[vtk-developers] Possible mistake in vtkRenderLargeImage

[John Biddiscombe]

Michael

I just did the sums and got the same equation as you for the adjustment 
to the view angle. Since I recently did some work on this class, I'll 
check in your fix.

Thanks. (I'd noticed a difference in appearance but hadn't given it any 
thought...good spot!)

JB

> Dear VTK developer community,
>
> I have come across what I believe to be a mistake in the  
> implementation of the class vtkRenderLargeImage in the Hybrid  
> package.  To illustrate the case, please consider the simple VTK  
> script found at http://www.science.uva.nl/~mscarpa/vtk/TestImage.py.   
> This script generates an image as http://www.science.uva.nl/~mscarpa/ 
> vtk/Image.png.
>
> Now, consider a modified version of the above script as found at  
> http://www.science.uva.nl/~mscarpa/vtk/TestRenderLargeImage.py.  The  
> only
> differences with the first script are that the render window has  
> smaller dimensions (a quarter width and a quarter height), and that  
> it uses the vtkRenderLargeImage class to generate an image of the  
> same size as the first script (500 by 500 pixels).  This script is  
> based on the example found with the VTK source code.  On my VTK  
> installation, the latter script generates an image as http:// 
> www.science.uva.nl/~mscarpa/vtk/RenderLargeImageOriginal.png.  This  
> is considerably different from the expected result, which (in this  
> simple case) should be identical to the first image.  I have looked  
> into the code of vtkRenderLargeImage (as can be found in CVS as of  
> this writing), and I think I
> found a mistake.  In the method vtkRenderLargeImage::RequestData(),  
> the following statement calculates the view angle incorrectly:
>
>   cam->SetViewAngle(asin(sin(viewAngle*3.1415926/360.0)/this- 
> >Magnification)
>                     * 360.0 / 3.1415926);
>
> I believe that the correct statement should be as follows:
>
>   cam->SetViewAngle(atan(tan(viewAngle*3.1415926/360.0)/this- 
> >Magnification)
>                     * 360.0 / 3.1415926);
>
> The only difference consists in replacing the sine and arcsine  
> functions with tangent and arctangent functions.  If the class is  
> recompiled with these changes, the two scripts above produce  
> identical images (see the image produced by the second script on my  
> VTK installation after applying the above changes at http:// 
> www.science.uva.nl/~mscarpa/vtk/RenderLargeImageModified.png).
>
> Please note that more complex scenes will not produce absolutely  
> identical images, but at least the images will be almost identical  
> and not differ so much due to different view angles.
>
> For convenience, this text can be found as HTML page at the following  
> location: http://www.science.uva.nl/~mscarpa/vtk/index.html
>
> Could someone please verify these findings and, if confirmed, could  
> these changes be taken over in the CVS repository?  Any comment is  
> obviously most welcome.
>
> Kind regards,
>
> Michael Scarpa
> Section Computational Science
> Universiteit van Amsterdam
> Kruislaan 403
> 1098 SJ Amsterdam, The Netherlands
> Room F2.15
> Tel: +31 20 525 7574
> Fax: +31 20 525 7419
> Web: http://www.science.uva.nl/~mscarpa/
>
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-- 
John Biddiscombe,                            email:biddisco @ cscs.ch
http://www.cscs.ch/about/BJohn.php
CSCS, Swiss National Supercomputing Centre  | Tel:  +41 (91) 610.82.07
Via Cantonale, 6928 Manno, Switzerland      | Fax:  +41 (91) 610.82.82