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Lost of precision while upscaling
Closed, InvalidPublic

Description

System Information
Operating system and graphics card

Blender Version
Broken: 2.72b and before

Short description of error
After scaling up the objects, the vertex coordinates, which was in the same point of space (snapped one to another) appear to be in different coordinates.

Exact steps for others to reproduce the error
Add two cubes, move one of them to random location, then snap another one to it. Scale them *100. Check the "shared" vertex coordinates of the second cube.

P.S.
If I will export the scaled cube (for example to collada file), I get broken uv unwrap because of the same issue, i think. All polygons will be detached one from another.

Details

Type
Bug

Event Timeline

Maxim Tkachenko (maleficmax) created this task.
Maxim Tkachenko (maleficmax) raised the priority of this task from to Needs Triage by Developer.

Hey @Maxim Tkachenko (maleficmax),
I'm not sure what you mean with "shared" vertex coordinates. I tried to reproduce this, but all the coordinates I found were exactly the same, except of the y-location in the Texture Space panel. Is this what you meant?

Julian Eisel (Severin) triaged this task as Needs Information from User priority.Dec 8 2014, 4:02 PM

Here we have a base cube:
http://s30.postimg.org/kfixd66wh/screenshot_2014_12_08_at_17_05_27.png
We move it to some random location (drag by mouse):
http://s30.postimg.org/nnneq7t69/screenshot_2014_12_08_at_17_07_01.png
Now we duplicate the cube (Shift+D) and then snap it to original. Any vertex to any vertex. Now some of their vertices share one coordinates.
http://s30.postimg.org/83g0zoj1t/screenshot_2014_12_08_at_17_08_41.png
Now they (vertices) are in the same point of space. They should be there even when we will...
...scale both boxes in 100 times (s - 100 - enter):
http://c2n.me/jqZ9tb.png
(sorry, had to change image hosting here :) )
Let's watch on the "shared" vertices from top:
http://c2n.me/jqZe9p.png
Lets zoooooom:
http://c2n.me/jqZfgz.png
This is the main thing what I am talking about. It can even be worse - the vertices of the same cube can be in different coordinates on any axis.

And the second thing is when i export such mesh to collada and it has a uvw unwrap, the coordinates of every polygon's vertex on it appears to be moved a little. And as a result - i have every polygon detached on the unwrap.

Julian Eisel (Severin) raised the priority of this task from Needs Information from User to Normal.Dec 16 2014, 4:49 PM
Lukas Toenne (lukastoenne) claimed this task.

This is a common floating point precision issue, not a bug. What you have to understand is that snapping works by adjusting the transform distance such that the 2 vertices coincide. Due to the limitation of the calculation the vertices don't coincide perfectly - and that should be fine. There is no logic demanding permanent alignment of objects through snapping, if you really need that you could try a parent with a shrinkwrap constraint perhaps. Even then, scaling by a factor of 10,000 is going to be an issue at some point, there is nothing we can do about this. Applying scales to objects helps, because it avoids multiplication rounding errors from matrix products.