USM锐化基本原理

USM锐化利用原图减去高斯模糊后的图，达到提升边缘清晰度的目的，基本公式如下

usm_color = gaussianBlurColor * -usmFactor + originColor * (1.0 + usmFactor)

usmFactor表示锐化程度，可以从0到1，超过1也是可以的

OpenGL Shader实现

高斯模糊实现

vec4 gaussianBlur(vec2 uv, float size) {
    float Pi = 6.28318530718; // Pi*2
    
    // GAUSSIAN BLUR SETTINGS {{{
    float Directions = 16.0; // BLUR DIRECTIONS (Default 16.0 - More is better but slower)
    float Quality = 4.0; // BLUR QUALITY (Default 4.0 - More is better but slower)
    float Size = size; // BLUR SIZE (Radius)
    // GAUSSIAN BLUR SETTINGS }}}
   
    vec2 Radius = Size/imageResolution.xy;
    
    // Pixel colour
    vec4 Color = texture(diffuseMap, uv);
    
    // Blur calculations
    for( float d=0.0; d<Pi; d+=Pi/Directions)
    {
        for(float i=1.0/Quality; i<=1.0; i+=1.0/Quality)
        {
            Color += texture( diffuseMap, uv+vec2(cos(d),sin(d))*Radius*i);
        }
    }
    
    // Output to screen
    Color /= Quality * Directions;
    return Color;
}

这里需要依赖图片的像素尺寸imageResolution，需要通过uniform传进来

USM实现

vec4 usm(vec4 originColor, float factor) {
    vec4 gaussianColor = gaussianBlur(vec2(fragUV.x, 1.0 - fragUV.y), 3);
    return gaussianColor * -factor + originColor * (1.0 + factor);
}