Metal每日分享，颜色转换滤镜效果

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本案例的目的是理解如何用Metal实现像素颜色转换滤镜，通过对像素颜色的不同读取方式获取到相应像素颜色，灰度图移除场景中除了黑白灰以外所有的颜色，让整个图像灰度化；

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Demo

• HarbethDemo地址
• iDay每日分享文档地址

对照图

invert	bgra	brga
gbra	grba	rbga

实操代码

// 转成灰度图滤镜
let filter = C7ColorConvert(with: .gray)

// 方案1:
let dest = BoxxIO.init(element: originImage, filter: filter)
ImageView.image = try? dest.output()

dest.filters.forEach {
    NSLog("%@", "\($0.parameterDescription)")
}

// 方案2:
ImageView.image = try? originImage.make(filter: filter)

// 方案3:
ImageView.image = originImage ->> filter

实现原理

• 过滤器

这款滤镜采用并行计算编码器设计.compute(kernel: type.rawValue)

/// 颜色通道`RGB`位置转换
public struct C7ColorConvert: C7FilterProtocol {
    
    public enum ColorType: String, CaseIterable {
        case invert = "C7ColorInvert"
        case gray = "C7Color2Gray"
        case bgra = "C7Color2BGRA"
        case brga = "C7Color2BRGA"
        case gbra = "C7Color2GBRA"
        case grba = "C7Color2GRBA"
        case rbga = "C7Color2RBGA"
    }
    
    private let type: ColorType
    
    public var modifier: Modifier {
        return .compute(kernel: type.rawValue)
    }
    
    public init(with type: ColorType) {
        self.type = type
    }
}

• 着色器

取出像素rgb值，然后根据对应像素颜色；灰度图则是取所有的颜色分量，将它们加权或平均；

// 颜色反转，1 - rgb
kernel void C7ColorInvert(texture2d<half, access::write> outputTexture [[texture(0)]],
                          texture2d<half, access::read> inputTexture [[texture(1)]],
                          uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    
    const half4 outColor(1.0h - inColor.rgb, inColor.a);
    
    outputTexture.write(outColor, grid);
}

// 转灰度图
kernel void C7Color2Gray(texture2d<half, access::write> outputTexture [[texture(0)]],
                         texture2d<half, access::read> inputTexture [[texture(1)]],
                         uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    
    const half3 kRec709Luma = half3(0.2126, 0.7152, 0.0722);
    const half gray = dot(inColor.rgb, kRec709Luma);
    const half4 outColor = half4(half3(gray), 1.0h);
    
    outputTexture.write(outColor, grid);
}

kernel void C7Color2BGRA(texture2d<half, access::write> outputTexture [[texture(0)]],
                         texture2d<half, access::read> inputTexture [[texture(1)]],
                         uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    
    const half4 outColor(inColor.bgr, inColor.a);
    
    outputTexture.write(outColor, grid);
}

kernel void C7Color2BRGA(texture2d<half, access::write> outputTexture [[texture(0)]],
                         texture2d<half, access::read> inputTexture [[texture(1)]],
                         uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    
    const half4 outColor(inColor.brg, inColor.a);
    
    outputTexture.write(outColor, grid);
}

kernel void C7Color2GBRA(texture2d<half, access::write> outputTexture [[texture(0)]],
                         texture2d<half, access::read> inputTexture [[texture(1)]],
                         uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    
    const half4 outColor(inColor.gbr, inColor.a);
    
    outputTexture.write(outColor, grid);
}

kernel void C7Color2GRBA(texture2d<half, access::write> outputTexture [[texture(0)]],
                         texture2d<half, access::read> inputTexture [[texture(1)]],
                         uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    
    const half4 outColor(inColor.grb, inColor.a);
    
    outputTexture.write(outColor, grid);
}

kernel void C7Color2RBGA(texture2d<half, access::write> outputTexture [[texture(0)]],
                         texture2d<half, access::read> inputTexture [[texture(1)]],
                         uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    
    const half4 outColor(inColor.rbg, inColor.a);
    
    outputTexture.write(outColor, grid);
}

• 权值法

const half3 kRec709Luma = half3(0.2126, 0.7152, 0.0722);
const half gray = dot(inColor.rgb, kRec709Luma);
const half4 outColor = half4(half3(gray), 1.0h);

• 平均值法

const float color = (inColor.r + inColor.g + inColor.b) / 3.0;
const half4 outColor = half4(color, color, color, 1.0h);

总结：
一般由于人眼对不同颜色的敏感度不一样，所以三种颜色值的权重不一样，一般来说绿色最高，红色其次，蓝色最低，最合理的取值分别为Wr ＝ 30％，Wg ＝ 59％，Wb ＝ 11％，所以权值法相对效果更好一点。

最后

• 关于颜色转换滤镜介绍与设计到此为止吧。
• 慢慢再补充其他相关滤镜，喜欢就给我点个星🌟吧。

✌️.