Monthly Archives: July 2010

Light Propagation Volumes

Posted by on July 18, 2010 19 comments

I've finally finished my lab course last week - thanks to my supervisor Matthäus G. Chajdas - you can read his blog here -, it wasn't your usual lab course with work sheets and boring homework, instead I've been allowed to implement a nice paper about a Global Illumination approximation algorithm called (Cascaded) Light Propagation Volumes. It's been developed by Crytek and you can find more information (including some presentations and videos) on their server. (Note: this is an implementation of the I3D paper, not the earlier SIGGRAPH one.)

Sponza scene (direct + indirect lighting w/ occlusion)

Sponza scene (only indirect lighting w/ occlusion)

Sponza scene (ony direct lighting)

Sponza scene (boosted indirect lighting w/ occlusion)

Sponza scene (boosted indirect lighting w/o occlusion)

The algorithm approximates global illumination by rendering the light into a reflective shadow map, injecting it into a volume (using a spherical harmonics representation) and propagates the light flux in this volume (hence the name of the algorithm) and taking into account occlusion as possible extension.

The whole algorithm is physically motivated but corners everywhere, of course, to be more efficient. The paper also contains a few errors and doesn't explain everything needed to implement it in great detail (like eg the solid angles of the side faces), so I've written two documents detailing the mistakes I've found and the additional calculations I've performed.

You can find the mistakes here (including suggested corrections) and the full annotations document here.

Finally I've also uploaded the whole prototype (including my code licensed under the FreeBSD license and the media files) here - it's 68 MB big (and it's been compressed with 7zip with a compression mode that might not be supported by WinZIP. The Sponza model is from Crytek, too. You can download the original model and textures here.
The project uses DirectX 10.1 and by default it won't run in DirectX 10, because it uses a texture format that is deprecated in D3D10 but supported again 10.1 (BGRA). See the comment by FatGarfield for the location that needs to be changed for it work in DX10, too. (However red and blue will be swapped then.)

I haven't implemented cascaded LPVs and I also use only one light/RSM and only inject its depth into the occlusion volume, but the results already look very nice in my opinion.

Stay tuned for more :-)
Cheers,
Andreas

Panorama Stitching

Posted by on July 17, 2010 7 comments

I've finally come around to "clean-up" some old project I've had lying around for a few months and upload it.
I'm talking about some Panorama Stitching code I wrote for our participation in Microsoft's Imagine Cup.
I'm suppressing all memories of it since it was an epic fail, but at least I still have learnt quite a bit about computer vision and image processing - enough to know that it's incredibly hard to come up with stable algorithms and kudos to anyone working in the field.

Here is the code dump: PanoramaStitching.zip

It contains many small projects which usually use multiple pictures as inputs or multiple webcams (depending on code or chosen preprocessor macros).

The most advanced prototype is the SnapshotHomographyConfigurator, which allows you to determine homographies between multiple cameras at once by marking shared points between the images.

Another one which works okay is the PanoramaStitching project. It creates panoramas using spherical or cylindircal projections of the input images. However, it is very sensitive to translations of the viewpoint. It works quite well with optimal/artificial images:

(Note: the small misalignment on the right stems from moving the player position slightly. Usually you use a deghosting algorithm to remove such misalignments.)

I've used OpenCV for image processing and yaml for loading and storing settings (and also rapidxml). OpenCV's C++ wrapper is pretty awesome. It's not perfect but it makes life a lot easier.

Stay tuned for more code/project uploads soon :-)

PS: Here are the links to some papers which proved useful to me (I didn't implement most of them though, and some are implemented in OpenCV already):

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