect(sourceImage.size), targetRect); [sourceImage drawInRect:imgRect];
    
    // Rotate the context
    CGPoint center = RectGetCenter(targetRect);
    CGContextTranslateCTM(context, center.x, center.y);
    CGContextRotateCTM(context, M_PI_4);
    CGContextTranslateCTM(context, -center.x, -center.y);
    // Create a string
    NSString *watermark = @"watermark";
    UIFont *font = [UIFont fontWithName:@"HelveticaNeue" size:48];
    CGSize size = [watermark sizeWithAttributes:@{NSFontAttributeName: font}];
    CGRect stringRect = RectCenteredInRect(SizeMakeRect(size), targetRect);
    
    // Retrieve the new image UIImage *image =
    UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

Retrieving Image Data

一般情况下,我们都可以通过PNG或者JPEG方式获取图片的data形式,但是这些data形式里除了图片的数据,还包括了图片的marker data等附加信息.如果我们需要获取纯图片的data数据 byte array,我们就需要直接使用context,来获取source data.

Creating Contexts

我们之前已经使用过CGBitmapContextCreate,这个方法需要如下几个参数:

void *data - 一般传入NULL.
size_t width and size_t height - image宽和高,通过CGBitmapContextCreate获取的图片的宽和高
size_t bitsPerComponent - 在UIKit中,我们一般使用的8-bit bytes,一般传入为8.
size_t bytesPerRow - multiply the size of the row by the bytes per component
CGColorSpaceRef colorspac e— You pass the color space Quartz should use to create the bitmap context, typically device RGB or device gray.
CGBitmapInfo bitmapInfo—This parameter specifies the style of alpha channel the bitmap uses.一般使用kCGImageAlphaPremultipliedFirst(彩色图片),或者kCGImageAlphaNone(grayscale)

Extracting Bytes的demo

NSData *BytesFromRGBImage(UIImage *sourceImage) {
    if (!sourceImage) return nil;
    // Establish color space
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    if (colorSpace == NULL){
        NSLog(@"Error creating RGB color space");
        return nil; }
    // Establish context
    int width = sourceImage.size.width;
    int height = sourceImage.size.height;
    CGContextRef context = CGBitmapContextCreate(NULL,
                                                 width,
                                                 height,
                                                 8, // bits per byte
                                                 width * 4, // bytes per row
                                                 colorSpace,
                                                 (CGBitmapInfo) kCGImageAlphaPremultipliedFirst);
    CGColorSpaceRelease(colorSpace);
    if (context == NULL){
        NSLog(@"Error creating context");
        return nil;
    }
    // Draw source into context bytes
    CGRect rect = (CGRect){.size = sourceImage.size};
    CGContextDrawImage(context, rect, sourceImage.CGImage);
    // Create NSData from bytes
    NSData *data = [NSData dataWithBytes:CGBitmapContextGetData(context) length:(width * height * 4)]; // bytes per image
    CGContextRelease(context);
    return data;
}

Creating Images from Bytes

与上一个步骤相反

#define BITS_PER_COMPONENT 8
#define ARGB_COUNT 4
UIImage *ImageFromBytes(NSData *data, CGSize targetSize) {
    // Check data
    int width = targetSize.width;
    int height = targetSize.height;
    if (data.length < (width * height * 4)) {
        NSLog(@"Error: Got %d bytes. Expected %d bytes", data.length, width * height * 4);
        return nil;
    }
    // Create a color space
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    if (colorSpace == NULL) {
        NSLog(@"Error creating RGB colorspace");
        return nil;
    }
    // Create the bitmap context
    Byte *bytes = (Byte *) data.bytes;
    CGContextRef context = CGBitmapContextCreate(bytes,
                                                 width,
                                                 height,
                                                 BITS_PER_COMPONENT, // 8 bits per component
                                                 width * ARGB_COUNT, // 4 bytes in ARGB
                                                 colorSpace,
                                                 (CGBitmapInfo)kCGImageAlphaPremultipliedFirst);
    CGColorSpaceRelease(colorSpace );
    if (context == NULL){
        NSLog(@"Error. Could not create context");
        return nil; }
    // Convert to image
    CGImageRef imageRef = CGBitmapContextCreateImage(context); UIImage *image = [UIImage imageWithCGImage:imageRef];
    // Clean up CGContextRelease(context); CFRelease(imageRef);
    return image;
}

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