Hi, 
I am adapting the ARToolkit for monoscopic
see-throughs. There are two questions I have got. They
both relate to gsub.c.

1. In argInit(...), does anyone know why the the
second row of the 3x4 projection matrix is subtracted
from a multiple("image height-1") of the third row?
This is equivalent to turning the image coordinate
system upside down, I think.

2. In argConvGLcpara2(...), after the QR
decomposition, when the projection matrix is converted
for OpenGL: Can anyone point me to a reference that
explains this operation, or tell me what and why it is
being done? It seems to be a basic operation, I don't
know.

Thanks,
Kasim

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Hi,

>1. In argInit(...), does anyone know why the the
>second row of the 3x4 projection matrix is subtracted
>from a multiple("image height-1") of the third row?
>This is equivalent to turning the image coordinate
>system upside down, I think.

Yes.


>2. In argConvGLcpara2(...), after the QR
>decomposition, when the projection matrix is converted
>for OpenGL: Can anyone point me to a reference that
>explains this operation, or tell me what and why it is
>being done? It seems to be a basic operation, I don't
>know.

In the ARToolKit, screen coordinates are represented in
PIXEL unit. For example, a image with 640x480 size has
[0-639] range of value in X axis and [0-479] range of value
in Y axis.
In order to use such camera parameters for openGL,
it should be convert for openGL representation.
Basically openGL uses [-1,1] range of value for normalized
clipping volume representation in X,Y,Z axis.
So [0-649] range and [0-479] range should be convert to [-1,1] range.

-- 
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  Hirokazu Kato
  Faculty of Information Sciences
  Hiroshima City University               Phone: 082-830-1705
  Email: kato@s ........................     Fax: 082-830-1435
  URL: http://www.sys.im.hiroshima-cu.ac.jp/people/kato/