[vtkusers] Re: Rendering slowed down after recent CVS update

[Paul Melis]

Hello Francois,

>Please do a "cvs -q -z9 up -PdA", performance should be back for your card. Just let me know.
>
>I modified VTK/Rendering/vtkOpenGLExtensionManager.cxx
>  
>
Thanks, that fixed it!

Paul

>Paul Melis wrote:
>>/ Hello Francois,
>/>/ 
>/>/ Well, I upgraded the driver to 96.43.01. As I expected it doesn't make
>/>/ the OpenGL version reported pre-1.4. In fact, it now reports it supports
>/>/ OpenGL 1.5.8 instead of 1.5.6 :)
>/>/ See the attached glxinfo output.
>/>/ 
>/>/ Paul
>/>/ 
>/>>/ Paul Melis wrote:
>/>>>/ / I am very doubtful it will make a difference. In NVidia's OpenGL
>/>>/ />/ extension specifications it says that this card (based on an NV25
>/>>/ chip)
>/>>/ />/ does not support the extension. See
>/>>/ />/
>/>>/ http://developer.download.nvidia.com/opengl/specs/nvOpenGLspecs.pdf   An
>/>>/ />/ earlier version of this document (dated somewhere in 2003) clearly
>/>>/ marks
>/>>/ />/ the extension as being emulated in software for NV2x cards, with a
>/>>/ note
>/>>/ />/ to expect slow performance.
>/>>/ />/ I attached the output anyway.
>/>>/ /
>/>>/ Thanks for your feedback.
>/>>/
>/>>/ Actually, I think it might make a difference :-) If the new driver
>/>>/ upgrade reports an OpenGL version less than 1.4 (currently it reports
>/>>/ OpenGL 1.5), and does not report "GL_EXT_blend_func_separate" in its
>/>>/ list of extensions, you will have your performance back without
>/>>/ changing any VTK code.
>/>>/ I'm explaining that later in this message.
>/>>/
>/>>/ Anyway, I'll take a look at pages 6 to 10 of the nvidia document to
>/>>/ see if there is any other functionality implemented in software and
>/>>/ try to make the vtkOpenGLExtensionManager aware of that.
>/>>/
>/>>>/ / Interestingly, the extension list doesn't
>/>>/ />/ have the blend_func_separate extension, while VTK obviously gets some
>/>>/ />/ function pointer when querying for this extension.
>/>>/ /
>/>>/
>/>>/ The VTK behavior is the right one.
>/>>/
>/>>/ Extension "GL_EXT_blend_func_separate" was promoted as a core feature
>/>>/ of OpenGL 1.4.
>/>>/ VTK first checks if this version of OpenGL is supported by the card,
>/>>/ if so, it uses the function pointer of "glBlendFuncSeparate".
>/>>/ If not, VTK gives a second chance by checking directly for the
>/>>/ extension. If this is supported, it uses the function pointer of
>/>>/ "glBlendFuncSeparateEXT".
>/>>/
>/>>/ The Linux driver of your graphics cards says it supports OpenGL 1.5 so
>/>>/ the VTK logic falls in the first case.
>/>>/
>/>>/ It has to happen in this order because some OpenGL implementations
>/>>/ only report the OpenGL version they support and they put only extra
>/>>/ extensions in the list of their extensions.
>/>>/
>/>>/ For instance, on the mac, if you want to use 3D textures, the graphics
>/>>/ card says it supports OpenGL 1.2 (which has GL_EXT_texture3D as a core
>/>>/ promoted extension) but it does not list
>/>>/ GL_EXT_texture3D in its list of extensions.
>/>>/
>/>>/ Ref: OpenGL 2.1 spec, Appendix G "Version 1.4", G.9 "Separate Blend
>/>>/ Functions", page 335 (pdf page 349).
