Shifting an equirectangular map north/south

[rdanhenry]

The energy requirements for altering a planet's rotation like that are up in "if you can do it, you don't need to" territory.

Venus rotates extremely slowly. Stopping it and starting the new rotation on a new axis would be less work than the inescapable need to spin it up to something much closer to an Earthlike rotational rate or Earth-based climate models won't even apply in the first place, as day/night would easily dominate over seasons.

[acrosome]

Venus rotates extremely slowly. Stopping it and starting the new rotation on a new axis would be less work than the inescapable need to spin it up to something much closer to an Earthlike rotational rate or Earth-based climate models won't even apply in the first place, as day/night would easily dominate over seasons.

I'm all over it, Brother.

[Azélor]

Venus rotates extremely slowly. Stopping it and starting the new rotation on a new axis would be less work than the inescapable need to spin it up to something much closer to an Earthlike rotational rate or Earth-based climate models won't even apply in the first place, as day/night would easily dominate over seasons.

Yes but no since he said he speeded up the rotation of the planet to have a 24h day.

[acrosome]

OK, now a question about GProjector.

When I load my PNG black-and-white height map into GProjector it looks like this:



But when GProjector displays it, it gets lighter, like this:



And as you can see it stays lighter when I try to save the image (after changing the axis).

Why?

Any ideas how to correct this?

[Mouse]

I don't know why it does that at all, but you could always open the finished product in GIMP and play with the colour curves to make it right again?

[waldronate]

It looks like it's doing a gamma correction on the file during load. As Mouse suggests, you might be able to use an image editor to apply an appropriate inverse gamma. You'd need to characterize the evil things that it's doing to your data to correct it, though (processing a known ramp might help).

[Charerg]

I think it must be like waldronate said: for some reason it changes the image into brighter. I experimented a bit, and it seems the original greyscale is still visible if you go under "shading" and "darken nightside" with 100% darkness:



I hope that will at least be helpful in reversing the gamma change.

Edit: Actually, I just realised that if you activate "darken nightside" and set it to 0% darkness, you'll get to keep the original greyscale image. Here's the image centered at 20S:



Hmm, very interesting. I wonder if GProjector applies some kind of "daylight gamma" automatically?

[acrosome]

Ah, thank you.

[waldronate]

If it's a daylight/nighttime correction factor, then it's probably an add followed by an exposure correction. Not quite the same as gamma, even though it can look similar.

[zhar2]

I wanted to do this and managed it with GIMP, with its polar coordinates filter.