disappearing skidmarks : hints

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I have now added two callbacks, as Paolo Leoncini suggested, for
skidamarks.  Namely:

int grSkidmarksPredrawCallback(ssgEntity *obj)
{
~    glDepthMask(GL_FALSE);
~    glPolygonOffset(-15, -20);
~    glEnable(GL_POLYGON_OFFSET_FILL);
}

int grSkidmarksPostdrawCallback(ssgEntity *obj)
{
~    glDisable(GL_POLYGON_OFFSET_FILL);
~    glDepthMask(GL_TRUE);
}


This however, is not sufficient to prevent skids from flickering on and
off depending on the viewing angle.

I noticed this:

When the skidmarks are not visible on a portion of the track, the part
of the car's shadow which has soft alpha blending disappears as well (so
it does not appear blurry any more).

So my guess is that this has something to do with thresholding, but I
have no other clues.

Any ideas?
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