Discussion Regarding limited range of planetary shadows

[Stebb]

I'm sure this is already well known in the community but I haven't seen it mentioned elsewhere. It is apparent that the shadows cast by planets have a very limited range of perhaps 1 or 2 planetary diameters.

The effect of this is that objects (i.e. moons and spacecraft) beyond that limiting distance from the planet are still fully illuminated by sunlight when the sun is out of sight, obscured by the parent planet.

I first noticed this while in orbit around one of Jupiter's moons. The moon moved into Jupiter's shadow during its orbit, so the sun was now hidden, but the moon and spacecraft were still fully illuminated by sunlight. Same thing happens with other moons much closer to their parent planet such as Phobos which remains fully illuminated when it passes into the shadow cast by Mars.

I don't know if this an issue that could be addressed within the scope of D3D9 development in pursuit of improved visual realism, or whether the limited shadow range effect is coded into Orbiter's core and can't be touched.

Anyway, thought I'd mention it in this forum for comments as it is a visualisation thing.

 

[Thunder Chicken]

I'm unsure about how it is implemented in Orbiter, but shadows from non-point sources do have distinct regions where an object may be in partial shadow or full shadow, depending on position, distance, and relative diameters of the source and light-blocking object.

Umbra, penumbra and antumbra - Wikipedia

en.wikipedia.org

I don't know to what degree that Orbiter has implemented these aspects.

 

[Stebb]

I didn't characterise the shadow effect (or lack of it) very well in my first post, but I've done some flight testing since then to get a better understanding of what causes the issue.

There are two parts to this :- (1) Moons and (2) Spacecraft.

(1) Moons:
Moons do not darken when they pass into the Umbra (shadow cone) of their parent planet. Period. I mean Full-Stop.

Of course this is not a 'bug' but rather a feature which many users would seldom if ever notice, unless like me they visit the moons of Mars and the big planets. It is therefore understandable that the simulation would omit the extra computational complexity required to model darkening of moons passing into the Umbra shadow.

(2) Spacecraft
Flight tests carried out using the standard DG.

As we all know spacecraft darken when passing into the Umbra of the parent planet, not just in low orbit but also out to large distances from the planet. But this darkening happens only under one very important condition, which is the Surface MFD (and related panel instruments) must be focused on the planet, which also allows the Map MFD to show the horizon loop on the planet map.

That is all fine in general but....

During an orbit transfer towards a moon, at some point the Surface MFD and related panel instruments will automatically become focused on that moon. When this occurs the horizon loop will no longer be shown on the Map MFD (still displaying the parent planet), and in this situation the DG will remain fully illuminated by Sunlight even when it passes into and is fully within the Umbra shadow of the parent planet, where of course the Sun cannot be seen. I'd describe this as a 'bug' rather than a feature.

Test it out by approaching Phobos and then when the Sun is getting close to setting behind Mars adopt an elliptical orbit round Mars of say 4000 by 6000 km. Check whether the DG remains illuminated once the Sun has set while the Surface MFD is still focused on Phobos. Also check how far the DG gets from Phobos before the Surface MFD focus changes back to the parent planet. I found that the DG travelled thousands of km from Phobos while still illuminated by Sunlight in the Umbra before the Surface MFD flipped back to Mars focus, at which point while still in the Umbra the DG was suddenly now in shadow as it should be.

I don't think there is a manual control to set the focus of the Surface MFD - if there was I'd use it often.

I know this is not a D3D9 issue - apologies for kicking this off in the wrong forum.

The only reason I want to mention all this, especially if this shadow effect is not well known, is in hope that it will be recognised as a significant issue and possibly something that can be examined by developers of Orbiter beta.