Terminator

[chris]

(Not the movie . . .)

The terminator is the boundary between the lit and unlit hemispheres of a planet. A new reference mark lets you precisely where it is:



The reference mark is only appropriate for ellipsoidal bodies; it won't work for irregular bodies such as asteroids. The current code also assumes that the sun is a point. On hot Jupiters, where the star fills a large region of the sky, there can be significant area on the 'dark' side of the terminator where some portion of the star is still visible.

There are a couple interesting places to turn on the terminator mark. One is Mercury: enabling both the planetographic grid and the terminator, and then accelerating time will make it clear just how odd the day-night cycle on this planet is. Relative to the grid, the terminator every so often appears to do a slight 'bounce': this is when the Mercury's 3:2 spin-orbit resonance produces a double sunrise for appropriately located Mercurians.

Another interesting spot is 2003 EL61. Its dramatically non-spherical shape means that its terminator is not the usual circle.

--Chris

[ElChristou]

It is in SVN already? One can display only the terminator?

[selden]

yes and yes.

[ElChristou]

yes and yes.

Nice! Tx!

[ElChristou]

Got an error in celx_frame.cpp, Command /usr/bin/gcc-4.0 failed with exit code 1...

[Vincent]

Chris,

This is a great educational addition to Celestia, thanks !

I got a link error when compiling with Visual C++ 2005 Express Edition and makerelease.
I fixed it by adding visibleregion.obj to the list of OBJS in engine.mak.

[duds26]

A better name would be Illumination boundary instead of terminator.
You see immediatelly what it is by its name.

[chris]

A better name would be Illumination boundary instead of terminator.
You see immediatelly what it is by its name.

Terminator is the the widely used term, and the appropriate name to use in Celestia:

http://en.wikipedia.org/wiki/Terminator_%28solar%29

(or terminateur in French: http://fr.wikipedia.org/wiki/Terminateur)

--Chris

[ElChristou]

:x Got an error in celx_frame.cpp, Command /usr/bin/gcc-4.0 failed with exit code 1...

Today svn just build fine. Tx for the fix.

No smooth lines for both terminator and planetographic grid?
One can also display planet grid and terminator on spacecraft; is this useful? (if yes, the term should change depending on the selected object ("planetographic" grid for a spacecraft... bof...))

Last remark, the contextual menu now display 6 options that could perhaps been displayed in a "Guides" submenu to make it less obtrusive... (and perhaps then, mark/unmark could also go under Guides...)

[ElChristou]

A better name would be Illumination boundary instead of terminator.
You see immediatelly what it is by its name.

H?h?... well sorry but if you talk about the terminator of a planet, perso I see immediately what you are talking about... Let's use the existing terms when they exist!

[chris]

:x Got an error in celx_frame.cpp, Command /usr/bin/gcc-4.0 failed with exit code 1...

Today svn just build fine. Tx for the fix.

No smooth lines for both terminator and planetographic grid?
One can also display planet grid and terminator on spacecraft; is this useful? (if yes, the term should change depending on the selected object ("planetographic" grid for a spacecraft... bof...))

It's easy enough the add line smoothing, but there was talking about whether it was actually an improvement. I usually run with full-scene antialiasing on and smooth lines off. I can turn on line smoothing though, and you can see which you like better.

I don't know if the grid is useful for spacecraft; it is interesting to enable it when body axes are turned on. Certainly, the name should change The terminator should probably be disabled for any non-ellipsoidal object--not just spacecraft, but asteroids as well.

Last remark, the contextual menu now display 6 options that could perhaps been displayed in a "Guides" submenu to make it less obtrusive... (and perhaps then, mark/unmark could also go under Guides...)

Agreed. In the other versions of Celestia, there is a reference marks submenu. The KDE and Qt4 versions have another submenu for markers that lets you select the marker shape.

--Chris

[chris]

I modified the terminator and planetographic grid reference marks so that there's less of a gap between the lines and the planet surface. I think that this improves their appearance significantly.

--Chris

[ElChristou]

It's easy enough the add line smoothing, but there was talking about whether it was actually an improvement. I usually run with full-scene antialiasing on and smooth lines off. I can turn on line smoothing though, and you can see which you like better...

