is TBN matrix really rotation matrix in nature?

Question

I've seen someguy's shader snippet, that the composed TBN matrix was used to transform the attribute a_pos.but obviously it's not related to bump-maping,or anything else. could this be rotation matrix made from input parameters of the shader? those vectors(normal,binormal,tangent) are nothing more than three bases, aren't they?

I mean the TBN matrix of the shader is not used to be the TBN matrix applied on bump-maping ever (as you know, to transform a vector to vector in tangent space,or to transform vector in tangent back using inverse of TBN matrix), and there 's no light calculation involved in. a little bit confusion in my mind.

Hope your guy help me get through it. sorry for my bad english,thank you!

Here's the shader, as the following:

#ifdef GL_ES
#else
#define highp
#define mediump
#define lowp
#endif
uniform   mat4 u_mvp;

attribute vec3 a_pos;
attribute vec3 a_normal;
attribute vec2 a_uv0;
attribute vec2 a_uv1;

varying   vec2 v_texcoord0;
varying   vec2 v_texcoordMap;

uniform highp float u_time;
uniform mediump float u_1DivLevelWidth;
uniform mediump float u_1DivLevelHeight;

void main()
{
    float mapPos = a_normal.x * 0.1 + a_normal.y * 0.6;
    vec2 v1 = vec2(cos(mapPos + u_time * 0.6), cos(mapPos + u_time * 0.9));
    mapPos = (a_normal.x + a_normal.y + a_pos.x + a_pos.y) * 3.0;
    vec2 v2 = vec2(cos(mapPos + u_time * 3.2 + (a_uv1.y * 0.1)), cos(mapPos + u_time * 2.9 + (a_uv1.y * 0.1))) * 0.02 * a_uv1.y;



    vec3 normal = normalize(vec3(v1 * (a_uv1.x * 0.2), 5.0));
    //vec3 normal = vec3(0.0, 0.0, 1.0);
    vec3 binormal = normalize(cross(normal, vec3(1.0, 0.0, 0.0)));
    vec3 tangent = normalize(cross(binormal, normal));


    // to transform the position using the above TBN matrix
    // what's the TBN matrix meaning for 
    vec4 pos = vec4((a_pos * mat3(tangent, binormal, normal)) + a_normal, 1.0) + vec4(v2 * (a_uv1.y * 0.2), 0.0, 0.0);

    gl_Position = u_mvp * pos;
    v_texcoord0 = a_uv0;
    v_texcoordMap = vec2(pos.x * u_1DivLevelWidth, pos.y * u_1DivLevelHeight);
}

"could this be rotation matrix made from some input parameters,just three basis?" It's not clear what you mean by this. — Nicol Bolas
hi,@Nicol Bolas. i've made that description clear and unambiguous。 — qxsl2000

Spektre Spektre · Accepted Answer · 2016-11-23T08:17:23

TBN matrix is transform matrix converting between rasterised primitive and world coordinates. The 3 vectors represent your rotation matrix and origin is usually placed to (0,0,0) or to one of the vertexes.

The answer to your question depends on the TBN usage. If you are using it only for vectors (like for directional light computations or bump mapping) then yes TBN can be just rotation matrix. But if you need transform also positions then you need full matrix with origin.

see related QA: Normal mapping gone horribly wrong where TBN is used as rotation 3x3 matrix.

is TBN matrix really rotation matrix in nature?

1 Answers