After a day of searching for answers I've found out that there seem to be 2 ways of doing this.

1. Calculate some kind of texel area of the triangle and divide this by the triangle area. (What exactly is meant by "texel area" and how do I calculate it using my texCoords[0,1] ?)

2. This: (GLSL code)

float mipmapLevel(vec2 uv, vec2 textureSize) { //rate of change of the pixels in u and v with respect to window space //approximate to au/ax, au/ay, av/ax, av/ay vec2 dx = dFdx( uv * textureSize.x); vec2 dy = dFdy( uv * textureSize.y); //select the LOD based on the maximum compression of an edge in texture space. //This corresponds to the maximum length of a side in texture space //max (sqrt(dUdx*dUdx + dVdx*dVdx), // sqrt(dUdy*dUdy + dVdy*dVdy)); float d = max( dot (dx, dx), dot( dy, dy)); //convert d length to power-of-two level of detail return 0.5*log2(d); }

I've been trying to get my head around these partial derivatives the entire day but I still don't quite get it.

The best I've come up with was something that "kind of" works but I'm sure it's not entirely correct:

I calculate the change of the texCoords between the 2x2 rasterized/shaded area:

float ddx = abs(uv_X.m128_f32[1] - uv_X.m128_f32[0]); float ddy = abs(uv_Y.m128_f32[2] - uv_Y.m128_f32[1]);

And then get the mip level like so:

ddx *= texSizeX; ddy *= texSizeY; Vector4_SSE vec0 = Vector4_SSE(ddx, ddy, 0, 0); float d = 0.5f * log2f(Vec4_SSE_Dot(vec0, vec0)); return std::max<unsigned int>(std::min<unsigned int>(static_cast<int>(d), 8), 0);

What I just don't understand is why the formular says au/ax, av/ax, au/ay, av/ay. Isn't the rate of change in window coordinates always going to be exactly 1 pixel ?? Doesn't this make the ax and ay variables completely void ? Or I'm just not getting it so please enlighten me

**Edited by lipsryme, 07 January 2014 - 02:21 PM.**