4 ms·

Why Geometry Shaders Are Slow (Unless You’re Intel)

99 points by mxfh 12y ago

bjwbell 12y ago
Does anyone know of a real world game or app that uses geometry shaders? Everything I've read (including this) says they're good in theory but not so much in practice.
- malkia 12y ago
  Assasin's Creed mentions of Geometry Shaders being used - (March, 2014, GDC) - https://bartwronski.files.wordpress.com/2014/03/ac4_gdc.pdf https://bartwronski.files.wordpress.com/2014/03/ac4_gdc.pdf possibly many others. EDIT: Here is much more updated version (4 versions actually with much more detailed images/video and more notes) - from the author's web-page - http://bartwronski.com/2014/05/09/digital-dragons-2014-slides/ http://bartwronski.com/2014/05/09/digital-dragons-2014-slide... oh, and if you are looking for some fun graphics failures :) http://igetyourfail.blogspot.com/2009/02/behold-power-of-geometry-shaders.html http://igetyourfail.blogspot.com/2009/02/behold-power-of-geo...
  - psykotic 12y ago
    For post-processing, Capcom's MT Framework used geometry shaders to generate primitives in lieu of scattered read-modify-writes to pixels. I'm not sure if the bang for the buck was there, but the concept was clever enough that it stuck with me.
- leohutson 12y ago
  They are used a lot for generating normal vectors on the fly, since you can take in a whole triangle, you just do a cross product of two of the edges and you're done. From my experience, they are fast enough as long as you don't emit more than 2x primitives that you take in. They are also useful for culling primitives. They are not suitable for tessellation, which funnily enough is better done by tessellation shaders. Tessellation shaders are actually pretty flexible, and can do most of the things that you might have thought geometry shaders would be suitable for.
westoncb 12y ago
I was working on a problem recently that's remarkably similar the author's rendering of hundreds of thousands of cubes (each arbitrarily oriented in space), and looked into geometry shaders some. There were several points where I thought I'd be able to use them to implement some resource saving trick that occurred to me—but was discouraged by further reading each time. I guess there are uses out there that they're more ideally suited for, but it'd be great if they did what they do... only fast :) Now I'm excited about this, though (https://www.khronos.org/vulkan)—which https://www.khronos.org/vulkan)—which I only noticed through the author's blog.
- frozenport 12y ago
  Ugg! Graphics and GPU compute were split by choice because OpenGL programming is an esoteric art, the only thing that can fix these is simplification - something I don't trust Khronos with because of my miserable experience with OpenCL when compared to CUDA.
  - greggman 12y ago
    Let's hope that since they had input from Value, Epic, and other users of the API instead of just producers they are doing a better job then they did with OpenCL.
    - wlesieutre 12y ago
      From what I've heard, Vulkan is largely based on Mantle. And while Khronos hasn't released anything unless you're a member of the secret club, you can take a look at Mantle here: http://www.amd.com/en-us/innovations/software-technologies/technologies-gaming/mantle#downloads http://www.amd.com/en-us/innovations/software-technologies/t...

99 points by mxfh 12y ago

Does anyone know of a real world game or app that uses geometry shaders? Everything I've read (including this) says they're good in theory but not so much in practice.

malkia 12y ago
Assasin's Creed mentions of Geometry Shaders being used - (March, 2014, GDC) - https://bartwronski.files.wordpress.com/2014/03/ac4_gdc.pdf https://bartwronski.files.wordpress.com/2014/03/ac4_gdc.pdf possibly many others. EDIT: Here is much more updated version (4 versions actually with much more detailed images/video and more notes) - from the author's web-page - http://bartwronski.com/2014/05/09/digital-dragons-2014-slides/ http://bartwronski.com/2014/05/09/digital-dragons-2014-slide... oh, and if you are looking for some fun graphics failures :) http://igetyourfail.blogspot.com/2009/02/behold-power-of-geometry-shaders.html http://igetyourfail.blogspot.com/2009/02/behold-power-of-geo...
- psykotic 12y ago
  For post-processing, Capcom's MT Framework used geometry shaders to generate primitives in lieu of scattered read-modify-writes to pixels. I'm not sure if the bang for the buck was there, but the concept was clever enough that it stuck with me.
leohutson 12y ago
They are used a lot for generating normal vectors on the fly, since you can take in a whole triangle, you just do a cross product of two of the edges and you're done. From my experience, they are fast enough as long as you don't emit more than 2x primitives that you take in. They are also useful for culling primitives. They are not suitable for tessellation, which funnily enough is better done by tessellation shaders. Tessellation shaders are actually pretty flexible, and can do most of the things that you might have thought geometry shaders would be suitable for.

westoncb 12y ago

I was working on a problem recently that's remarkably similar the author's rendering of hundreds of thousands of cubes (each arbitrarily oriented in space), and looked into geometry shaders some. There were several points where I thought I'd be able to use them to implement some resource saving trick that occurred to me—but was discouraged by further reading each time. I guess there are uses out there that they're more ideally suited for, but it'd be great if they did what they do... only fast :) Now I'm excited about this, though (https://www.khronos.org/vulkan)—which https://www.khronos.org/vulkan)—which I only noticed through the author's blog.

frozenport 12y ago
Ugg! Graphics and GPU compute were split by choice because OpenGL programming is an esoteric art, the only thing that can fix these is simplification - something I don't trust Khronos with because of my miserable experience with OpenCL when compared to CUDA.
- greggman 12y ago
  Let's hope that since they had input from Value, Epic, and other users of the API instead of just producers they are doing a better job then they did with OpenCL.
  - wlesieutre 12y ago
    From what I've heard, Vulkan is largely based on Mantle. And while Khronos hasn't released anything unless you're a member of the secret club, you can take a look at Mantle here: http://www.amd.com/en-us/innovations/software-technologies/technologies-gaming/mantle#downloads http://www.amd.com/en-us/innovations/software-technologies/t...