我们常用osgDB::readImage或者osg::Image::allocateImage()方式创建Image对象，
跟深一步的带Mipmap的Image怎样创建呢？ 偶然在分析osgParticle::PrecipitationEffect.cpp中的代码时有所收获
不废话，直接贴代码
以下这段代码是申请一个带mipmap的宽高相同(size)的Image的方法，image的总大小totalSize的计算方法以及mipmapData中要存储的各级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语言 n>>=1 中的>>=意思是先将变量n的各个二进制位顺序右移1位，最高位补二进制0，然后将这个结果再复制给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个字节
}
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分级数据
for(i=0; s>0; s>>=1, ++i)
{
fillSpotLightImage(ptr, centerColour, backgroudColour, s, power);
ptr += s*s*4;
}
return image;
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);
}
}
}
