CGContextSetBlendMode - CGBlendM

//正常；也是默认的模式。前景图会覆盖背景图
kCGBlendModeNormal

// 正片叠底；混合了前景和背景的颜色，最终颜色比原先的都暗
kCGBlendModeMultiply

//滤色；把前景和背景图的颜色先反过来，然后混合
kCGBlendModeScreen

// 覆盖；能保留灰度信息，结合
kCGBlendModeOverlay

//覆盖；能保留灰度信息，结合kCGBlendModeSaturation能保留透明度信息，
//在imageWithBlendMode方法中两次执行drawInRect方法实现我们基本需求";  
kCGBlendModeOverlay: 

kCGBlendModeDarken// 变暗
kCGBlendModeLighten// 变亮
kCGBlendModeColorDodge// 颜色变淡
kCGBlendModeColorBurn//颜色加深
kCGBlendModeSoftLight// 柔光
kCGBlendModeHardLight// 强光
kCGBlendModeDifference//插值
kCGBlendModeExclusion//排除
kCGBlendModeHue// 色调
kCGBlendModeSaturation//饱和度
kCGBlendModeColor// 颜色
kCGBlendModeLuminosity// 亮度

 //Apple额外定义的枚举  
            //R: premultiplied result, 表示混合结果  
            //S: Source, 表示源颜色(Sa对应透明度值: 0.0-1.0)  
            //D: destination colors with alpha, 表示带透明度的目标颜色(Da对应透明度值: 0.0-1.0)  
R表示结果，S表示包含alpha的原色，D表示包含alpha的目标色，Ra，Sa和Da分别是三个的alpha。明白了这些以后，就可以开始寻找我们所需要的blend模式了。相信你可以和我一样，很快找到这个模式

kCGBlendModeClear: R = 0
kCGBlendModeCopy: R = S
kCGBlendModeSourceIn: R = S*Da //就是说目标色 = 原色*目标色的透明度
kCGBlendModeSourceOut: R = S*(1 - Da)
kCGBlendModeSourceAtop: R = S*Da + D*(1 - Sa)
kCGBlendModeDestinationOver: R = S*(1 - Da) + D
kCGBlendModeXOR: R = S*(1 - Da) + D*(1 - Sa)
kCGBlendModePlusDarker: R = MAX(0, (1 - D) + (1 - S)
kCGBlendModePlusLighter: R = MIN(1, S + D)（最后一种混合模式）

- (void)changeBlendMode:(UIGestureRecognizer*)gesture {  
    static CGBlendMode blendMode = kCGBlendModeNormal;  
    NSString *strMsg;  
    switch (blendMode) {  
        case kCGBlendModeNormal: {  
            strMsg = @"kCGBlendModeNormal: 正常；也是默认的模式。前景图会覆盖背景图";  
            break;  
        }  
        case kCGBlendModeMultiply: {  
            strMsg = @"kCGBlendModeMultiply: 正片叠底；混合了前景和背景的颜色，最终颜色比原先的都暗";  
            break;  
        }  
        case kCGBlendModeScreen: {  
            strMsg = @"kCGBlendModeScreen: 滤色；把前景和背景图的颜色先反过来，然后混合";  
            break;  
        }  
        case kCGBlendModeOverlay: {  
            strMsg = @"kCGBlendModeOverlay: 覆盖；能保留灰度信息，结合kCGBlendModeSaturation能保留透明度信息，在imageWithBlendMode方法中两次执行drawInRect方法实现我们基本需求";  
            break;  
        }  
        case kCGBlendModeDarken: {  
            strMsg = @"kCGBlendModeDarken: 变暗";  
            break;  
        }  
        case kCGBlendModeLighten: {  
            strMsg = @"kCGBlendModeLighten: 变亮";  
            break;  
        }  
        case kCGBlendModeColorDodge: {  
            strMsg = @"kCGBlendModeColorDodge: 颜色变淡";  
            break;  
        }  
        case kCGBlendModeColorBurn: {  
            strMsg = @"kCGBlendModeColorBurn: 颜色加深";  
            break;  
        }  
        case kCGBlendModeSoftLight: {  
            strMsg = @"kCGBlendModeSoftLight: 柔光";  
            break;  
        }  
        case kCGBlendModeHardLight: {  
            strMsg = @"kCGBlendModeHardLight: 强光";  
            break;  
        }  
        case kCGBlendModeDifference: {  
            strMsg = @"kCGBlendModeDifference: 插值";  
            break;  
        }  
        case kCGBlendModeExclusion: {  
            strMsg = @"kCGBlendModeExclusion: 排除";  
            break;  
        }  
        case kCGBlendModeHue: {  
            strMsg = @"kCGBlendModeHue: 色调";  
            break;  
        }  
        case kCGBlendModeSaturation: {  
            strMsg = @"kCGBlendModeSaturation: 饱和度";  
            break;  
        }  
        case kCGBlendModeColor: {  
            strMsg = @"kCGBlendModeColor: 颜色";  
            break;  
        }  
        case kCGBlendModeLuminosity: {  
            strMsg = @"kCGBlendModeLuminosity: 亮度";  
            break;  
        }  
            //Apple额外定义的枚举  
            //R: premultiplied result, 表示混合结果  
            //S: Source, 表示源颜色(Sa对应透明度值: 0.0-1.0)  
            //D: destination colors with alpha, 表示带透明度的目标颜色(Da对应透明度值: 0.0-1.0)  
        case kCGBlendModeClear: {  
            strMsg = @"kCGBlendModeClear: R = 0";  
            break;  
        }  
        case kCGBlendModeCopy: {  
            strMsg = @"kCGBlendModeCopy: R = S";  
            break;  
        }  
        case kCGBlendModeSourceIn: {  
            strMsg = @"kCGBlendModeSourceIn: R = S*Da";  
            break;  
        }  
        case kCGBlendModeSourceOut: {  
            strMsg = @"kCGBlendModeSourceOut: R = S*(1 - Da)";  
            break;  
        }  
        case kCGBlendModeSourceAtop: {  
            strMsg = @"kCGBlendModeSourceAtop: R = S*Da + D*(1 - Sa)";  
            break;  
        }  
        case kCGBlendModeDestinationOver: {  
            strMsg = @"kCGBlendModeDestinationOver: R = S*(1 - Da) + D";  
            break;  
        }  
        case kCGBlendModeDestinationIn: {  
            strMsg = @"kCGBlendModeDestinationIn: R = D*Sa；能保留透明度信息";  
            break;  
        }  
        case kCGBlendModeDestinationOut: {  
            strMsg = @"kCGBlendModeDestinationOut: R = D*(1 - Sa)";  
            break;  
        }  
        case kCGBlendModeDestinationAtop: {  
            strMsg = @"kCGBlendModeDestinationAtop: R = S*(1 - Da) + D*Sa";  
            break;  
        }  
        case kCGBlendModeXOR: {  
            strMsg = @"kCGBlendModeXOR: R = S*(1 - Da) + D*(1 - Sa)";  
            break;  
        }  
        case kCGBlendModePlusDarker: {  
            strMsg = @"kCGBlendModePlusDarker: R = MAX(0, (1 - D) + (1 - S))";  
            break;  
        }  
        case kCGBlendModePlusLighter: {  
            strMsg = @"kCGBlendModePlusLighter: R = MIN(1, S + D)（最后一种混合模式）";  
            break;  
        }  
        default: {  
            break;  
        }  
    }