今天有人問我問題，說怎麼實現背景虛化，換句話說，就是把某張圖片的背景顏色（比如白色）給弄沒了，不然貼在螢幕上有白色背景。下面是解決方法。用shader處理了畫素，使黑色背景透明。

Shader.h

#ifndef __TestShader__ShaderSprite__
#define __TestShader__ShaderSprite__

#include "cocos2d.h"
USING_NS_CC;

class ShaderSprite : public CCSprite {
    
public:
    static ShaderSprite* create(const char* pszFileName);
    virtual bool initWithTexture(CCTexture2D *pTexture, const CCRect& rect);
    virtual void draw(void);
};

#endif /* defined(__TestShader__ShaderSprite__) */
 

#include "ShaderSprite.h"

static CC_DLL const GLchar *transparentshader =
#include "tansparentshader.h"

ShaderSprite* ShaderSprite::create(const char *pszFileName)
{
    ShaderSprite *pRet = new ShaderSprite();
    if (pRet && pRet->initWithFile(pszFileName)) {
        pRet->autorelease();
        return pRet;
    }
    else
    {
        delete pRet;
        pRet = NULL;
        return NULL;
    }
}

bool ShaderSprite::initWithTexture(CCTexture2D *pTexture, const CCRect& rect)
{
    do{
//        CCLog("override initWithTexture!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");
        CC_BREAK_IF(!CCSprite::initWithTexture(pTexture, rect));
        
        // 載入頂點著色器和片元著色器
        m_pShaderProgram = new  CCGLProgram();
        m_pShaderProgram ->initWithVertexShaderByteArray(ccPositionTextureA8Color_vert, transparentshader);
        
        CHECK_GL_ERROR_DEBUG();
        
        // 啟用頂點著色器的attribute變數，座標、紋理座標、顏色
        m_pShaderProgram->addAttribute(kCCAttributeNamePosition, kCCVertexAttrib_Position);
        m_pShaderProgram->addAttribute(kCCAttributeNameColor, kCCVertexAttrib_Color);
        m_pShaderProgram->addAttribute(kCCAttributeNameTexCoord, kCCVertexAttrib_TexCoords);
        
        CHECK_GL_ERROR_DEBUG();
        
        // 自定義著色器連結
        m_pShaderProgram->link();
        
        CHECK_GL_ERROR_DEBUG();
        
        // 設定移動、縮放、旋轉矩陣
        m_pShaderProgram->updateUniforms();
        
        CHECK_GL_ERROR_DEBUG();
        
        return true;
        
    }while(0);
    return false;
}

void ShaderSprite::draw(void)
{
//    CCLog("override draw!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");
    CC_PROFILER_START_CATEGORY(kCCProfilerCategorySprite, "CCSprite - draw");
    
    CCAssert(!m_pobBatchNode, "If CCSprite is being rendered by CCSpriteBatchNode, CCSprite#draw SHOULD NOT be called");
    
    CC_NODE_DRAW_SETUP();
    
    //
	// 啟用attributes變數輸入，頂點座標，紋理座標，顏色
	//
	ccGLEnableVertexAttribs( kCCVertexAttribFlag_PosColorTex );
    ccGLBlendFunc(m_sBlendFunc.src, m_sBlendFunc.dst);
    
    m_pShaderProgram->use();
    m_pShaderProgram->setUniformsForBuiltins();
    
    // 繫結紋理到紋理槽0
    ccGLBindTexture2D(m_pobTexture->getName());


    
#define kQuadSize sizeof(m_sQuad.bl)
	long offset = (long)&m_sQuad;
    
	// vertex
	int diff = offsetof( ccV3F_C4B_T2F, vertices);
	glVertexAttribPointer(kCCVertexAttrib_Position, 3, GL_FLOAT, GL_FALSE, kQuadSize, (void*) (offset + diff));
    
	// texCoods
	diff = offsetof( ccV3F_C4B_T2F, texCoords);
	glVertexAttribPointer(kCCVertexAttrib_TexCoords, 2, GL_FLOAT, GL_FALSE, kQuadSize, (void*)(offset + diff));
    
	// color
	diff = offsetof( ccV3F_C4B_T2F, colors);
	glVertexAttribPointer(kCCVertexAttrib_Color, 4, GL_UNSIGNED_BYTE, GL_TRUE, kQuadSize, (void*)(offset + diff));
    
    
	glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
    
	CHECK_GL_ERROR_DEBUG();

    CC_INCREMENT_GL_DRAWS(1);
    CC_PROFILER_STOP_CATEGORY(kCCProfilerCategorySprite, "CCSprite - draw");
}
 

tansparentshader.h

                                                       
#ifdef GL_ES                                            
precision lowp float;                                   
#endif                                                  
varying vec4 v_fragmentColor;                           
varying vec2 v_texCoord;                                
uniform sampler2D u_texture;                            
void main()                                             
{                                                       
    float ratio=0.0;                                    
    vec4 texColor = texture2D(u_texture, v_texCoord);   
    ratio = texColor[0] > texColor[1]?(texColor[0] > texColor[2] ? texColor[0] : texColor[2]) :(texColor[1] > texColor[2]? texColor[1] : texColor[2]);                                      \n\
if (ratio != 0.0)                                          
{                                                          
    texColor[0] = texColor[0] /  ratio;                    
    texColor[1] = texColor[1] /  ratio;                    
    texColor[2] = texColor[2] /  ratio;                    
    texColor[3] = ratio;                                   
}                                                          
else                                                      
{                                                          
    texColor[3] = 0.0;                                     
}                                                          
gl_FragColor = v_fragmentColor*texColor;                   
}";
 

然後ratio是指在rgb中找最大的，如果ratio為0直接將alpha設為0，否則alpha設為ratio，然後各rgb除以ratio，這裡是為了做漸變，否則變化太生硬。

上圖看看吧

改變前：

改變後：

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