Unity5.x Surface Shader使用alpha blend

#pragma surface surfaceFunction lightModel [optionalparams]

在Unity4.x中的Surface Shader，只要加上Blend SrcAlpha OneMinusSrcAlpha即可实现alpha blend的效果。

而在Unity5.x中，必须在#pragma中加上alhpa:blend或alpha:fade等才可实现效果。

如果不加这些，加上Blend SrcAlpha OneMinusSrcAlpha 在#pragma中加上keepalpha也可实现alpha blend的效果。

总结：要在Unity5.x的Surface Shader中实现alhpa blend的效果，

要么:#pragma surface surfaceFunction lightModel alpha:auto(blend,fade等等)

要么：Blend SrcAlpha OneMinusSrcAlpha

#pragma surface surfaceFunction lightModel keepalpha

Optional parameters

Transparency and alpha testing is controlled by alpha and alphatest directives. Transparency can typically be of two kinds: traditional alpha blending (used for fading objects out) or more physically plausible “premultiplied blending” (which allows semitransparent surfaces to retain proper specular reflections). Enabling semitransparency makes the generated surface shader code contain blending commands; whereas enabling alpha cutout will do a fragment discard in the generated pixel shader, based on the given variable.

• alpha or alpha:auto - Will pick fade-transparency (same as alpha:fade) for simple lighting functions, and premultiplied transparency (same as alpha:premul) for physically based lighting functions.
• alpha:blend - Enable alpha blending.
• alpha:fade - Enable traditional fade-transparency.
• alpha:premul - Enable premultiplied alpha transparency.
• alphatest:VariableName - Enable alpha cutout transparency. Cutoff value is in a float variable with VariableName. You’ll likely also want to use addshadow directive to generate proper shadow caster pass.
• keepalpha - By default opaque surface shaders write 1.0 (white) into alpha channel, no matter what’s output in the Alpha of output struct or what’s returned by the lighting function. Using this option allows keeping lighting function’s alpha value even for opaque surface shaders.
• decal:add - Additive decal shader (e.g. terrain AddPass). This is meant for objects that lie atop of other surfaces, and use additive blending. See Surface Shader Examples
• decal:blend - Semitransparent decal shader. This is meant for objects that lie atop of other surfaces, and use alpha blending. See Surface Shader Examples