>/>>/ http://www.opengl.org/registry/doc/glspec21.20061201.pdf
>/>>/
>/>>/
>/>>/ To sum up, please still give a try to a driver upgrade and let me know
>/>>/ about it.
>/>>/
>/>>/ Regards.
>/>>/
>/>>/
>/>>>>/ / Paul Melis wrote:
>/>>/ />>>/ / Paul Melis wrote:
>/>>/ />>/ />/ />>/ Sean McBride wrote:
>/>>/ />>/ />>/
>/>>/ />>/ />>>/ /On 1/16/08 4:24 PM, Paul Melis said:
>/>>/ />>/ />>/ />/
>/>>/ />>/ />>/ />>/And another reply to myself...
>/>>/ />>/ />>/ />>/After going back to a CVS version of VTK of October 1st the
>/>>/ />>/ />>/ problem />>/seems to be gone. So something changed in VTK
>/>>/ between
>/>>/ />>/ />>/ october and today />>/that influences the render performance,
>/>>/ />>/ probably
>/>>/ />>/ />>/ due to a software />>/fallback suddenly being used in the
>/>>/ NVidia
>/>>/ />>/ driver.
>/>>/ />>/ />>/ />>/    />>/
>/>>/ />>/ />>/ />/
>/>>/ />>/ />>/ />/Next try Nov 1, Dec 1, etc. until you have it narrowed
>/>>/ down to
>/>>/ />>/ />>/ between 2
>/>>/ />>/ />>/ />/days... tedious, but easy.  You'll probably find the problem
>/>>/ />>/ this way.
>/>>/ />>/ />>/ />/  />/
>/>>/ />>/ />>/ /Well, I made a little script that did all the hard work of
>/>>/ checking
>/>>/ />>/ />>/ out a CVS version for specific date, configuring it with cmake,
>/>>/ />>/ />>/ building it, setting appropriate environment variables and then
>/>>/ />>/ />>/ running a little test application. The results indicate that
>/>>/ the
>/>>/ />>/ />>/ problems start with commits done on October 27. I suspect the
>/>>/ />>/ changes
>/>>/ />>/ />>/ relating to alpha handing (for bug 2347) in
>/>>/ />>/ vtkOpenGLRenderWindow.cxx
>/>>/ />>/ />>/ and friends, although I don't see anything suspicious in the
>/>>/ code.
>/>>/ />>/ />>/ Most of the changes seem related to depth peeling and using
>/>>/ />>/ different
>/>>/ />>/ />>/ blending, but I'm not enabling depth peeling and are also
>/>>/ not using
>/>>/ />>/ />>/ any transparent geometry.
>/>>/ />>/ />>/
>/>>/ />>/ />>/  />>/
>/>>/ />>/ />/ />/ Okay, further testing shows it is indeed related to using a
>/>>/ />>/ different
>/>>/ />>/ />/ blending function. If I take the faulty revision and only
>/>>/ comment out
>/>>/ />>/ />/ the calls to glBlendFuncSeparate in vtkOpenGLRenderer.cxx and
>/>>/ />>/ />/ vtkOpenGLRenderWindow.cxx the problem goes away, i.e.
>/>>/ performance is
>/>>/ />>/ />/ restored. So it looks like a software fallback is used as
>/>>/ />>/ side-effect of
>/>>/ />>/ />/ this call. Yuck. The card (actually driver) says it's OpenGL 1.5
>/>>/ />>/ />/ capable, and glBlendFuncSeparate was introduced in 1.4. But it
>/>>/ />>/ doesn't
>/>>/ />>/ />/ seem to be hw-accelerated...
>/>>/ />>/ />/ />/ I guess there's not much of a work-around for this,
>/>>/ except a card
>/>>/ />>/ />/ upgrade :-/
>/>>/ />>/ />/ />/ Paul
>/>>/ /
>/>>/
>/>>/ -- 
>/>>/ François Bertel, PhD  | Kitware Inc. Suite 204
>/>>/ 1 (518) 371 3971 x113 | 28 Corporate Drive
>/>>/                      | Clifton Park NY 12065, USA
>/>>/
>/>/ 
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>/
>
>-- 
>François Bertel, PhD  | Kitware Inc. Suite 204
>1 (518) 371 3971 x113 | 28 Corporate Drive
>                      | Clifton Park NY 12065, USA
>