[Bf-committers] Bump mapping computation issue

[Jonathan Merritt]

Hi Stephane,

I have a couple of quick questions.  You seem to be quite familiar with 
this, so I thought I'd ask first before digging into the code... :-)

Stephane SOPPERA wrote:

> So if I'm not wrong, textures in blender already returns a vector that 
> contains the coordinates in texture space of the vector to add to the 
> normal:
>
> tex->nor[0]= (val1-val2);
> tex->nor[1]= (val1-val3);


Do you know where these come from?  How are they calculated?

I'm asking because a common approach for bump mapping in REYES based 
renderers is to perturb the micropolygon coordinates and calculate the 
effective surface normals numerically.  The function "calculatenormal()" 
is provided specifically for this purpose:
    Pshad = P + displacement * N;
    N = calculatenormal(Pshad);
where:
    Pshad - the "effective" location of the point used for shading 
calculations
    displacement - scalar displacement (eg: from a single channel texture)
    N - surface normal vector
    calculatenormal(Pshad) = dPshad/du (cross product with) dPshad/dv

In practice this can be done because entire grids of micropolygons are 
shaded at once, so the Pshad values are all set (stored in the 
micropolygon data shading structures) before calculatenormal() is called.

So, maybe nor[0] and nor[1] already incorporate dP/du and dP/dv?

Jonathan Merritt.