Hello,

In my application I want to handle textures in a same way, even if they are applied in a different manner (projection, cylindrical, spherical, box). So this means I don't want to use automatic texture coord. generation, but I want to calculate them myself, when the object is created.

Usually, people calculate one texture coordinate (u,v) for each vertex (x,y,z). So each vertex will always have assigned the same texture coordinate.

But in my understanding, this cannot work when using cylindrical/spherical texture mapping:

suppose that one of the triangle (with vertices A, B, C) closing the cylinder has following angular coordinates (in polar coordinates, in degrees): 340, 350, 10

So those 3 vertices will be mapped following 3 u-texture coordinates: 0.9, 0.95, 0.05 (approximately!)

But that is not correct: the texture fragment would be spanning across almost the whole texture!

we should rather have following u-texture coordinates: 0.9, 0.95, 1.05

I could assign the vertex C the u-texture coordinate "1.05" to make that triangle look ok, but the problem is that the same vertex C is also part of other triangles, which might rather require the u-texture coordinate "0.05".

I hope I explained myself clearly enough ;)

So the only solution to this would be to have 3 texture coordinates for each triangle, rather than to have one texture coordinate for each vertex. Do people usually do that?

Is there another option?

Thanks for any advice!

Thank you for the reply dpadam450,

I didn't use the right word I guess. With "texture fragment" I meant "one textured triangle part of the cylinder".
I attached a picture to this reply, it should clarify my problem.

Thanks

Thanks again.

Obviously that's one way to do. But it is very troublesome if you are not directly in control of what geometry is being used (i.e. the user can edit and manipulate the geometry)
The way I am doing it now (see my "funny" explanation above) works, but instead of calculating the texture coordinates on a vertex basis, I am calculating the texture coordinates on a triangle basis:

Basically, instead of one pair of texture coordinate per vertex, I use 3 pair of texture coordinate per triangle

I just wanted to confirm that this method is also being used sometimes, since I will have to be able to export the whole thing at some point, and provide something "standard" ;)

Well, then here is a stupid question maybe:

With the specially prepared cylinder (with correct seam) you will still need special handling at the seam points: indeed every seam point is part of a triangle "opening" the cylinder, and a triangle "closing" the cylinder, thus every seam point will need 2 differentiated texture coordinates... unless you create the cylinder open (i.e. two overlapping seams, each seam point is present twice).

Please refer to the picture: points B & C in the upper part, will require different texture coordinates!

The second form, the "open" cylinder, is the common solution used in almost every model exporter available. The reason is exactly as you have mentioned.
Texture coordinates are not the only attribute that can differ per vertex on the model. Multiple normals are also commonly available, forcing even more splitting into multiple "single attribute" vertices.

Thank you Ignifex!

That makes sense.

Cheers