Jump to content

  • Log In with Google      Sign In   
  • Create Account

atmospheric scattering and sky geometry


Old topic!
Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.

  • You cannot reply to this topic
3 replies to this topic

#1 Irbis_88   Members   -  Reputation: 107

Like
0Likes
Like

Posted 29 November 2013 - 04:06 PM

I'm trying to implement an atmospheric scattering in my graphics (game) engine based on the gpu gems article: http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter16.html An example implementation from that article uses a skydome. My scene is different - I don't render a whole earth with an atmosphere which can be also visible from the space but some finite flat (rectangle) area with objects, for example a race track. In fact this is the most common scenario in many games.

 

Now I wonder how to render a sky in such case:

 

1.What kind of geometry I should use: skydome, skybox or a full screen quad - then I have to move almost all calculations to the fragment shader, but I don't know if it makse sense in terms quality/performance ?

2.How to place sky geometry on the scene ? My idea: I have a hemisphere (skydome) geometry with radius = 1 and center in vec3(0, 0, 0) - in object space. Those vertices are sent to the atmospheric scattering vertex shader:

layout(location=0) in vec3 inPosition;

Next, In the vertex shader I transform vertex this way:

v3Pos = inPosition * 0.25f + 10.0f;

Uniform v3CameraPos = vec3(0.0f, 10.0f, 0.0f), uniform fInnerRadius = 10.0f, uniform fCameraHeight = 10.0f

Then I have correct an inner/outer radius propotion (10/10.25),right? I also send to the vertex shader a model matrix which sets a position
of the hemisphere to the postion of the mobile camera vec3(myCamera.x, myCamera.y, myCamera.z):

vec4 position = ProjectionMatrix * ViewMatrix * ModelMatrix * vec4(inPosition, 1.0);
gl_Position = position.xyww; // always fails depth test.

The hemisphere moves together with the camera (encloses only some space around camera with radius = 1, but it also always fails a depth test.) Unfortunately a sky color which I get is not correct: http://img17.imageshack.us/img17/6930/un5u.png

 

3.What about a "sky curve"? Here is a picture which demonstrate what I mean: http://imageshack.us/a/img5/4968/fccw.png

 

Debugging: In the vertex shader I assigned to v3Pos position of the "highest" vertex in the hemisphere:

vec3 v3Pos = vec3(0.0f, 10.25f, 0.0f);

Now the whole sky contains a color of that vertex: http://img27.imageshack.us/img27/7241/a6ud.png


Edited by Irbis_88, 29 November 2013 - 04:10 PM.


Sponsor:

#2 Kaptein   Prime Members   -  Reputation: 2179

Like
0Likes
Like

Posted 01 December 2013 - 04:17 AM

That's the article that shows an implentation of simulated Rayleigh scattering, and it requires a skydome to be the top part of basically a planet's atmosphere.

The skydome must be created as in the drawing shown:

http://http.developer.nvidia.com/GPUGems2/elementLinks/16_atmospheric_03.jpg

 

I have an implementation that works here:

https://github.com/fwsGonzo/cppcraft/blob/master/source/atmosphere.cpp

 

Some downsides:

1. I still haven't bothered to index the dome.

2. His implementation of scattering isn't the best

3. It doesn't work well if your player can go up/down, as in flying (but that isn't unusual at all)

 

Otherwise, it's fairly ok



#3 ankhd   Members   -  Reputation: 1358

Like
0Likes
Like

Posted 02 December 2013 - 04:55 AM

Heres a good read



#4 Irbis_88   Members   -  Reputation: 107

Like
0Likes
Like

Posted 14 December 2013 - 04:49 PM

ankhd - thank you for the link but I didn't found there informations about a sky geometry. I think that there is also used skydome which is the same as in the GPU Gems 2 article . Please let me know if I wrong.

 

Kaptein - thank you for sharing the code. First I generated a mesh with innerRadius = 10 and outerRadius = 10,25. It looks very flat compared to the hemisphere:

http://imageshack.us/a/img189/223/yn8w.png

http://imageshack.us/a/img11/6931/zcdv.png

 

When I use above mesh I get correct results. I attached that skydome to the camera as I described in the first post. I also tried another approach: I generated a mesh with innerRadius = 100000 and outer radius = 102500. Then camera height can be variable. Here are screens:

cameraHeight = innerRadius (100000): http://imageshack.us/a/img713/9215/fmx1.png

cameraHeight = 101250: http://imageshack.us/a/img560/3236/k5fp.png As you can see there a little mismatch between a sun and a brighter area on the sky which should be around the sun. Is it possible to fix that ?

 

Moreover I removed the last line

gl_FragColor.a = gl_FragColor.b;

from the SkyFromAtmosphere.frag and a sky color is more realistic.

 

 

Conslusion: a hemisphere as a sky geometry doesn't work with that algorithm.

 

Which atmospheric scattering algorithm do you use in your implementations ? How you represent a sky geometry ? How it is done in games ? (There can be some differences between the "standard scenes" and the "flight simulators scenes")


Edited by Irbis_88, 14 December 2013 - 04:59 PM.





Old topic!
Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.



PARTNERS