Ogre-渐变背景色（gradient background）的实现

转载自：http://blog.csdn.net/hefee/article/details/6287341

背景色在ogre里面是通过ViewPort类中的setBackgroundColour（）这个成员函数实现的，不过此类只提供给单一颜色的设置！！

不过，我们可以通过间接的方法去创建渐变的背景色，下面详细介绍：

1. 重构Ogre::Rectangle2D类：（OgreColourRectangle2D.h）

#ifndef OGRECOLOUREDRECTANGLE2D_H
#define OGRECOLOUREDRECTANGLE2D_H

#include "OgreRectangle2D.h"

class ColouredRectangle2D : public Ogre::Rectangle2D
{
public:
   ColouredRectangle2D(bool includeTextureCoordinates = false);
   ~ColouredRectangle2D();

   void setColours(const Ogre::ColourValue &topLeft, const Ogre::ColourValue &bottomLeft, const Ogre::ColourValue &topRight, const Ogre::ColourValue &bottomRight);

};

#endif // OGRECOLOUREDRECTANGLE2D_H

2. 重构类的实现: (OgreColourRectangle2D.cpp)

#include "OgreColouredRectangle2D.h"

#include "OgreHardwareBufferManager.h"

//#define POSITION_BINDING 0
//#define NORMAL_BINDING 1
//#define TEXCOORD_BINDING 2
#define COLOUR_BINDING 3

ColouredRectangle2D::ColouredRectangle2D(bool includeTextureCoordinates /*= false*/):Ogre::Rectangle2D(includeTextureCoordinates)
{
   Ogre::VertexDeclaration* decl = mRenderOp.vertexData->vertexDeclaration;

   decl->addElement(COLOUR_BINDING, 0, Ogre::VET_COLOUR, Ogre::VES_DIFFUSE);
   Ogre::VertexBufferBinding* bind = mRenderOp.vertexData->vertexBufferBinding;

    Ogre::HardwareVertexBufferSharedPtr vbuf = 
        Ogre::HardwareBufferManager::getSingleton().createVertexBuffer(
        decl->getVertexSize(COLOUR_BINDING),
        mRenderOp.vertexData->vertexCount,
        Ogre::HardwareBuffer::HBU_STATIC_WRITE_ONLY);

    // Bind buffer
    bind->setBinding(COLOUR_BINDING, vbuf);
}

ColouredRectangle2D::~ColouredRectangle2D()
{
}

void ColouredRectangle2D::setColours(const Ogre::ColourValue &topLeft, const Ogre::ColourValue &bottomLeft, const Ogre::ColourValue &topRight, const Ogre::ColourValue &bottomRight)
{
   Ogre::HardwareVertexBufferSharedPtr vbuf = 
      mRenderOp.vertexData->vertexBufferBinding->getBuffer(COLOUR_BINDING);
   unsigned int* pUint32 = static_cast<unsigned int*>(vbuf->lock(Ogre::HardwareBuffer::HBL_DISCARD));

   const Ogre::VertexElementType srcType = Ogre::VertexElement::getBestColourVertexElementType();

   *pUint32++ = Ogre::VertexElement::convertColourValue( topLeft, srcType );

   *pUint32++ = Ogre::VertexElement::convertColourValue( bottomLeft, srcType );

   *pUint32++ = Ogre::VertexElement::convertColourValue( topRight, srcType );

   *pUint32++ = Ogre::VertexElement::convertColourValue( bottomRight, srcType );

   vbuf->unlock();
}

3. 重构类应用

// Create background material
      Ogre::MaterialPtr material = Ogre::MaterialManager::getSingleton().create("Background", "General");
      material->getTechnique(0)->getPass(0)->setDepthCheckEnabled(false);
      material->getTechnique(0)->getPass(0)->setDepthWriteEnabled(false);
      material->getTechnique(0)->getPass(0)->setLightingEnabled(false);
       
      // Create background rectangle covering the whole screen
      ColouredRectangle2D* rect = new ColouredRectangle2D();
      rect->setCorners(-1.0, 1.0, 1.0, -1.0);
      rect->setMaterial("Background");
      
      // Set the colours
      rect->setColours( Ogre::ColourValue::Red, Ogre::ColourValue::Green, Ogre::ColourValue::Blue, Ogre::ColourValue::Black ); 

      // Render the background before everything else
      rect->setRenderQueueGroup(Ogre::RENDER_QUEUE_BACKGROUND);
       
      // Use infinite AAB to always stay visible
      Ogre::AxisAlignedBox aabInf;
      aabInf.setInfinite();
      rect->setBoundingBox(aabInf);
       
      // Attach background to the scene
      SceneNode* node = mSceneMgr->getRootSceneNode()->createChildSceneNode("Background");
      node->attachObject(rect);

完成以上三步，就可以实现一个viewport的渐变背景色。基本原理是:创建s一个拥有渐变颜色的2D Rectangle，并使其作为背景渲染。有几点值得注意的时候：

1. #define COLOUR_BINDING 3，以及后续用到COLOUR_BINDING的各个地方。由于Ogre的各个版本的差别（本人使用的是1.7版本的），一些低版本的Ogre没有实现像cpp中注释的前三种，即POSITION、Normal、TEXCoord，此时如果冒然使用Colour，就会出现意想不到的错误。（当前的Mogre版本就没有实现3个，具体哪几个不慎清楚）

2. rect->setRenderQueueGroup(Ogre::RENDER_QUEUE_BACKGROUND);这句很重要。

3. 如果要使用某个特定的图片作为背景，则简单的多:有兴趣的可以自己试试