How to map text on a sphere?

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Old 04 April 2011   #1
How to map text on a sphere?

Hi, I'm a photo- retoucher with a very limited experience in 3d. Last week I was asked to retouch a photo and to composite writings on two billiard spheres.

I asked the photographer to put a piece of square tape on the sphere so that I would have a reference for the distortion. Obviously it wasn't precise enough so it was useless.

I often see that in movies they have some kind of grids (points) that they can use for reference. Where can I obtain further information about this technique?

What is the best method to create geometrical references of the surface of 3d objects?





My next attempt was to just create a 3d sphere in photoshop and to map my text on it.


The distortion is obviously wrong. The square looks now like a rectangular. It is way to much stretched on the X- axis.

The proportion is more or less 1.5 : 1.0

Why doesn't Photoshop distort it correctly?



To correct it I've just distorted the texture on it's V-axis....

 
Old 04 April 2011   #2
part2

The resulting text looks now like this:





I thought it was distorted correctly this way, but the Client wasn't of the same opinion. He told me that it isn't correct that the text is following the parallels. It rather has to be distorted on both sides.
Like this:



After a discussion we decided to not distort it this way, because it just wouldn't make sense.

The last request he made was this:



He insisted that the letter which are further away have to be smaller than the one that are nearer.

I think it is wrong and doesn't look right...

I don't really think he had a very precise idea what he was talking about, but unfortunately I wasn't sure either. So in the end as it is often the case I just tried to make him happy...

Next time I'll be better prepared...

Thank you very much in advance

Sebastian

Last edited by s3bi : 04 April 2011 at 10:15 AM.
 
Old 04 April 2011   #3
If you want to map a square patch (block of text) on a sphere more accurately you don’t want to use the standard globe grid that you’re showing (large spheres on left in image below). You’ll want to use 2 sets of just the parallel circles (latitude lines on a globe), one set at 90° to the other as shown in the right sphere examples below. *The red lines in (A) represent the tape on the ball.
(B) shows the spheres in orthographic front view (no perspective) to compare the distortion of the standard globe projection to the other.

You also have to take into consideration the lens focal length used and or it's distance from the subject.

Right click to see larger image.

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Last edited by Quadart : 04 April 2011 at 11:04 AM.
 
Old 04 April 2011   #4
Hi Quadart,

thank you very much for your reply. Basically you are saying that the approach of using a globe do determine the distortion is wrong, right?

Do you have any experience with the build in 3D features of Photoshop? What Software did u use to create the Spheres in your screenshot?

Do u have any idea how to work with tracking points?

Thanks!
 
Old 04 April 2011   #5
In your case, where you are trying to map a square patch of text on a sphere (similar to the tape patch), the globe projection grid is going to produce horizontal bloating. Although the vertical lines are actually straight (relatively speaking ), they don’t appear to be from a front on view. If you wanted to represent actual perpendicular straight lines on a sphere, you would need two sets of the globes longitude lines (circles rotated on the vertical axis [y]), one set at 90° to the other (to get the ‘bloat' all around). The standard globe projection works fine if you are running a band of text completely around the ‘equator’ of a sphere. The grid I used (which is not available in PS 3d, afaik) best represents a square patch projected on a sphere, which works fine for a square not much bigger than the taped patch. Actually, a square projected on a sphere is a puckered square, with it’s side lines arced toward the center, if presented on a 2d plane. Note the buckling of the tape as it tries to comply with the round surface.
Either projection method works, depending on what you are trying to achieve.


As far as PS’s 3d features, I have no real experience with them as I use Cinema 4D, primarily, for all things 3d. I used C4D for the examples shown.

I’m not an animator (just a babbler) and don’t work with tracking points.
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Last edited by Quadart : 04 April 2011 at 01:54 PM.
 
Old 04 April 2011   #6
Hey! Thx!

So basically you are saying that there doesn't exist just one "correct" projection, right? Do u have any idea how you would achieve the projection without the 3d functionality in photoshop or with Cinema 4d?

Btw. your portfolio is awesome!
 
Old 04 April 2011   #7
Thanks for the compliment.

The correct method you choose to make your grid depends on what you want to achieve.

If the 3d app or 2d app wrapping methods were not available, I would use that photographed tape method in your example, as long as the text block is no bigger than the tape area in your example. I would print out a sample of the needed text with a light grid of horizontal and vertical lines overlaying it for reference (printed to actual size). I would then apply some double-sided tape to the cropped print, apply it to the ball and photograph it. I’d use the photo in my psd file to carefully warp the text block to the photo ref using the Warp tool. This should work as long as the text does not creep to far around the side of the ball.

The other alternative is to tediously create a perspective line drawing to geometrically construct a grid on a sphere.

You may want to ask this question in the Photoshop forum to get more feedback.
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Last edited by Quadart : 04 April 2011 at 05:10 PM.
 
Old 04 April 2011   #8
The examples show another type of spherical projection called equirectangular projection . If your 2d map is not properly ‘distorted’ it will be distorted on the ball. This is one reason why you’re having a problem with the text not looking right.
Imagine if the dots were letters.

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Old 04 April 2011   #9
Originally Posted by s3bi: The proportion is more or less 1.5 : 1.0


The proportion of a texture map for a perfect sphere is 2:1, not 1.5:1.
 
Old 04 April 2011   #10
Hi Quadart! Thanks for your reply! Very interesting!

Should I post a new thread in the Photoshop- forum or just link to the existing one?

Thanks for your reply Creed but I didn't say anything like that. I just had to re-transform the text-map to make it look right on the sphere (150% on the v- axis)
 
Old 04 April 2011   #11
At this point I would just start a new thread in the Photoshop forum and link to this thread, instead of asking to have this one moved there.
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Old 04 April 2011   #12
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