Hi,

I was trying to make the UV of a plane slide according to its orientation relative to the down vector (0, -1, 0) (just like a liquid would flow on a surface).

For this, I use the tangent and bitangent in order to deduce how much they are colinear to this down vector, and make UV slide according to the dot values. When I am applying a rotation to the geometry **before computing the tangent**, the shader gives nice results. If I apply the rotation to the mesh instead, it does not work anymore, even though I projected every vectors into the base of the normalMatrix.

here are two images explaining the problem:

- When the transform is applied on the geometry
**before computing the tangents.**

- When the transform is applied on the
**mesh**.

You can see the shaders below:

```
uniforms: THREE.UniformsUtils.merge(
[
THREE.UniformsLib.lights,
{
normalMap: { value: null },
uTime: { value: 0 }
}
]
),
vertex: [
'attribute vec4 tangent;',
'varying vec3 vNormal;',
'varying vec3 vTangent;',
'varying vec3 vBinormal;',
'varying vec2 vUv;',
'varying vec3 vViewPosition;',
'varying vec3 vDown;',
'const vec3 DOWN = vec3(0.0, -1.0, 0.0);',
'void main() {',
'vec4 mvPosition = modelViewMatrix * vec4( position, 1.0 );',
'vViewPosition = -mvPosition.xyz;',
'vNormal = normalize(normalMatrix * normal);',
'vTangent = normalize(normalMatrix * tangent.xyz);',
'vBinormal = normalize(cross(vNormal, vTangent) * tangent.w);',
'vDown = normalize(normalMatrix * DOWN);',
'vUv = uv;',
'gl_Position = projectionMatrix * mvPosition;',
'}'
].join( '\n' ),
fragment: [
'#define USE_NORMALMAP',
'varying vec3 vNormal;',
'varying vec3 vTangent;',
'varying vec3 vBinormal;',
'varying vec2 vUv;',
'varying vec3 vViewPosition;',
'varying vec3 vDown;',
'uniform float uTime;',
//THREE.ShaderChunk.common,
//THREE.ShaderChunk.lights_pars,
THREE.ShaderChunk.normalmap_pars_fragment,
'void main() {',
//THREE.ShaderChunk.normal_flip,
//THREE.ShaderChunk.normal_fragment,
'float uSlide = dot(vTangent, vDown);',
'float vSlide = dot(vBinormal, vDown);',
'vec2 slideUV = vUv + vec2(-uSlide, -vSlide) * uTime;',
'//#if NUM_DIR_LIGHTS > 0',
'//for( int i = 0; i < NUM_DIR_LIGHTS; i++ ) {',
'gl_FragColor = vec4(texture2D( normalMap, slideUV ).xyz, 0.65);',
'//gl_FragColor = vec4(vec3(vTangent), 1.0);',
//THREE.ShaderChunk.linear_to_gamma_fragment,
'}'
].join( '\n' )
```

I know the normalMatrix take the view into account, but the normalMatrix should be orthonormal and so projecting each vectors using it should not change the dot product. However, when I display the `uSlide`

dot product, it is clear that is depend on the view.

If someone can help me figure out what’s the problem, it would be very nice, Thanks!