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;
}
