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# Bezier rasterisation.

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I''ve created a c graphics program that creates a bezier patch surface and i''m trying to rasterise it. At present im using an average of the polygon triangle surounding each pount on the patch to aquire the vertex normals, these normals seem correct but when i rasterise the points using a phong shading and illumination model the patch doesnt appear smooth on the screen. Im quite new to graphics programming so i was wondering if i should be using the tangents of the bezier surface to find the normals and if this would make a difference? if this is so how do i do it!?! or if using the polygon average was a sufficient method and it was my rasterisation that was wrong. thanks.

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No worries i''ve sorted it now i had a mistake in my phong interpolation and it works now.

but is it better to use the tangnets of the bezier patch to find the normals than my current polygon averaging method? i presume the averaging method as it requires less in the way of processing power. but hey what do i know.

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You take the derivative with respect to one of the parameters to find a tangent vector. There are two parameters so that gives you two vectors. It might be easiest just to try it with a plane defined by two independant vectors first. Any point on the plane is u*v1+v*v2 and the derivative with respect to u is v1 and with respect to v is v2. Since both vectors are tangent their cross product is a normal for the tangent plane.

Well, I guess that would be a plane through the origin but u*v1+v*v2+v3 wouldn't change the derivatives.

[edited by - LilBudyWizer on October 20, 2002 2:56:43 PM]

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