Bug in default shader with normal map

Description of the problem

There are rending issues due to the default shader (checked with MeshStandardMaterial and MeshPhongMaterial) when using the normal map if all the texture coordinates for the face are exactly the same.

I had exactly the same issue with other shaders and this was related to the dFdx(uv) and dFdy(uv) shader functions returning 0 and leading to a division by 0.

Live example:

• jsfiddle (dev branch)

Three.js version

• Dev
• r108
• r104

Browser

• All of them
• Chrome
• Firefox
• Internet Explorer

OS

• All of them
• Windows
• macOS
• Linux
• Android
• iOS

Hardware Requirements (graphics card, VR Device, …)

OpenGL vendor: NVIDIA Corporation (0x10de)
OpenGL renderer: GeForce RTX 2060/PCIe/SSE2 (0x1f08)
OpenGL version: 4.6.0 NVIDIA 430.40

[WestLangley]

This is what I believe is happening…

Even if the uv’s at each vertex are identical, interpolation of uv’s across the face of the primitive can result in different values due to numerical roundoff.

Consequently,

vec2 st0 = dFdx( vUv.st );

may not be the zero vector we would hope it would be.

The next line

float scale = sign( st1.t * st0.s - st0.t * st1.s );

will return - 1, 0, or 1.

And the following line is normalizing something that is very close to zero

vec3 S = normalize( ( q0 * st1.t - q1 * st0.t ) * scale );

which just amplifies the numerical errors since it returns a unit-length vector.

[WestLangley]

Just wanna say that additional if statements in shader code can degrade performance.

@Mugen87 The OP is only trying to help locate the issue at this point.