Re: [Algorithms] Terrain Normals

[Klaus H. <k_h...@os...>]

Hi,

I made two screen shots for you. These images are not supposed to look good,
but they show that lightmaps don't introduce those Gouraud shading
artifacts. The lightmap contains only a single color (at varying
intensities), and two directional light sources have been used. There's
exactly one texel per data point in the height field.

Please ignore those 'lines' that look like a bug... these are texture seams,
and it's sort of difficult to get rid of them.

http://www.thecore.de/TheCore/img1.jpg
http://www.thecore.de/TheCore/img2.jpg

Niki

----- Original Message -----
From: Pai-Hung Chen <pa...@ac...>
To: <gda...@li...>
Sent: Monday, August 14, 2000 6:53 PM
Subject: Re: [Algorithms] Terrain Normals


> Hi,
>
> Thanks for the help!
>
> > Personally, I use pre-lit textures or lightmaps (using Phong shading) to
> > eliminate this effect, but the following algorithm to compute the vertex
> > normals should already reduce (or maybe even eliminate) those seams
> (Thanks
> > to Peter Dimov for the algorithm :).
>
> Wouldn't the same artifacts still be visible in the pre-lit lightmaps
since
> the lightmaps' color/intensity are still affected by the _faulty_ normals
> when Gouraud shading is used (at least in D3D's case)?
>
> Also, could you briefly explain the reasoning of the normal-generating
> algorithm you suggested as follows?  A high-level, general idea is fine.
:-)
> Thank you.
>
> Pai-Hung Chen
>
>
> > void VertexNormal(
> >     Vector3& normal,
> >     long col,
> >     long row,
> >     float gridSpacing)
> > {
> >     float nx, nz, denom;
> >
> >     if (col > 0 && col < m_fieldSize - 1)
> >     {
> >         nx = GetElev(col - 1, row) - (col + 1, row);
> >     }
> >     else if (col > 0)
> >     {
> >         nx = 2.0f * (GetElev(col - 1, row) - GetElev(col, row));
> >     }
> >     else nx = 2.0f * (GetElev(col, row) - GetElev(col + 1, row));
> >
> >     if (row > 0 && row < m_fieldSize - 1)
> >     {
> >         nz = GetElev(col, row - 1) - GetElev(col, row + 1);
> >     }
> >     else if (row > 0)
> >     {
> >         nz = 2.0f * (GetElev(col, row - 1) - GetElev(col, row));
> >     }
> >     else nz = 2.0f * (GetElev(col, row) - GetElev(col, row + 1));
> >
> >     gridSpacing *= 2.0f;
> >
> >     denom = 1.0f / sqrt(nx * nx + gridSpacing * gridSpacing + nz * nz);
> >
> >     normal.x = nx * denom;
> >     normal.y = gridSpacing * denom;
> >     normal.z = nz * denom;
> > }
> >
> > "normal" is the unit vertex normal that is returned by the function.
> "(col,
> > row)" represents the data point in the height field for which you want
to
> > compute the vertex normal, and "gridSpacing" is the spacing between two
> > adjacent vertices (normally, gridSpacing is 1, unless you decide to
scale
> > the height field). Finally, GetElev() return a elevation of a particular
> > data point in the height field.
> >
> > HTH,
> > Niki
> >
> > ----- Original Message -----
> > From: Pai-Hung Chen <pa...@ac...>
> > To: <gda...@li...>
> > Sent: Monday, August 14, 2000 8:22 AM
> > Subject: [Algorithms] Terrain Normals
> >
> >
> > > Hi,
> > >
> > > I've written a routine to read a bitmap heightfield and construct a
> > terrain
> > > mesh from it.  I partition each set of four pixels into two triangles
> > along
> > > the upper-right-to-lower-left diagonal line.  Therefore, for example,
a
> > 128
> > > x 128 bitmap will produce (128-1) x (128-1) x 2 = 32258 triangles
using
> my
> > > method.  For each triangle I caculate its unit face normal.  For each
> > vertex
> > > of a triangle, I calculate its vertex normal by adding and averaging
the
> > > face normals of all triangles that share the vertex (in my case a
vertex
> > can
> > > be shared by at most six triangles and at least one triganle) and then
> > > normalize the averaged normal, which is used in redering.  My problem
is
> > > that there are some *horizontal dimmed banding* effects along the
> triangle
> > > edges of the mesh when rendering in D3D with light cast on it.  Also,
> > there
> > > are very visible dimmed seams between triangles when light is cast on
> it.
> > > All these artifacts seem to manifest only when light is cast in large
> > angles
> > > (i.e. not directly).  Is it because the arrangement of triangles in my
> > mesh
> > > is too *uniform* (actually they are extremely uniform)?  Or is my
> > > calculation of vertex normals incorrect?
> > >
> > > Thanks in advance,
> > >
> > > Pai-Hung Chen
> > >
> > >
> > >
> > > _______________________________________________
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> > >
> >
> >
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