Sunday, September 28, 2008

State of Rasterization

I was down at Siggraph when the new OpenGL 3.0 specification was released. I was excited for the OpenGL BoF later that week to see all the specified updates they had promised. Monday of that same week, I read a slashdot article that kind of broke the news to me that the original enhancements to the specification like the object model were butchered. I was a little shocked, but was sincerely hoping it was like many slashdot articles, where the commenters simply read the summary and didn't bother reading the article. I had some naive hope that the webmaster accidentally posted the old specification up using the new file name, but after scanning the entire specification, I can see that wasn't the case. Many people were angry that they were left in the dark so long about the drop in features and the news that Khronos would drop the ball like ARB has done. Although I think Khronos screwed up, I still think a much-needed object model will is still on the road map and through these frequent "updates", I can see more state-friendly API. nVidia already released a new extension that provides texture building with binding that texture. Even though it's just an extension, I hope to eventually see many other functions go down that road.

nvidia has done a good job supporting OpenGL in various ways including graphics demos, presentations, APIs, etc. Although I don't see much use with some of the new additions to the pipeline--nvidia itself has told developers NOT to use the geometry shader--I do like some of the things presented in this new API.

nvidia's slides on OpenGL features on the GeForce 8 architecture. I'm linking them straight from the nvidia page until they tell me I can't

One of the cooler additions to the rendering pipeline IMHO is the transform feedback buffer. Basically when you send vertex data down the pipeline and it gets transformed by various matrices in the vertex shader, you don't really have an idea of what those vertices actually get transformed to. You could do some GPGPU thing where you print the vertex information to the screen and read it back, but this is tedious to implement and debug. A better solution would just be to store these transformations in the rendering API. One of the pesky things about shadow volumes is you have to compute the edge silhouettes. This has been solved in a respect in "Shadow Volumes on Programmable Graphics Hardware" by Brabec et al., and now can be implemented trivially using this new transform feedback buffer. Other work has been down to use silhouettes to provide some effect, so I can see Brabec's implementation applied to a whole slew of visual effects that require light or camera silhouette information. In their presentation, they use the geometry shader for silhouette detection, which I think is a mistake. Much like Barnes and Noble dedicating a section to Vampire Romance might be considered a mistake. Maybe that's a stretch.

Another exciting extension is the EXT_direct_state_access. Although this isn't the "object model" many people were expecting, I believe this kind of extension will generally lead into the state-friendly programming model we all wanted. How many times have we not known the state of OpenGL coming into a given function and changing the state would possibly break the assumptions a proceeding function would have? Once again kudos to nvidia for a great support and presentations.

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