cocos2d-x3.2中用shader使图片背景透明

今天有人问我问题，说怎么实现背景虚化，换句话说，就是把某张图片的背景颜色（比如白色）给弄没了，不然贴在屏幕上有白色背景。下面是解决方法。用shader处理了像素，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__) */

Shader.cpp

#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));

        // 加载顶点着色器和片元着色器
        m_pShaderProgram = new  CCGLProgram();
        m_pShaderProgram ->initWithVertexShaderByteArray(ccPositionTextureA8Color_vert, transparentshader);

        CHECK_GL_ERROR_DEBUG();

        // 启用顶点着色器的attribute变量，坐标、纹理坐标、颜色
        m_pShaderProgram->addAttribute(kCCAttributeNamePosition, kCCVertexAttrib_Position);
        m_pShaderProgram->addAttribute(kCCAttributeNameColor, kCCVertexAttrib_Color);
        m_pShaderProgram->addAttribute(kCCAttributeNameTexCoord, kCCVertexAttrib_TexCoords);

        CHECK_GL_ERROR_DEBUG();

        // 自定义着色器链接
        m_pShaderProgram->link();

        CHECK_GL_ERROR_DEBUG();

        // 设置移动、缩放、旋转矩阵
        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();

    //
	// 启用attributes变量输入，顶点坐标，纹理坐标，颜色
	//
	ccGLEnableVertexAttribs( kCCVertexAttribFlag_PosColorTex );
    ccGLBlendFunc(m_sBlendFunc.src, m_sBlendFunc.dst);

    m_pShaderProgram->use();
    m_pShaderProgram->setUniformsForBuiltins();

    // 绑定纹理到纹理槽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");
}

片段着色器代码

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]);                                      \nif (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;
}";

注意shader编程没有隐士数据类型转换，所以都显示为float了。

然后ratio是指在rgb中找最大的，如果ratio为0直接将alpha设为0，否则alpha设为ratio，然后各rgb除以ratio，这里是为了做渐变，否则变化太生硬。

上图看看吧

改变前：

改变后：