Blender MixColor 节点算法解析

最近在写Blender内的材质，对于MixColor这个节点产生了一些困惑：

比如上图Multi,Add，Mix对应的Fac值是如何参与计算的。Multi,Add，Mix等本身的计算公式还好。

有：function mix = mix(col1,col2,fac) = col1 * (1-fac) + col2 * fac;

忍不住去翻了下Blender源码。

Mix Color定义在：

https://github.com/blender/blender/blob/2864c20302513dae0443af461d225b5a1987267a/intern/cycles/kernel/osl/shaders/node_mix_color.osl#L9

/* SPDX-FileCopyrightText: 2011-2022 Blender Foundation
 *
 * SPDX-License-Identifier: Apache-2.0 */

#include "node_color.h"
#include "node_color_blend.h"
#include "stdcycles.h"

shader node_mix_color(string blend_type = "mix",
                      int use_clamp = 0,
                      int use_clamp_result = 0,
                      float Factor = 0.5,
                      color A = 0.0,
                      color B = 0.0,
                      output color Result = 0.0)
{
  float t = (use_clamp) ? clamp(Factor, 0.0, 1.0) : Factor;

  if (blend_type == "mix")
    Result = mix(A, B, t);
  if (blend_type == "add")
    Result = node_mix_add(t, A, B);
  if (blend_type == "multiply")
    Result = node_mix_mul(t, A, B);
  if (blend_type == "screen")
    Result = node_mix_screen(t, A, B);
  if (blend_type == "overlay")
    Result = node_mix_overlay(t, A, B);
  if (blend_type == "subtract")
    Result = node_mix_sub(t, A, B);
  if (blend_type == "divide")
    Result = node_mix_div(t, A, B);
  if (blend_type == "difference")
    Result = node_mix_diff(t, A, B);
  if (blend_type == "exclusion")
    Result = node_mix_exclusion(t, A, B);
  if (blend_type == "darken")
    Result = node_mix_dark(t, A, B);
  if (blend_type == "lighten")
    Result = node_mix_light(t, A, B);
  if (blend_type == "dodge")
    Result = node_mix_dodge(t, A, B);
  if (blend_type == "burn")
    Result = node_mix_burn(t, A, B);
  if (blend_type == "hue")
    Result = node_mix_hue(t, A, B);
  if (blend_type == "saturation")
    Result = node_mix_sat(t, A, B);
  if (blend_type == "value")
    Result = node_mix_val(t, A, B);
  if (blend_type == "color")
    Result = node_mix_color(t, A, B);
  if (blend_type == "soft_light")
    Result = node_mix_soft(t, A, B);
  if (blend_type == "linear_light")
    Result = node_mix_linear(t, A, B);

  if (use_clamp_result)
    Result = clamp(Result, 0.0, 1.0);
}

在node_color_blend.h头文件中：

color node_mix_add(float t, color col1, color col2)
{
  return mix(col1, col1 + col2, t);
}

color node_mix_mul(float t, color col1, color col2)
{
  return mix(col1, col1 * col2, t);
}
...

所以就mul/add而言，他们与Mix的不同就是算法中的颜色2参数。

补充dark 、light算法：

color node_mix_dark(float t, color col1, color col2)
{
  return mix(col1, min(col1, col2), t);
}

color node_mix_light(float t, color col1, color col2)
{
  return mix(col1, max(col1, col2), t);
}