[csu540-f05-rpf] [csu540-f05] SP3 question

[Philip Lee]

Robert Futrelle wrote:
> If you get a hit on a triangle and N dot L is positive, then your shadow 
> ray, from e', will be directed away from the surface, which is fine.  As 
> I mentioned in class, there's a slight chance that due to roundoff, it 
> will hit the surface it is supposedly directed *away* from - the reason 
> I suggested adjusting e' to be slightly away from the surface by adding 
> a few pixels in the N direction.
> 
> I'm not absolutely sure why Matt's strategy worked - have to look at his 
> code. But if it worked and shadows appeared, great!
> 
> In my TinyRayTrace, I made d so large that if t < 1.0, the hit was 
> inside the basic volume containing the scene. You could also make |d| = 
> 1.0, a unit
> vector and test for t being less than some value such as 2,000, again 
> meaning that the hit was inside your basic volume.  The nice thing about 
> making |d| = 1.0 is that t then becomes a measure of how far along the 
> ray from the eyepoint your hit is, measured in pixels.
> 
> The gamma and beta tests are unchanged by your choice of |d| and the t 
> limit test.
> 
>   -- RPF
> 
> 
> 
>> Philip,
>>
>> I had the same problem you did.  The way I was able to get around it 
>> was by adding a check on the T-value in the same function that checks 
>> the alpha, beta, and gamma values.  Make sure that it is between 0 and 
>> 1.  As soon as I did this it worked.  I THINK what was happening is 
>> that the same triangle was getting hit, and generating a negative 
>> t-value.  Hope that helps.
>>
>> - Matt
>>
>> On 12/6/05, Robert Futrelle 
>> <<mailto:futrelle at ccs.neu.edu>futrelle at ccs.neu.edu> wrote:
>>
>> Here's the answer to the question a number of you have asked me.
>>
>> Q.  How do you get the coordinates of the point where the first ray
>> hits a triangle?  (Needed because that's the new eyepoint for
>> launching a ray in the direction of the light source.)
>>
>> A. At the top of pg. 205, you see that the ray you start with has the 
>> form:
>> p = e + td, where p, e, and d are all points/vectors in 3-space. You
>> know the values of p, e, and d.
>>
>> When you hit a triangle, you can figure out the value of t.  You
>> simply use that value in the equation for p and that gives you the
>> point in 3-space where the ray hits the triangle.  So if T is the
>> value of t where the first hit occurs, then that point is p = e + Td.
>> That point becomes the new eyepoint, e', for launching the ray in the
>> direction of the light, so e' = e + Td.
>>
>> If L is a unit vector toward the light source, you can write your new 
>> ray as
>>
>> p = e' + wL
>>
>> using w instead of t, and L instead of d.
>>
>> Hope this helps.
>>
>>   - Prof. Futrelle
>>
>>
>>
>>> I'll answer you in about an hour, soon as I get home.
>>>
>>>    rpf
>>>
>>>> I have a quick question. When you do the ray trace and your ray
>>>> hits a point in a triangle, how do you get the z-coordinate of that
>>>> point? I am having difficulties trying to ray trace back to the
>>>> light source from a point on a triangle.
>>>>
>>>> Thanks.
>>>>
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>>
>>
>> -- 
>> Matthew Rancourt
>> <mailto:MRancourt at gmail.com> MRancourt at gmail.com
>> <mailto:rancourt at ccs.neu.edu>rancourt at ccs.neu.edu
>>
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> 
> 
> 
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It seems like I can't find a way to fix this problem. I don't know maybe 
I should just send as it is? and explain what is going on in my readme 
file? I will give it a try tomorrow again, I've been trying to solve it 
for hours now and it is too late now 1:12.

Philip