GLSL常用函数

• genType abs (genType x) 绝对值 Returns x if x >= 0, otherwise it returns –x.
• genType sign (genType x) 符号 Returns 1.0 if x > 0, 0.0 if x = 0, or –1.0 if x < 0
• genType floor (genType x) 向下取整 Returns a value equal to the nearest integer that is less than or equal to x
• genType ceil (genType x) 向上取整 Returns a value equal to the nearest integer that is greater than or equal to x
• genType fract (genType x) 正数取小数，负数取(小数部分+1)Returns x – floor (x)
• genType mod (genType x, genType y) 取模 Modulus. Returns x – y ∗ floor (x/y)

如果要对参数截断到[0.0,1.0], 正确的表达式是factor = mod(factor, 1.0 + EQUAL_THRESHOLD);
测试发现，shader精度为highp时，EQUAL_THRESHOLD为1e-8无效，至少要1e-7 

• genType min (genType x, genType y) 最小值 Returns y if y < x, otherwise it returns x
• genType max (genType x, genType y) 最大值 Returns y if x < y, otherwise it returns x.
• genType clamp (genType x,genType minVal, genType maxVal) 截断 Returns min (max (x, minVal), maxVal) Results are undefined if minVal > maxVal.
• genType mix (genType x,genType y,genType a) 混合(1-alpha) Returns the linear blend of x and y: x(1-a)+y*a
• genType step (genType edge, genType x) 是阶梯或平地则返回1 Returns 0.0 if x < edge, otherwise it returns 1.0
• genType smoothstep (genType edge0,genType edge1,genType x)
  顺滑返回0~1的值 Returns 0.0 if x <= edge0 and 1.0 if x >= edge1 and performs smooth Hermite interpolation between 0 and 1 when edge0 < x < edge1. This is useful in cases where you would want a threshold function with a smooth transition. This is equivalent to: genType t; t = clamp ((x – edge0) / (edge1 – edge0), 0, 1); return t * t * (3 – 2 * t); Results are undefined if edge0 >= edge1.