Why reverse the element order when sampling from normal map

[unknownue]

Here is the code of sampling normal from normal map:

tinyrenderer/model.cpp

Lines 86 to 93 in 907bb56

	Vec3f Model::normal(Vec2f uvf) {
	Vec2i uv(uvf[0]*normalmap_.get_width(), uvf[1]*normalmap_.get_height());
	TGAColor c = normalmap_.get(uv[0], uv[1]);
	Vec3f res;
	for (int i=0; i<3; i++)
	res[2-i] = (float)c[i]/255.f*2.f - 1.f;
	return res;
	}

I can't understand why the element order of normal is reversed, that is, why not write the code like this:

for (int i=0; i<3; i++)
    res[i] = (float)c[i]/255.f*2.f - 1.f;

[ssloy]

It is a simple question of color packing in TGA images; they are stored in the BGR order and not RGB.