Pitfalls with Cylinder Unwrap

The video quickly runs through a couple of weirdnesses with Cylinder Unwrapping. It is more a demo than a tutorial, but it yet contains some hopefully useful information for your daily work.


This is a quick glance at unwrapping.
Delete the default cube.
Add a cylinder and keep the cap ends open.
Now go to the UV editing screen layout and
do a simple unwrap.

The result is a circular shape. Why that ?
The unwrap tool tries to keep adjacent edges
of the model also adjacent in the UV map.
Hence this UV map is a perfect solution, because
it does not contain any seams.

let us now create a simple test pattern.
Go to image, new image. Also check UV test grid.
Open the properties sidebar in the 3D view.
locate the display section.
and enable textured solid.

go to object mode and inspect the cylinder surface.
So, there is no seam as expected, but the test pattern shows distortions.

Well, actually we want to map the cylinder to a flat rectangular surface.
Lets try this by adding a seam.
Select one vertical edge.
mark it as seam.
Now select all vertices and unwrap again.

The result looks much less distorted,
although we still see a mismatch at the seam.

We can do that better by using a cylinder unwrap.
The unwrap should work smoothly because we indeed have a perfect cylinder here.

unwrap, then select Cylinder projection.
the result is almost perfect,
we no longer see any distortions,
and only the vertical scaling seams broken.
But this is just because the circumference of
the cylinder is 3.14 times larger than its height.
Lets ignore this for a moment and lets check one other annoyance.
The UV map is not exactly aligned to the texture bounds.
While this is not important in practice, nevertheless it is unexpected.

Here is a trick to fix that.
Ensure that you have selected all vertices,
then Zoom into your model.
and move the entire object a tiny bit to the left.
unwrap again.
And now the UV map aligns perfectly.
I do not know why that happens, but it works.

Now scale your cylinder down a bit.
unwrap again. I expected that the UV Map would keep the same.
but the scaling on the Y axis is changed.
You can fix that by calling:
UV, transform, scale to bounds.

Now scale the cylinder up.
unwrap again. Is that expected behaviour?
Well i personally did not expect that to happen.

I hope this little weirdness collection helps a bit,
to get around some pitfalls with texturing.
Thank you for watching.

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