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hi there, some of you seem to be perfect for helping me out with my current problem:

i want to do a vertex interpolation in the vertex shader

i am using a fixed pipeline attribute for giving the shader a secondary coordinate -> gl_Normal

i am somehow encountering a very strange problem, which i thought has to do with the "w" component, but this does not seem to be the issue, what is the problem ?

the problem is that when i put the same coordinate in
GL.vertex3f(mycoordinate)
GL.normal(mycoordinate)

they are NOT THE SAME ... i am blending linearly between them:

vec3 vertex=mix(gl_Vertex.xyz,gl_Normal.xyz,interpolationvalue);
 
 and after the interpolation i calculate the final position with:

gl_Position = gl_ModelViewProjectionMatrix * vec4(vertex,1);

and, i thought the position should not change the slightest, but it is somehow strange, because the points actually move ...

any ideas !?

Not sure if this is relevant here, but there is a mathematical difference between transforming

- a location in space

- a normal vector

both seem to be the same kind of 3-component vector, but they have different meanings. The former simply moves as the transformation says. But the latter is really the orientation of a tangent plane, and so the normal vector cannot simply be transformed by the same matrix as a location in space. It must remain perpendicular to the surface.

Therefore another matrix is derived from the original transformation (the transpose of the adjoint? I am not sure offhand), and normal vectors are instead transformed by that derived matrix.


In other words, the mismatch might come from the fact that two identical input vectors are multiplied with different transformation matrices.

arhh, lol this was the error, thank you for pointing!!!