[ios2] 关于CGBitmapContextCreate【转】

CGContextRef CGBitmapContextCreate （
　　void *data，
　　size_t width，
　　size_t height，
　　size_t bitsPerComponent，
　　size_t bytesPerRow，
　　CGColorSpaceRef colorspace，
　　CGBitmapInfo bitmapInfo
　　）;参数data指向绘图操作被渲染的内存区域，这个内存区域大小应该为（bytesPerRow*height）个字节。如果对绘制操作被渲染的内存区域并无特别的要求，那么可以传递NULL给参数date。
　　 参数width代表被渲染内存区域的宽度。
　　 参数height代表被渲染内存区域的高度。
　　 参数bitsPerComponent被渲染内存区域中组件在屏幕每个像素点上需要使用的bits位，举例来说，如果使用32-bit像素和RGB颜色格式，那么RGBA颜色格式中每个组件在屏幕每个像素点上需要使用的bits位就为32/4=8。
　　 参数bytesPerRow代表被渲染内存区域中每行所使用的bytes位数。
　　 参数colorspace用于被渲染内存区域的“位图上下文”。
　　 参数bitmapInfo指定被渲染内存区域的“视图”是否包含一个alpha（透视）通道以及每个像素相应的位置，除此之外还可以指定组件式是浮点值还是整数值。获取图片中单个点的颜色：- (UIColor*) getPixelColorAtLocation:(CGPoint)point {UIColor* color = nil;CGImageRef inImage = self.image.CGImage;// Create off screen
bitmap context to draw the image into. Format ARGB is 4 bytes for
each pixel: Alpa, Red, Green, BlueCGContextRef cgctx = [self createARGBBitmapContextFromImage:inImage];if (cgctx == NULL) { return nil; }size_t w = CGImageGetWidth(inImage);size_t h = CGImageGetHeight(inImage);CGRect rect = {{0,0},{w,h}};// Draw the image to
the bitmap context. Once we draw, the
memory// allocated for the
context for rendering will then contain
the// raw image data in
the specified color space.CGContextDrawImage(cgctx, rect, inImage);// Now we can get a
pointer to the image data associated with the bitmap//
context.unsigned char* data = CGBitmapContextGetData (cgctx);if (data != NULL) {//offset locates the
pixel in the data from x,y.//4 for 4 bytes of
data per pixel, w is width of one row of data.@try {int offset = 4*((w*round(point.y))+round(point.x));NSLog(@"offset:
%d", offset);int alpha = data[offset];int red = data[offset+1];int green = data[offset+2];int blue = data[offset+3];NSLog(@"offset: %i colors: RGB A %i %i %i
%i",offset,red,green,blue,alpha);color = [UIColor colorWithRed:(red/255.0f) green:(green/255.0f) blue:(blue/255.0f) alpha:(alpha/255.0f)];}@catch (NSException * e) {NSLog(@"%@",[e reason]);}@finally {}}// When finished,
release the contextCGContextRelease(cgctx);// Free image data
memory for the contextif (data) { free(data); }return color;}创建取点图片工作域：- (CGContextRef) createARGBBitmapContextFromImage:(CGImageRef) inImage {CGContextRef
context = NULL;CGColorSpaceRef colorSpace;void *bitmapData;intbitmapByteCount;intbitmapBytesPerRow;// Get image width,
height. We'll use the entire image.size_t pixelsWide = CGImageGetWidth(inImage);size_t pixelsHigh = CGImageGetHeight(inImage);// Declare the number
of bytes per row. Each pixel in the bitmap in this// example is
represented by 4 bytes; 8 bits each of red, green, blue,
and// alpha.bitmapBytesPerRow = (pixelsWide * 4);bitmapByteCount
= (bitmapBytesPerRow * pixelsHigh);// Use the generic RGB
color space.colorSpace = CGColorSpaceCreateDeviceRGB();if (colorSpace == NULL){fprintf(stderr, "Error allocating color
space\n");return NULL;}// Allocate memory for
image data. This is the destination in memory// where any drawing
to the bitmap context will be rendered.bitmapData = malloc( bitmapByteCount );if (bitmapData == NULL){fprintf (stderr, "Memory
not allocated!");CGColorSpaceRelease( colorSpace );return NULL;}// Create the bitmap
context. We want pre-multiplied ARGB,
8-bits// per component.
Regardless of what the source image format
is// (CMYK, Grayscale,
and so on) it will be converted over to the format// specified here by
CGBitmapContextCreate.context = CGBitmapContextCreate (bitmapData,pixelsWide,pixelsHigh,8,
// bits per
componentbitmapBytesPerRow,colorSpace,kCGImageAlphaPremultipliedFirst);if (context == NULL){free (bitmapData);fprintf (stderr, "Context
not created!");}// Make sure and
release colorspace before returningCGColorSpaceRelease( colorSpace );return context;}

