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Archive Notice: This thread is old and no longer active. It is here for reference purposes. This thread was created on an older version of the flipcode forums, before the site closed in 2005. Please keep that in mind as you view this thread, as many of the topics and opinions may be outdated.
 
Martin Beck

March 28, 1999, 05:47 AM

Hi out there!

Just a little question:
Why do I read almost everywhere that one uses the formula

eyedist/z=y'/y
=>
y'=eyedist*y/z
?

In school we've just learned, that it has to be like this:

eyedist/(z+eyedist)=y'/y
=>
y'=eyedist*y/(z+eyedist)

I think, if one uses bigger eye distances, i.e. 500, there is somehow
a difference...

 
Jaap Suter

March 28, 1999, 06:47 AM

Hiya!

It depends on what Z value you use. In this formula:

---- eyedist/z=y'/y

---- y'=eyedist*y/z

z means the z value from the camera to the point;

In the nex formula however:
---- eyedist/(z+eyedist)=y'/y
---- y' = eyedist*y/(z+eyedist)

z means the z value from projection plane to the point;
Since eyedist is the z value from camera to projection plane these
equations are exectly the same.

If still not clear mail me!

Jaap Suter

 
Conor Stokes

March 29, 1999, 06:34 AM



Jaap Suter wrote:
>>Hiya!
>>
>>It depends on what Z value you use. In this formula:
>>
>>---- eyedist/z=y'/y
>>
>>---- y'=eyedist*y/z
>>
>>z means the z value from the camera to the point;
>>
>>In the nex formula however:
>>---- eyedist/(z+eyedist)=y'/y
>>---- y' = eyedist*y/(z+eyedist)
>>
>>z means the z value from projection plane to the point;
>>Since eyedist is the z value from camera to projection plane these
>>equations are exectly the same.
>>
>>If still not clear mail me!
>>
>>Jaap Suter
>>
Well, another way to do it is to cast all your points to the back frustum plane. Then you only
have to do one divide for all your points. And of course you just have to rotate and transform
lots of unit vectors too. But I have been discussing a bit on this with Phantom, with our
co-developed RVS scheme.

Conor Stokes

 
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