Question

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 ?

Answer 1

I am learning OpenGL NeHe Production.When I read lesson22 Bump-Mapping

Why? The NeHe tutorials are terribly outdated, and the Bump Mapping technique outlined there completely obsolete. It’s been superseeded by shader based normal mapping for well over 13 years (until 2003 texture combiners were used instead of shaders).

Also instead of BMPs you should use a image file format better suited for textures (with alpha channel). Like:

• TGA
• PNG
• OpenEXR

Also the various compressed DX texture formats are a good choice for several applications.

My question is :why the index of Image->data is a*3???

Extracting the red channel of a RGB DIB.