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texture mapping on a sphere

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chad demoss
texture mapping on a sphere
on Apr 25, 2018 at 12:06:55 am

I tried putting a texture map on a sphere and it did what I though it would do as in "pinching" the 2d map together where the edges meet. I got the uv map using BP - UV edit and used "Sphere" projection to get the UV mesh. maybe I should of used a different projection? I see what needs to be done (I need to squash the very top and bottom of the psd file I made but I'm wondering if there's an easier way in cinema 4d?

attached is a screen grab of the render and the psd file. I put down the red dots to see where the problem was.





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Brian Jones
Re: texture mapping on a sphere
on Apr 25, 2018 at 12:41:07 am

images on a sphere should (at least with Spherical mapping) be twice as wide as tall (2x1 aspect ratio) and everything gets stretched sideways toward the poles (in the image) because it's going to be pinched in as it gets wrapped onto the sphere. That's why everything is too big the farther north and south you go with a Mercator projection map.
Take a look at the images of the mars https://nasa3d.arc.nasa.gov/detail/mar0kuu2 or the moon here http://www.celestiamotherlode.net/catalog/moon.php

that's what your texture should do, all features should be stretched sideways, more and more towards what should be the poles.


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chad demoss
Re: texture mapping on a sphere
on Apr 25, 2018 at 5:43:08 pm

ok I get it.
in BP - UV I made a new material at 1024x2048.

thanks for your help.


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Steve Bentley
Re: texture mapping on a sphere
on May 2, 2018 at 9:03:01 pm

Here's another trick Chad.
Find the texture you like (Brian is right - ratio should be 2:1) then take it into Pshop. Change the image size so that its square (yes it will distort). Then use the Polar coordinates effect (rect to polar) and that will show you what the image will look like mapped on a sphere (from the top down). Do a little touch up painting (or stamping) to clean up the pinch point in the middle - don't change the edges! Then use the polar effect again, but choose polar to rect. This will get you back to your square but with some weird looking smeary things at the top. Thats normal.

Now flip the image upside down and go through the same process again - the polar filter can only do one pole at a time- the top one- so you have to flip the image if you are going to see both poles at some point.

Once that's done flip your image right side up again and then change the image size back to the original size. You will need do to a little sharpening because the polar effect makes things a little soft due to the inherent lack of resolution when converting from cartesian to polar.

Use a spherical map for this texture in C4D - a UVW map will also work with a sphere object but not as well because the UVW will "eat" a lot of what's at the poles due to the polys getting smaller and the polar effect distorts things assuming a sphere. If you mapped a globe with a UVW, Antarctica would simply be gobbled up in the pinch and north and south america would be too close to the poles as a result - russia and canada would probably touch where Santa lives. While the antarctica will look odd in the pshop file it will map correctly if you use spherical mapping.

This is also a perfect effect for reflection maps so that you don't see the pinch point at the top of the environment reflected in your scene objects.



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chad demoss
Re: texture mapping on a sphere
on May 3, 2018 at 11:23:54 pm

I ended up just making a square 2048x2048 throwing down a bunch of evenly spaced guides and scaling the tops and bottom to the 2:1 ration in 10 steps so that as it got closer to the center each section was scaled less . It worked for the most part except the extreme top and bottom was still stretched. Ill try what you said with polar coordinates and using spherical mapping.



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Steve Bentley
Re: texture mapping on a sphere
on May 4, 2018 at 2:41:37 am

To keep sharpness of the texture overall you can do our polar trick and then just reveal the unpinched polar part on top of the the original 2:1 image in pshop so only the stuff that is at the extreme poles gets soft due to the back and forth with the polar effect.



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