Would be cool for those who don't have full-scene AA; I suppose we are not a lot but...

[Cham]

The planetary grid needs some refinements. It's still crude at close range (too much polygonal feel), and the degree symbol should be shown for the coordinates.

There's also some 'interferences' between the clouds layer (in the case of Earth) and the terminator circle and the planetary grid.

[chris]

The planetary grid needs some refinements. It's still crude at close range (too much polygonal feel), and the degree symbol should be shown for the coordinates.

I disagree about the degree symbol--it would just add visual clutter when it's already quite clear what the numbers indicate. I'm planning on adding more segments to the grid at close range, but should probably implement some sort of culling strategy in order to avoid drawing a lot of unseen line segments.

There's also some 'interferences' between the clouds layer (in the case of Earth) and the terminator circle and the planetary grid.

I'm trying to decide what to do about that--should the grid be displayed over the cloud layer when clouds are enabled and on the ground otherwise? Selden and I discussed this issue briefly and agreed that that seemed the best option.

--Chris

[Cham]

I'm trying to decide what to do about that--should the grid be displayed over the cloud layer when clouds are enabled and on the ground otherwise? Selden and I discussed this issue briefly and agreed that that seemed the best option.

I think it would be much better to see the grid above the clouds and the ground, in all cases.

Also, the grid color used should be a parameter in the start script (as all other colors...), since there may be some confusion between the grid lines and spacecraft trajectories (especially in the case of labels and grid coordinates).

[chris]

I'm trying to decide what to do about that--should the grid be displayed over the cloud layer when clouds are enabled and on the ground otherwise? Selden and I discussed this issue briefly and agreed that that seemed the best option.

I think it would be much better to see the grid above the clouds and the ground, in all cases.

Also, the grid color used should be a parameter in the start script (as all other colors...), since there may be some confusion between the grid lines and spacecraft trajectories (especially in the case of labels and grid coordinates).

Setting the color is already possible with Vincent's latest celx checkin.

Here's an interesting usage of visibility circles. The visible regions of GPS satellites are drawn in cyan, and the the visible regions of ISS and Hubble are green:

visregion.png

(There's no UI for this right now; I had to write a very simple script. For ISS and Hubble, this is all that I had to do:

Code: Select all

earth = celestia:find("Sol/Earth")
iss = celestia:find("Sol/Earth/ISS")
hubble = celestia:find("Sol/Earth/Hubble")

earth:addreferencemark{type = "visible region", color = "green", target = iss, tag = "vr iss" }
earth:addreferencemark{type = "visible region", color = "green", target = hubble, tag = "vr hubble" }


Credit goes to Vincent for implementing the celx commands to add and remove reference marks.

--Chris

[Cham]

Setting the color is already possible with Vincent's latest celx checkin.

Chris, please, could you give the proper celx command to change the color of the planetary grid ?

[Vincent]

Setting the color is already possible with Vincent's latest celx checkin.

Chris, please, could you give the proper celx command to change the color of the planetary grid ?

Martin,

The colors of the planetographic grid elements can be changed using the setlinecolor/setlabelcolor methods:

Code: Select all

sel = celestia:getselection()

celestia:setlinecolor("planetographicgrid", 0.35, 0.35, 0)
celestia:setlinecolor("planetequator", 0.7, 0.5, 0)
celestia:setlabelcolor("planetographicgrid", 0.6, 0.4, 0)

sel:addreferencemark{type = "planetographic grid"}

[chris]

Setting the color is already possible with Vincent's latest celx checkin.

Chris, please, could you give the proper celx command to change the color of the planetary grid ?

setlinecolor and setlabelcolor, just as for other colors.

I don't advocate actually using these particular colors, but here's an example:

Code: Select all

celestia:setlinecolor("planetographicgrid", 0.5, 0.5, 0.8)
celestia:setlinecolor("planetequator", 0.9, 0.9, 1.0)
celestia:setlabelcolor("planetographicgrid", 0.0, 1.0, 1.0)


We should probably make the terminator color settable as well.

--Chris