Load bmp file as texture using auxDIBImageLoad in OpenGL - opengl

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 The NeHe tutorials are terribly outdated, and the Bump Mapping technique outlined there completely obsolete.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.

Answer 2

It's the channel offset. The RGB data is stored as three consecutive bytes. Here 'a' represents which pixel (group of 3 bytes, one for R, one for G, one for B).
Think of a*3 as a pointer to an array of 3 bytes:
char* myPixel = Image->data + (a*3);
char red = myPixel[0];
char green = myPixel[1];
char blue = myPixel[2];