Re: Heat hazes

[Katie Lucas <katie@fysh.org>]

On Sun, Sep 22, 2002 at 08:10:15AM -0500, Steve Baker wrote:
> Adam D. Moss wrote:
> >Katie Lucas wrote:
> >
> >>I've been playing "Jak and Daxter" on the PS2 recently, and there's
> >>this very nice effect with heat hazes rising from torches and
> >>things. The PS2 is structured around OpenGL,
> >
> >
> >I wish it were true that the PS2 is structured around OpenGL!
> >Is it, indeed?
> 
> I wouldn't say it was structured around OpenGL - but there is a
> reasonably efficient partial OpenGL implementation for it.
> 
> >>so it may be an OpenGL
> >>effect - any ideas how it's done?
> >>I can think how to do it with a per-pixel texture processor (look at
> >>the existing image and distort it, but is there a reasonably fast way
> >>without that?
> >
> >Without having seen it firsthand, the way I imagine it's done is that
> >the scene is drawn approximately back-to-front and that when it comes
> >to render the torch the 'behind the torch' framebuffer contents are
> >sucked into a texture (PS2 has pretty fast paths for fb->texture
> >and render-to-texture) and drawn as a billboarded mesh with its
> >vertices wiggled.
> 
> Yes - that seems most likely.

Pretty much what I figured - the effect appears to be "overlayable" in
that several patches of distortion can overlap.

I'll have to have a play with trying it at some point...

> Another alternative is that they might concievably render the polygons
> that are behind the torch directly into a texture map.  On some hardware,
> the data path from frame buffer to texture is very slow and it can actually
> be faster just to re-render those polygons directly into the texture map.
> 
> Which of those things you do depends on the ratio of the speed of rendering
> to the speed of the frame-buffer-to-texture pathway and the amount of
> overdraw in the area behind the torch.

PS2 seems to have a disturbing amount of drawing power... although we
have seen geometry pop-up and a few render glitches in J&D, and
apparently in one particularly complex bit (Which I haven't got to yet
but Duncan has) it broke frame, it does draw Quite A Bit Of Stuff..