美圖秀秀濾鏡之色溫（白平衡）

　　色溫色溫是表示光源光譜質量最通用的指標。

　　GPUImage中我們通過GPUImageWhiteBalanceFilter來實現

　　頂點著色

uniform sampler2D inputImageTexture;
varying highp vec2 textureCoordinate;
 
uniform lowp float temperature;
uniform lowp float tint;

const lowp vec3 warmFilter = vec3(0.93, 0.54, 0.0);

const mediump mat3 RGBtoYIQ = mat3(0.299, 0.587, 0.114, 0.596, -0.274, -0.322, 0.212, -0.523, 0.311);
const mediump mat3 YIQtoRGB = mat3(1.0, 0.956, 0.621, 1.0, -0.272, -0.647, 1.0, -1.105, 1.702);

void main()
{
    lowp vec4 source = texture2D(inputImageTexture, textureCoordinate);
    
    mediump vec3 yiq = RGBtoYIQ * source.rgb; //adjusting tint
    yiq.b = clamp(yiq.b + tint*0.5226*0.1, -0.5226, 0.5226);
    lowp vec3 rgb = YIQtoRGB * yiq;

    lowp vec3 processed = vec3(
        (rgb.r < 0.5 ? (2.0 * rgb.r * warmFilter.r) : (1.0 - 2.0 * (1.0 - rgb.r) * (1.0 - warmFilter.r))), //adjusting temperature
        (rgb.g < 0.5 ? (2.0 * rgb.g * warmFilter.g) : (1.0 - 2.0 * (1.0 - rgb.g) * (1.0 - warmFilter.g))), 
        (rgb.b < 0.5 ? (2.0 * rgb.b * warmFilter.b) : (1.0 - 2.0 * (1.0 - rgb.b) * (1.0 - warmFilter.b))));

    gl_FragColor = vec4(mix(rgb, processed, temperature), source.a);
}

 色溫演算法較複雜，它與溫度有關

具體應用

+ (UIImage *)changeValueForWhiteBalanceFilter:(float)value image:(UIImage *)image
{
    GPUImageWhiteBalanceFilter *filter = [[GPUImageWhiteBalanceFilter alloc] init];
    filter.temperature = value;
    filter.tint = 0.0;
    [filter forceProcessingAtSize:image.size];
    GPUImagePicture *pic = [[GPUImagePicture alloc] initWithImage:image];
    [pic addTarget:filter];
    
    [pic processImage];
    [filter useNextFrameForImageCapture];
    return [filter imageFromCurrentFramebuffer];
}

效果

　　　　　　　　　　高溫度 　　　　　　　　　　　　　　　　　　　　　　　　　　低溫度