[gameprogrammer] Re: issues with Calculating texture coordinates

Perhaps a description of the effect your are striving for might help.

In any case, (and I can't be sure without knowing better your goal) it sounds to me like you are trying too hard with this whole thing... If you are trying to map a texture onto screen coordinates, then you merely find how far across the screen the vertex is, and figure the ratio, and that's basically (assuming you don't have any blank space around the texture area) your texture coordinate.

Now, if you're trying to determine the texture coordinates for, say, a sphere or a cylinder, then I'm afraid I cannot help you. That's something I have yet to investigate; though it seems to me you would not need to get any information about the view-space for calculating texture coordinates on a regular surface... So perhaps I'm very confused about your purposes.

If nothing else, we can get this thread kick-started so you can get as much help as you need. ^_^

--Scott

On May 31, 2006, at 10:41 PM, Gautam Narain wrote:

Hi,

I am trying to figure out how to calculate texture coordinates and was looking into some google sites. I also went through Bob's Texture mapping information in gameprogrammer.org page and the PCGPE article. However I still have some doubts :-

Firstly it says to have view space coordinates - I am getting the windows screen coordinates
using the gluProject function OpenGL. Then it says to calculate the P, M, N vectors(i.e. the vertex in the texture space) and then to calculate the 9 magic numbers.

From what I read from the 2 articles - if I have 3 vectors v1, v2 and v3 then

P = v1 ;
M = v2 - P;
N = P - v3;

Now after I calculate the 9 magic numbers and then I calculate the fx and fy(framebuffer vectors) which is the screen space or view space vector v1 -> v1.x / v1.z and v1.y / v1.z.

Using these 9 magic numbers and the frame buffer vector we calculate the values of U and V right and that is the index into the texture to use ?

However this does not seem to work. I have a window coordinate as <320.000000, 240.000000, 0.900901> so if I divide by 0.9 it will give me a value around 355 and in turn the U and V will be a large value.

Is my understanding of view space correct in assuming that its screen coordinates or window coordinates ? And if so then what exactly am I missing.

Also lastly to calculate the UV for each vertex for a triangle the magic numbers needs to be computed once only right and the different fx and fy values give the different UV coordinates ?

Thanks
Gautam



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