[Radiance-general] Transform from 3D to 2D Coordinates

Wed, 30 Apr 2003 18:58:43 +0100

Dear Greg,

Thanks a lot for you quick reply to my last question. Im sorry to bother
you again with what is probably another obvious question.

 I have been spending a lot of time trying to use your 'viewloc()'
function in my program. I believe i know what most of the parameters
should be but i was hoping you could confirm (or correct) a few of my
assumptions as I cannot seem to produce any correct output. 

Firstly, viewloc() is passed three variables ip, v and p. p is obviously
the input co-ordinates in the xyz used by the radiance coordinate
system. v is the details of the view. ip therefore must be the output
coordinates - are ip[0] and ip[1] the new x and y coordinates?

the only v setttings that i cannot figure out the origin of is hoff and
voff (the horizontal and vertical offsets). As i do not change many
rendering parameters with rpict (only -vp and -vd) i am assuming these
default to 0 - is this correct? i assume the rest of the v variables
(i.e. hvec, vvec, hn2 etc.) are calculated in the 'setview' function.

Finally, I have been passing ip[0] and ip[1] to the loc2pix function to
convert this location to a pixel coordinate. My resolution is the
default 512X512 and my -vv and -vh are also the default 45. Does this
mean that loc2pix will give me an (x,y) between 0 and 512?

Again sorry to bother you with this - i wouldnt ask unless i really
couldnt sort it out myself

Your help is greatly appreciated

Jon Bright

ps as a matter of interest i hear you are visiting my university
(Bristol) on sunday to meet, among other people, my supervisor (Alan
Chalmers). Small world isnt it!

> Hi Jon,
> 
> Focal length is specified with the -vh and -vv options of rpict,
> which 
> also controls the image aspect ratio.  Knowing one from the other 
> requires a specific image (film) size and format.  For 35mm film, the
> 
> formula is:
> 
> focal_length = 35/2 / tan(vv_angle/2)
> 
> Where vv_angle is the vertical view angle for a landscape picture. 
> The 
> horizontal angle is then given by:
> 
> hh_angle = 2 * atan(vv_angle/2 * 3/2)
> 
> To check your calcs, a 50mm lens in 35mm photography is equivalent to
> a 
> Radiance view specification of:
> 
> -vh 49.6 -vv 35.3
> 
> (Remember that many trigonometric libraries including those used by 
> Radiance's "calc" program deal in radians, and you need to convert
> this 
> using 180/PI.)
> 
> I hope the above exercise makes it clear why I didn't choose focal 
> length in definining views in rpict -- they don't make a lot of
> sense.
> 
> There are also Radiance library routines you can access from C-code. 
> 
> Check out the viewloc() function in src/common/image.c and some of
> the 
> other routines there.
> 
> Good luck!
> -Greg
> 
> > From: [email protected]
> > Date: Mon Apr 28, 2003  12:25:41  PM US/Pacific
> > To: [email protected]
> > Subject: [Radiance-general] Transform from 3D to 2D Coordinates
> > Reply-To: [email protected]
> >
> > Dear Radiance list,
> >
> > I am currently using radiance to render some pictures which I would
> 
> > like
> > to apply some post processing to. This will involve pasting an
> image
> > onto a specific location in the .pic file i have created.
> >
> > In order to do this i need to know the location of certain points.
> I
> > know where they are in my 3D scene, but cannot work out a 3D to 2D
> > coordinate transform. I know that with a traditional pinhole 
> > perspective
> > projection i can work out new 2D coordinates if i know the old
> > coordinates, the focal point and the focal length. But i cant find
> any
> > way of determining what the focal length used by rpict is!
> >
> > If anyone has, ideally, an equation for transforming 3D radiance
> > coordinates into 2D .pic coordinates based on an (x,y,z), view
> point 
> > and
> > view direction i would be incredibly grateful if you could let me
> know.
> > And if anyone could even help me determine the focal length i need
> that
> > would be a big help
> >
> > Thanks a lot for your time
> >
> > Jon Bright
> > _______________________________________________
> > Radiance-general mailing list
> > [email protected]
> > http://www.radiance-online.org/mailman/listinfo/radiance-general
> >
> 
> _______________________________________________
> Radiance-general mailing list
> [email protected]
> http://www.radiance-online.org/mailman/listinfo/radiance-general
> 
> 