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# From uv to local position

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4 replies to this topic

### #1tpastor  Members

Posted 27 December 2012 - 03:56 PM

Hi,

Iam processing a light map, and I need to recover the position from a model from the uv mapping.
What is the right way to do this ?
This is what I did ...

Rasterize all the triangles of my model and constructed a dictionary that mapped every uv to the generated position and normal.
Is there another more clever way to achieve this ?

Another question related.

I did a radiosity simulation, how can I save the results to a light map ( I already have the uv mapping ) I used the model triangles as the initial patches, but after I needed to tessellated some of them.
In the end I got looks of triangles with their radiosity, how can I map the tessellated triangles color to a texture.

Again I Rasterized the initial triangles and for each "texel" generated I looked into the tessellated triangles to find the right color ....
Is this a clever way ?

### #2LorenzoGatti  Members

Posted 28 December 2012 - 08:56 AM

What are you trying to do? You have a model, consisting of vertices which already have UV coordinates (I assume it's a detailed model, with texture seams assigned by you in a modeling tool, and actual UV coordinates computed automatically with criteria such as close triangle angles in the model and in the texture), and you are computing a light map for some representative points of your model's surface: each point has known UV coordinates (interpolated from the ones of the enclosing triangle's vertices) and the computation results should be just placed at those coordinates in the light map texture (with suitable interpolation, since the UV coordinates are fractional and you are conceptually resampling a continuous function).<br /><br />I don't see any place in this process for "dictionaries" other than the final light map texture and the UV coordinates in your model.<br /><br />The second question isn't particularly clear, but if you are worried by mismatches between a simplified model and an automatically tessellated one you only need to ensure that "original" vertices retain their UV coordinates after tessellating the model and new vertices have suitably interpolated UV coordinates, which should be the default behaviour of any sensible tool.

Omae Wa Mou Shindeiru

### #3tpastor  Members

Posted 28 December 2012 - 10:09 AM

the explanation i gave was not good at all .......

i need to process a lightmap (like a post process but in cpu).

for each texel, i need to recover the model position that "generated" it (the triangle position that this texel will be "pasted" ).

so i did:

Rasterized all the triangles and discovered each UV, world position and normal for all rasterized "pixel", then i add it in a associative array (uv -> world position/Normal)

then when processing th lightmap (image processing ..) i just used this map to get the properties i needed !!!

is this a reasonable way ? (for me it looked like a hack)

The second part

Just extending the UV to the tessellated triangles fixed resolved. Thanks

### #4LorenzoGatti  Members

Posted 29 December 2012 - 11:25 AM

Rasterizing triangles is rather wrong because you want to compute a world position for each light map texel: screen space fragments are either too many or too few.<br />You can iterate over the triangles in the model and compute the world-space coordinates of that vertex and the set of light map texels with UV coordinates that are contained in that triangle. The world-space position of the point that would be painted with each of those lightmap texels can be interpolated from the three vertex positions by UV coordinates.<br />At the end of the process, every texel has been assigned a world space position if it is used in a triangle; if lightmap texels are reused in more than one point of the model the result is ambiguous, while unused UV ranges have no position.

Omae Wa Mou Shindeiru

### #5MJP  Moderators

Posted 29 December 2012 - 01:07 PM

Rasterizing triangles is rather wrong because you want to compute a world position for each light map texel: screen space fragments are either too many or too few.<br />You can iterate over the triangles in the model and compute the world-space coordinates of that vertex and the set of light map texels with UV coordinates that are contained in that triangle. The world-space position of the point that would be painted with each of those lightmap texels can be interpolated from the three vertex positions by UV coordinates.<br />At the end of the process, every texel has been assigned a world space position if it is used in a triangle; if lightmap texels are reused in more than one point of the model the result is ambiguous, while unused UV ranges have no position.

Actually rasterizing the triangles in UV space gives you exactly what you want as long as you use conservative rasterization. Rasterization is just an algorithm that computes a set of pixels covered by a triangle, which is exactly what you suggest doing.

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