iOS從背景圖中取色的代碼

void *bitmapData; //內存空間的指針，該內存空間的大小等于圖像使用RGB通道所占用的字節數。

static CGContextRef CreateRGBABitmapContext (CGImageRef inImage)
{
    CGContextRef context = NULL;
    CGColorSpaceRef colorSpace;
    int bitmapByteCount;
    int bitmapBytesPerRow;

    size_t pixelsWide = CGImageGetWidth(inImage); //獲取橫向的像素點的個數
    size_t pixelsHigh = CGImageGetHeight(inImage);

    bitmapBytesPerRow   = (pixelsWide * 4); //每一行的像素點占用的字節數，每個像素點的ARGB四個通道各占8個bit(0-255)的空間
    bitmapByteCount = (bitmapBytesPerRow * pixelsHigh); //計算整張圖占用的字節數

    colorSpace = CGColorSpaceCreateDeviceRGB();//創建依賴于設備的RGB通道
    //分配足夠容納圖片字節數的內存空間
    bitmapData = malloc( bitmapByteCount );
    //創建CoreGraphic的圖形上下文，該上下文描述了bitmaData指向的內存空間需要繪制的圖像的一些繪制參數
    context = CGBitmapContextCreate (bitmapData,
                                     pixelsWide,
                                     pixelsHigh,
                                     8,
                                     bitmapBytesPerRow,
                                     colorSpace,
                                     kCGImageAlphaPremultipliedLast);
    //Core Foundation中通過含有Create、Alloc的方法名字創建的指針，需要使用CFRelease()函數釋放
    CGColorSpaceRelease( colorSpace );
    return context;
}

// 返回一個指針，該指針指向一個數組，數組中的每四個元素都是圖像上的一個像素點的RGBA的數值(0-255)，用無符號的char是因為它正好的取值范圍就是0-255
static unsigned char *RequestImagePixelData(UIImage *inImage)
{
    CGImageRef img = [inImage CGImage];
    CGSize size = [inImage size];
    //使用上面的函數創建上下文
    CGContextRef cgctx = CreateRGBABitmapContext(img);
    CGRect rect = {{0,0},{size.width, size.height}};
    //將目標圖像繪制到指定的上下文，實際為上下文內的bitmapData。
    CGContextDrawImage(cgctx, rect, img);
    unsigned char *data = CGBitmapContextGetData (cgctx);
    //釋放上面的函數創建的上下文
    CGContextRelease(cgctx);
    return data;
}

//設置背景原圖片,即取色所用的圖片
- (void)setSourceImage:(NSString *)sourceImage ImageWidth:(int)_width ImageHeight:(int)_height {
    //生成指定大小的背景圖
    UIImage *im = [UIImage imageNamed:sourceImage];
    UIImage *newImage;
    UIImageView *view = [[UIImageView alloc] initWithImage:im];
    view.frame = CGRectMake(0, 0, _width, _height);
    UIGraphicsBeginImageContext(CGSizeMake(_width, _height)); //size 為CGSize類型，即你所需要的圖片尺寸
    [im drawInRect:CGRectMake(0, 0, _width, _height)]; //newImageRect指定了圖片繪制區域
    newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    width = newImage.size.width;
    height = newImage.size.height;
    //將解析背景圖為像素，供取色用
    imgPixel = RequestImagePixelData(newImage);
}

//計算顏色
-(UIColor*)calColor:(CGPoint)aPoint {
    int i = 4 * width * round(aPoint.y+imageView.frame.size.height/2) + 4 * round(aPoint.x+imageView.frame.size.width/2);
    int _r = (unsigned char)imgPixel[i];
    int _g = (unsigned char)imgPixel[i+1];
    int _b = (unsigned char)imgPixel[i+2];
    NSLog(@"(%f,%f)",aPoint.x,aPoint.y);
    NSLog(@"Red : %f   Green: %f   Blue: %f",_r/255.0,_g/255.0,_b/255.0);
    return [UIColor colorWithRed:_r/255.0f green:_g/255.0f blue:_b/255.0f alpha:1.0];
}   

- (void)changColor:(UIColor *)color{
    int width_;
    if (![Util isIpad]) {
        width_ = 30;
    } else {
        width_ = 70;
    }

    UIGraphicsBeginImageContext(CGSizeMake(width_, width_));
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGContextMoveToPoint(ctx, 20, 20);
    CGContextSetFillColorWithColor(ctx, color.CGColor);
    if (![Util isIpad]) {
        CGContextAddArc(ctx, width_/2, width_/2, 14.5, 0, 6.3, 0);
    } else {
        CGContextAddArc(ctx, width_/2+0.5, width_/2, 31.3, 0, 6.3, 0);
    }
    CGContextFillPath(ctx);
    self->pickedColorImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
}
//該片段來自于http://outofmemory.cn