Hans Fugal

> Sent: 26 July 2007 14:57
> To: FlightGear developers discussions
> Subject: Re: [Flightgear-devel] Broken Clouds
> 
> 
> On 7/26/07, Vivian Meazza <[EMAIL PROTECTED]> wrote:
> > Hans Fugal
> 
> > I have never seen or heard of any problem with osg and 
> clouds of any 
> > sort. Xnview reports no errors in cirrus.rgba, or with any of the 
> > textures. Perhaps you have corrupted the clouds textures 
> locally? Did 
> > you open them in any editor before you saw the weird effects? Of 
> > course, people might be just accepting the weirdness, and not 
> > reporting it.
> 
> Jester, jentron, and I did some investigating last night. The 
> textures in question seem to be a less-common format that 
> some programs understand and others do not. For example, 
> IrfanView and ImageMagick both treat cirrus.rgba similarly - 
> it looks something like this: 
> http://hans.fugal.net/tmp/fg/cirrus-abstractart.png . I think 
> we can agree this is not what the sky should look like. Some 
> programs refuse to open it at all, complaining that they 
> don't recognize the format.
> 
> Jester has a better understanding about just what this format 
> is, but I believe the salient features are 2-channel: 
> greyscale with alpha. ImageMagick's identify(1) reports them 
> as PseudoColor images (vs
> DirectColor)

That's what I would expect, but Melchior is more knowledgeable than I.
 
> Gimp opens them fine, as does "osgviewer --image 
> cirrus.rgba". The latter is most important, it means that OSG 
> itself can handle this format. So we likely are looking at a 
> memory corruption bug in OSG when this particular type of 
> image is loaded. Something about the way things work on intel 
> macs is causing the corruption to manifest itself where as it 
> goes unnoticed on linux by sheer luck. Just a theory anyway.

On Windows, too.
 
> Incidentally, I have observed in both linux and osx in fg/osg 
> that cloud layers don't work quite like I'd expect. If the 
> weather is scattered clouds at 3000 ft and overcast at 5000, 
> then when I'm sitting on the runway I see blue sky between 
> the scattered clouds. It's not until I go above 3000 that 
> suddenly there's overcast sky above me.
> 

I'm rather afraid that this last is a "feature" rather than a "bug". As I
understand it, it is a consequence of the way osg orders/handles
transparencies. Not good at all. 

Vivian


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