創建帶Mipmap的osg::Image

我們常用osgDB::readImage或者osg::Image::allocateImage()方式創建Image對象，

跟深一步的帶Mipmap的Image怎樣創建呢？ 偶然在分析osgParticle::PrecipitationEffect.cpp中的代碼時有所收獲

不廢話，直接貼代碼

以下這段代碼是申請一個帶mipmap的寬高相同(size)的Image的方法，image的總大小totalSize的計算方法以及mipmapData中要存儲的各級mipmap的大小

osg::Image* image = new osg::Image;
 osg::Image::MipmapDataType mipmapData;

unsigned int s = size;
unsigned int totalSize = 0;

unsigned i;
    
//C語言 n>>=1 中的>>=意思是先將變量n的各個二進制位順序右移1位，最高位補二進制0，然后將這個結果再復制給n。
for(i=0; s>0; s>>=1, ++i)   
{
        
    if (i>0) mipmapData.push_back(totalSize);
 
    totalSize += s*s*4;   // 4 代表RGBA方式占4個字節
}
unsigned char* ptr = new unsigned char[totalSize];   
image->setImage(size, size, size, GL_RGBA, GL_RGBA, GL_UNSIGNED_BYTE, ptr, osg::Image::USE_NEW_DELETE,1);   
image->setMipmapLevels(mipmapData);

s = size;

   // 填充image分級數據
    for(i=0; s>0; s>>=1, ++i)
    {
        fillSpotLightImage(ptr, centerColour, backgroudColour, s, power);
        ptr += s*s*4;
    }

return image;

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static void fillSpotLightImage(unsigned char* ptr, const osg::Vec4& centerColour, const osg::Vec4& backgroudColour, unsigned int size, float power)
{
    if (size==1)
    {
        float r = 0.5f;
        osg::Vec4 color = centerColour*r+backgroudColour*(1.0f-r);
        *ptr++ = (unsigned char)((color[0])*255.0f);
        *ptr++ = (unsigned char)((color[1])*255.0f);
        *ptr++ = (unsigned char)((color[2])*255.0f);
        *ptr++ = (unsigned char)((color[3])*255.0f);
        return;
    }

    float mid = (float(size)-1.0f)*0.5f;
    float div = 2.0f/float(size);
    for(unsigned int r=0;r<size;++r)
    {
        //unsigned char* ptr = image->data(0,r,0);
        for(unsigned int c=0;c<size;++c)
        {
            float dx = (float(c) - mid)*div;
            float dy = (float(r) - mid)*div;
            float r = powf(1.0f-sqrtf(dx*dx+dy*dy),power);
            if (r<0.0f) r=0.0f;
            osg::Vec4 color = centerColour*r+backgroudColour*(1.0f-r);
            *ptr++ = (unsigned char)((color[0])*255.0f);
            *ptr++ = (unsigned char)((color[1])*255.0f);
            *ptr++ = (unsigned char)((color[2])*255.0f);
            *ptr++ = (unsigned char)((color[3])*255.0f);
        }
    }
}