step函数

        step(edge,x)：当x>edge时返回1，否则返回0

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;

void main(){
  vec2 st = gl_FragCoord.xy/u_resolution.xy;

  float f = step(0.5, st.x);

  gl_FragColor = vec4(f, 0, 0, 1.0); 
}

 类似下面的javaScript代码

float myStep(float edge, float x){
  if(x>= edge){
    return 1.0;
  }else{
    return 0.0;
  }
}