Re: [Flightgear-devel] Re: night slowdown with nvidia GeForce 6200 on linux

[Curtis L. O. <cur...@fl...>]

Frederic Bouvier wrote:

>Quoting Alex Romosan :
>
>  
>
>>Erik Hofman writes:
>>
>>    
>>
>>>Alex Romosan wrote:
>>>
>>>      
>>>
>>>>still trying to figure out the real reason why the nvidia driver is
>>>>slow when we enable GL_POINT_SMOOTH in fgfs (and learning a lot more
>>>>about openGL then i ever wanted to know).
>>>>        
>>>>
>>>If I'm correct then you're not using points larger than one pixel in
>>>this test application. Maybe that's the reason?
>>>      
>>>
>>the point size is set to 5. that's not it.
>>    
>>
>
>FWIW, Curt doesn't draw points for the airport lights, but triangles in point
>mode, where two of the three have alpha 0 and with an environment map / texgen
>to make the points fade when not viewed in the runway axis.
>
>That's a bit trickier than simple points ( the ground point for example )
>

Let me just add a bit more information so you all don't think I'm 
completely nuts for doing it this way. :-)

1. Note that in real life, runway lights are directional.  They are 
mounted so they point up along the standard approach path.  If you look 
at a runway light from behind, you really don't see it.  There is a 
separate set of lights pointing in the opposite direction.  As your 
perspective becomes more perpendicular to the direction of the light, 
the apparent brightness of the light fades.  If you look at a runway 
from directly above or from the side, they can often be very dim and 
hard to pick out.

2. If you use a simple billboard or a simple point for a light, it will 
look exactly the same from any direction.

3. You can use a neat environment mapping opengl trick to change the 
color of the light based on what direction you are looking relative to 
the 'sweet spot' of the light.  By cleverly creating an environment map 
that has brighter colors near the sweet spot, and dimmer colors near the 
edges, we can give the effect of a directional light.

4. Unfortunately, the environment mapping effect happens even when you 
look at the object from 'behind', it's just that the mapping gets 
inverted from what it is when viewed from the 'front'.

5. To make our lights invisible from behind, I draw them as a triangle 
and take advantage of back face culling.  The triangle is created so it 
faces the direction of the light's sweet spot.  But this creates it's 
own problems.  Polygon objects start flickering in and out when they go 
subpixel in size ... and there are other issues with drawing lights as 
triangles too.  So instead, we switch opengl into 'point drawing' mode 
when we draw the triangles.  This forces opengl to only draw the 
verticies of objects (as points) rather than coloring them in with a 
texture as we do normally.  But we only want one vertex instead of 3 so 
we set the alpha of two of them to be zero (or completely transparent.)

This is a really round about way to get to the end result, but it works 
with simple, basic, well supported opengl features.  Lights are 
directional and lights disappeare when viewed from behind.

I'm sure it's possible to do something similar with shaders, but the one 
person who did some work on runway lights with shaders, didn't do it in 
a way that could be easily integrated with our code and still support 
non-shader hardware so we weren't able to integrate his work into 
FlightGear.

Perhaps we need to find someone with a bit of opengl knowledge to work 
on better lights, the code is structured so it should be pretty easy to 
call a different light generation function.  But if you do want to 
tackle this, please work closely with us core developers so we can 
ensure compatibility and integratability.  I hate turning down code 
submissions, but sometimes the code is such a mess or would cause so 
many problems to integrate, we just can't do it.  But by working with 
the core developers from the start we can usually avoid these problems.

Regards,

Curt.

-- 
Curtis Olson        http://www.flightgear.org/~curt
HumanFIRST Program  http://www.humanfirst.umn.edu/
FlightGear Project  http://www.flightgear.org
Unique text:        2f585eeea02e2c79d7b1d8c4963bae2d