Using VirtualProgram to Make the model transparent, but no effect

[iamzheng00]

I want to make the osg model - "cow" transparent. The version of osgEarth is 3.2.
I use virtualprogram to do that, but the color.a value seems don't work. Could anyone please give some help,thanks!
the main relevant code is as follows:

const char* modelTransparencyFrag = R"(

		 #version 330`
		 uniform float modelTransparencyValue;    
		 void modelTransparencyFunc(inout vec4 color)
		{        
			color=vec4(color.r, color.g, color.b, color.a * modelTransparencyValue);   
		})";

osg::ref_ptr<osg::Node> cow= osgDB::readRefNodeFile("E:/data_osg/cow.osg");

osgEarth::VirtualProgram* vp = osgEarth::VirtualProgram::getOrCreate(cow->getOrCreateStateSet());
vp->setFunction("modelTransparencyFunc", modelTransparencyFrag, osgEarth::ShaderComp::LOCATION_FRAGMENT_COLORING);
osg::Uniform* transparentUniform = new osg::Uniform("modelTransparencyValue", 0.3f);
cow->getOrCreateStateSet()->addUniform(transparentUniform);
mapNode->addChild(cow);