Free 3d tutorials: from 3d tips & tricks to advanced 3D software tutorials in 4 seconds.

Making of Varga

“Making of Varga ” by Paul Tosca
Now lets say that i will cut the UVs along the edge and will rotate one shell 90 degrees CCW like in the image below.Because of the rotation in UV space the tangent space for the first triangle will be different than in the first situation ( the RGB triad for the first triangle has rotated also 90 deg along blue axis )
Because of this , the orientation of the uv shells will affect the colors you will see in the normal map;if you rotate the uvs of your model ( or the uvs for some shells ) lets say 90 degrees ClockWise and recompute the normal map the colors in the new normal map will be completely different ( not because the entire image will be rotated by 90 degrees … you can rotate it 90 CCW in photoshop and compare it with the first version to check …. they will be different ) Tangent space per vertex will be then computed and after that you can compute the tangent space vectors for any arbitrary point on the surface by interpolating vertex tangent space vectors. Now ,for computing a normal map the raytracing algorithm will cast a ray from the current point of the low poly mesh to the hign poly and at the point of intersection with the high will compute the normal for the high surface in world space ( WS ) then this normal is transformed in the tangent space ( TS ) of the low poly version and stored in the normal map.So you see that the normal computed is dependent to the tangent space used for the low poly version and the tangent space normal map must be created using the same normal/tangent/binormal as the game uses ( or the application you will use to display the normal maps) ,otherwise the normals will be misinterpreted and you will see discontinuities on border uv shells , hence the problems you will have when trying to display a normal map computed with one app in another app. It will not be a problem if different apps will use different methods for tangent space vector calculations , the problem is that they are not making that info available so that someone interested could write a plugin/tool that will remedy the problem …the only app i know so far to made the formulae public is Mudbox , on their online help pages you can find how they are calculating tangent space vectors. Now you know why the problem occur but there is little to do about it … at least you should know that you aren’t doing anything wrong , the app is to blame 🙂 . I will show an example and explain how the normal map will be computed and how to interpret it channel by channel… this will help if you want to alter the normal maps in photoshop , overlay multiple normal maps , paint them directly in photoshop etc … I will bake the normals from the plane and cylinder below to a single quad plane highlighted in green.As i explained earlier the tangent space will be : normal pointing up( blue one) tangent from left to right ( 3-4 red one) and bitangent/binormal down-up (3-1 the green one) . The cylinder has all the edges hard so the normals will change in steps, it will be easier to analyze.I have numbered the faces on the cylinder from 1 to 7.
Now lets suppose we are looking at the objects directly from front , it will look something like in the image below.
     

Pages: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Leave a Reply