- (UIColor *) colorOfPoint:(CGPoint)point
{
unsigned char pixel[4] = {0};
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(pixel, 1, 1, 8, 4, colorSpace, kCGImageAlphaPremultipliedLast);

CGContextTranslateCTM(context, -point.x, -point.y);

[self.layer renderInContext:context];

CGContextRelease(context);
CGColorSpaceRelease(colorSpace);

//NSLog(@"pixel: %d %d %d %d", pixel[0], pixel[1], pixel[2], pixel[3]);

UIColor *color = [UIColor colorWithRed:pixel[0]/255.0 green:pixel[1]/255.0 blue:pixel[2]/255.0 alpha:pixel[3]/255.0];

return color;
}

//图片压缩

iOS自带的提供了一个API如下[html] view plaincopyNSData *UIImageJPEGRepresentation(UIImage *image, CGFloat compressionQuality);在Iphone上有两种读取图片数据的简单方法: UIImageJPEGRepresentation和UIImagePNGRepresentation. UIImageJPEGRepresentation函数需要两个参数:图片的引用和压缩系数.而UIImagePNGRepresentation只需 要图片引用作为参数.通过在实际使用过程中,比较发现: UIImagePNGRepresentation(UIImage* image) 要比UIImageJPEGRepresentation(UIImage* image, 1.0) 返回的图片数据量大很多.譬如,同样是读取摄像头拍摄的同样景色的照片, UIImagePNGRepresentation()返回的数据量大小为199K ,而 UIImageJPEGRepresentation(UIImage* image, 1.0)返回的数据量大小只为140KB,比前者少了50多KB.如果对图片的清晰度要求不高,还可以通过设置 UIImageJPEGRepresentation函数的第二个参数,大幅度降低图片数据量.譬如,刚才拍摄的图片, 通过调用UIImageJPEGRepresentation(UIImage* image, 1.0)读取数据时,返回的数据大小为140KB,但更改压缩系数后,通过调用UIImageJPEGRepresentation(UIImage* image, 0.5)读取数据时,返回的数据大小只有11KB多,大大压缩了图片的数据量 ,而且从视角角度看,图片的质量并没有明显的降低.因此,在读取图片数据内容时,建议优先使用UIImageJPEGRepresentation,并可 根据自己的实际使用场景,设置压缩系数,进一步降低图片数据量大小。[html] view plaincopyUIImage *imageNew = [info objectForKey:@"UIImagePickerControllerOriginalImage"];imageNew = [self imageWithImage:imageNew scaledToSize:CGSizeMake(100, 100)];NSData *imageData = UIImageJPEGRepresentation(imageNew, 0.0001);m_selectImage = [UIImage imageWithData:imageData];

.h具体code[html] view plaincopy#import <Foundation/Foundation.h>@interface UIImage (UIImageExt)- (UIImage *)scaleToSize:(UIImage *)img size:(CGSize)size;- (UIImage *)imageByScalingAndCroppingForSize:(CGSize)targetSize;@end

.m具体code

[html] view plaincopy#import "UIImageExt.h"@implementation UIImage (UIImageExt)- (UIImage *)scaleToSize:(UIImage *)img size:(CGSize)size{// 创建一个bitmap的context// 并把它设置成为当前正在使用的contextUIGraphicsBeginImageContext(size);// 绘制改变大小的图片[img drawInRect:CGRectMake(0, 0, size.width, size.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;}- (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize{UIImage *sourceImage = self;UIImage *newImage = nil;CGSize imageSize = sourceImage.size;CGFloat width = imageSize.width;CGFloat height = imageSize.height;CGFloat targetWidth = targetSize.width;CGFloat targetHeight = targetSize.height;CGFloat scaleFactor = 0.0;CGFloat scaledWidth = targetWidth;CGFloat scaledHeight = targetHeight;CGPoint thumbnailPoint = CGPointMake(0.0,0.0);if (CGSizeEqualToSize(imageSize, targetSize) == NO){CGFloat widthFactor = targetWidth / width;CGFloat heightFactor = targetHeight / height;if (widthFactor > heightFactor)scaleFactor = widthFactor; // scale to fit heightelsescaleFactor = heightFactor; // scale to fit widthscaledWidth = width * scaleFactor;scaledHeight = height * scaleFactor;// center the imageif (widthFactor > heightFactor){thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;}elseif (widthFactor < heightFactor){thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;}}UIGraphicsBeginImageContext(targetSize); // this will cropCGRect thumbnailRect = CGRectZero;thumbnailRect.origin = thumbnailPoint;thumbnailRect.size.width = scaledWidth;thumbnailRect.size.height = scaledHeight;[sourceImage drawInRect:thumbnailRect];newImage = UIGraphicsGetImageFromCurrentImageContext();if(newImage == nil)NSLog(@"could not scale image");//pop the context to get back to the defaultUIGraphicsEndImageContext();return newImage;}@end