I am learning OpenGL NeHe Production.When I read lesson22 Bump-Mapping、Multi-texture,I got a problem.
When I load logo bmp file,I need to load two bmp files:one stores color information ,and another stores alpha information. here is the two bmp files: OpenGL_Alpha.bmp:
and OpenGL.bmp :
Here is the code:
if (Image=auxDIBImageLoad("Data/OpenGL_ALPHA.bmp")) {
alpha=new char[4*Image->sizeX*Image->sizeY];
for (int a=0; a<Image->sizeX*Image->sizeY; a++)
alpha[4*a+3]=Image->data[a*3]; //???????
if (!(Image=auxDIBImageLoad("Data/OpenGL.bmp"))) status=false;
for (a=0; a<Image->sizeX*Image->sizeY; a++) {
alpha[4*a]=Image->data[a*3];//??????????
alpha[4*a+1]=Image->data[a*3+1];
alpha[4*a+2]=Image->data[a*3+2];
}
glGenTextures(1, &glLogo);
glBindTexture(GL_TEXTURE_2D, glLogo);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, Image->sizeX, Image->sizeY, 0, GL_RGBA, GL_UNSIGNED_BYTE, alpha);
delete alpha;
}
My question is :why the index of Image->data is a*3???
Could someone interpret for me ?