学步园

转载自：http://renderingpipeline.com/2012/06/photoshop-blendmodi-glsl/

I once wrote a texturing tool in which you could paint on a 3D model on multiple layers. These layers were composing the final texture just as you know it from photoshop, complete with alpha, opacity slider and: blend modes. these blend modes are quite simple
to implement and it can be useful to use the exact same modes as they are familiar to artists and it’s possible to prototype the final effects in PS (or even write importers and exporters to psd files). The hardest part actually was collecting all the formulas,
so this is why I’m collecting them here once again:

I’m assuming the ‘lower’ color is stored in base and the ‘upper’ color is stored in blend.

• multiply:

  result = base * blend;

• screen:

  result = vec4(1.0) - ((vec4(1.0) - blend) * (vec4(1.0) - base));

• darken:

  result = min( blend, base );

• lighten:

  result = max( blend, base );

• difference:

  result = abs( base - blend );

• negation:

  result = vec4(1.0) - abs( vec4(1.0) - base - blend );

• exclusion:

  result = base + blend - (2.0*base*blend);

• overlay:

  # per channel:
  if (base < 0.5) {
      result = 2.0 * base * blend;
  } else {
      result = vec4(1.0) - 2.0 * (vec4(1.0) - blend) * (vec4(1.0) - base);
  }

• hard light:

  # per channel:
  if (blend < 0.5) {
      result = 2.0 * base * blend;
  } else {
      result = vec4(1.0) - 2.0 * (vec4(1.0) - blend) * (vec4(1.0) - base);
  }

• soft light:

  # per channel:
  if (blend < 0.5) {
      result = 2.0 * base * blend + base*base - 2.0 * base*base*blend;
  } else {
      result = 2.0 * sqrt(base) * blend - sqrt(base) + 2.0 * base - 2.0 * base*blend;
  }

• dodge:

  result = base / (vec4(1.0) - blend);

• burn:

  result = vec4(1.0) - (vec4(1.0) - base) / blend;

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