[EMAIL PROTECTED] wrote:
> Hi,everyone, in Opengl document, it says that gluPickMatrix creates a
> projection matrix that can be used to restrict drawing to a small region
> of the viewport. the implementation of gluPickMatrix in the Mesa3D is
> the following:
> =========
> void APIENTRY gluPickMatrix( GLdouble x, GLdouble y,
> GLdouble width, GLdouble height,
> const GLint viewport[4] )
> {
> GLfloat m[16];
> GLfloat sx, sy;
> GLfloat tx, ty;
>
> sx = viewport[2] / width;
> sy = viewport[3] / height;
> tx = (viewport[2] + 2.0 * (viewport[0] - x)) / width;
> ty = (viewport[3] + 2.0 * (viewport[1] - y)) / height;
>
> #define M(row,col) m[col*4+row]
> M(0,0) = sx; M(0,1) = 0.0; M(0,2) = 0.0; M(0,3) = tx;
> M(1,0) = 0.0; M(1,1) = sy; M(1,2) = 0.0; M(1,3) = ty;
> M(2,0) = 0.0; M(2,1) = 0.0; M(2,2) = 1.0; M(2,3) = 0.0;
> M(3,0) = 0.0; M(3,1) = 0.0; M(3,2) = 0.0; M(3,3) = 1.0;
> #undef M
>
> glMultMatrixf( m );
> }
> ==========
> Thought it over for many times, I still can not understand the theory of
> this implementation, I can not deduce this result.
> Does anyone can explain the theory of pickmatrix function? How it works?
> Can anyone give me a deduction in detail.
The gluPickMatrix man page is at
http://manpages.unixforum.co.uk/man-pages/linux/suse-linux-10.1/3/gluPickMatrix-man-page.html
among other places.
The Mesa/progs/samples/select.c is a demonstration.
gluPickMatrix really just applies a scale and translation to the
projection matrix so that a small part of the scene gets projected into
the viewport. Most of the scene will get clipped away. What doesn't
get clipped is a candidate for 'picking'.
glRenderMode(GL_SELECT) is usually used in conjuction with gluPickMatrix
to implement picking with the mouse.
-Brian
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