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：

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+ (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;

}

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+ (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：

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- (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：

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return [[UIImage imageWithColor:[UIColor purpleColor] size:originImage.size] changeOpacityByFactor:0.5];;

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

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

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









其中，overlay算法如下：

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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));

}