iOS中的图像处理（三）——混合运算

有时候，单独对一张图像进行处理是很难或者根本达不到我们想要的效果的。一个好的滤镜效果的诞生，往往要经过很多复杂步骤、细致微调、图片应用效果观察以及很多图层叠加。

我在JSWidget上发现了一些常用混合算法，对应着一些常用混合模式，通过这些blend modes，我们可以指定两张图像如何混合。

不过在此之前，我们需要纯颜色图像和渐变图像来做辅助：

[cpp] 
+ (UIImage *)imageWithColor:(UIColor *)color size:(CGSize)size 
{ 
    // http://stackoverflow.com/questions/1213790/how-to-get-a-color-image-in-iphone-sdk 
     
    //Create a context of the appropriate size 
    UIGraphicsBeginImageContext(size); 
    CGContextRef currentContext = UIGraphicsGetCurrentContext(); 
     
    //Build a rect of appropriate size at origin 0,0 
    CGRect fillRect = CGRectMake(0, 0, size.width, size.height); 
     
    //Set the fill color 
    CGContextSetFillColorWithColor(currentContext, color.CGColor); 
     
    //Fill the color 
    CGContextFillRect(currentContext, fillRect); 
     
    //Snap the picture and close the context 
    UIImage *colorImage = UIGraphicsGetImageFromCurrentImageContext(); 
    UIGraphicsEndImageContext(); 
     
    return colorImage; 
} 

[cpp] 
+ (UIImage *)imageWithGradient:(UIImage *)image startColor:(UIColor *)startColor endColor:(UIColor *)endColor 
{ 
    UIGraphicsBeginImageContextWithOptions(image.size, NO, image.scale); 
    CGContextRef context = UIGraphicsGetCurrentContext(); 
    CGContextTranslateCTM(context, 0, image.size.height); 
    CGContextScaleCTM(context, 1.0, -1.0); 
     
    CGContextSetBlendMode(context, kCGBlendModeNormal); 
    CGRect rect = CGRectMake(0, 0, image.size.width, image.size.height); 
    CGContextDrawImage(context, rect, image.CGImage); 
     
    // Create gradient 
    NSArray *colors = [NSArray arrayWithObjects:(id)endColor.CGColor, (id)startColor.CGColor, nil]; 
    CGColorSpaceRef space = CGColorSpaceCreateDeviceRGB(); 
    CGGradientRef gradient = CGGradientCreateWithColors(space, (CFArrayRef)colors, NULL); 
     
    // Apply gradient 
    CGContextClipToMask(context, rect, image.CGImage); 
    CGContextDrawLinearGradient(context, gradient, CGPointMake(0,0), CGPointMake(0, image.size.height), 0); 
    UIImage *gradientImage = UIGraphicsGetImageFromCurrentImageContext(); 
    UIGraphicsEndImageContext(); 
     
    CGGradientRelease(gradient); 
    CGColorSpaceRelease(space); 
     
    return gradientImage; 
} 

而且在第一篇文章中提到的透明度滤镜（作用域像素的alpha值上）是没效果的，可以通过Quartz 2D来实现：
[cpp]
- (UIImage *)setAlpha:(CGFloat)alpha 
{ 
    // http://stackoverflow.com/questions/5084845/how-to-set-the-opacity-alpha-of-a-uiimage  
     
    UIGraphicsBeginImageContextWithOptions(self.size, NO, 0.0f); 
     
    CGContextRef ctx = UIGraphicsGetCurrentContext(); 
    CGRect area = CGRectMake(0, 0, self.size.width, self.size.height); 
     
    CGContextScaleCTM(ctx, 1, -1); 
    CGContextTranslateCTM(ctx, 0, -area.size.height); 
     
    CGContextSetBlendMode(ctx, kCGBlendModeMultiply); 
     
    CGContextSetAlpha(ctx, alpha); 
     
    CGContextDrawImage(ctx, area, self.CGImage); 
     
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext(); 
     
    UIGraphicsEndImageContext(); 
     
    return newImage; 
} 

在此基础上，通过下面四行代码，可以分别得到四种不同效果：
[cpp]
return [[UIImage imageWithColor:[UIColor purpleColor] size:originImage.size] changeOpacityByFactor:0.5];; 

[cpp]
return [UIImage imageWithGradient:originImage startColor:[UIColor whiteColor] endColor:[UIColor yellowColor]]; 

[cpp] 
return [[originImage tintWithMaxRGBA:(RGBA){190, 190, 230} minRGBA:(RGBA){50, 35, 10}] overlayWithImage:[[UIImage imageWithColor:[UIColor purpleColor] size:originImage.size] changeOpacityByFactor:0.3]]; 

[cpp] 
return [originImage softlightWithImage:[[UIImage imageWithColor:[UIColor yellowColor] size:originImage.size] changeOpacityByFactor:0.8]]; 

 
其中，overlay算法如下：

[cpp] 
double calcOverlay(float b, float t)  
{ 
    return (b > 128.0f) ? 255.0f - 2.0f * (255.0f - t) * (255.0f - b) / 255.0f: (b * t * 2.0f) / 255.0f; 
} 
 
void filterOverlay(UInt8 *pixelBuf, UInt8 *pixedBlendBuf, UInt32 offset, void *context) 
{ 
    int r = offset; 
    int g = offset+1; 
    int b = offset+2; 
     
    int red = pixelBuf[r]; 
    int green = pixelBuf[g]; 
    int blue = pixelBuf[b]; 
     
    int blendRed = pixedBlendBuf[r]; 
    int blendGreen = pixedBlendBuf[g]; 
    int blendBlue = pixedBlendBuf[b]; 
     
        pixelBuf[r] = SAFECOLOR(calcOverlay(red, blendRed)); 
    pixelBuf[g] = SAFECOLOR(calcOverlay(green, blendGreen)); 
    pixelBuf[b] = SAFECOLOR(calcOverlay(blue, blendBlue)); 
}