Hello,
I would like drawn an oxygen molecul like this : https://upload.wikimedia.org/wikipedia/ ... 3D-vdW.png
How to intermingle the spheres?

Many thanks ! Olivier

Inkscape is not set up to calculate the intersections of 3D objects. It is strictly 2D vector drawing. Your best bet is to get a simple 3D modeler to make the picture and trace it in Inkscape.
If you really want work from scratch, the intersection line of two spheres is always a circle, seen in perspective as an ellipse. So those curves are portions of ellipses, the spheres are circular arcs, the shading is a radial gradient, and Bob's your uncle (I don't know what the French is for this idiom)!

SO here is a little diagram of a simple way to do this, using the Boolean functions division and union to cut away the unneeded parts of three circles to simulate a 3D render.



Oops the text that says "Cheating Ellipse! Should be a straight line" is too small to read. Basically it means that correctly the join between the two same size spheres should look like a straight line but a curve is visually more interesting so I made a shallow curve that is not too far from true.

druban wrote: and Bob's your uncle (I don't know what the French is for this idiom)!

somethin like 'et voilà [c'est fait]' or
'le tour est joué' or both 'et voilà le tour est joué'
Or '[c'était] du gâteau' (which is quite a literral translation of '[it was]piece of cake')

more familiar you can use 'les doigts dans le nez' or the frenglished version 'fingers in the nose'

hey I was going to use Voila but thought what if that has become politically incorrect for whatever reason..

Nice diagram!

Just a note, even if it's for "eyeballing" an acceptable look.
The cutline of the two spheres being an ellipse, right.
Cutline is the same as if the spheres were split into two pieces by a plane.

Now, let's look at the sphere on the left alone with that cutline=circle and ellipse. You couldn't give those a fill for a shaded look as they don't make much sense in 3D.
The ellipse positioned in the depicted way would definitely be tangent to that circle as well.


Not saying constructing the accurate image would be easy though. Probably that would be more error prone than simple eyeballing.

Almost all the time you are dead on, Lazur, and probably this time too! But I don't understand what you mean about the gradient fill... Inasmuch as a radial gradient can approximate a highlight on a sphere for inaccurate but convincing illustrations I don't see the problem with any of these fills - I know the real highlight would not be circular but curved, of course.
As for the tangency thing I kind of thought that too when starting out but realized it's only true for spheres of the same size or when the plane of intersection is parallel to the line of sight. And also not considering perspective which adds some more complications.

I will try to do an extreme case illustration when I have time if I have expressed this badly...



ET VOILA!

Thank you but I might have jumped too soon this time.
The cut line-ellipse can be non-tangent to the sphere/circle as well.
With such a height/width ratio on the ellipse I'd expected a view from a narrower angle, which would result in tangents.

Based on the image in the original post, quick eyeballing:

The ellipse is tangent to both circles:

Another note.
If the three cutlines intersect in one point at the front, they are intersecting at the back in one point as well.
Three cutlines intersecting:=three planes can be layed on each cutline, which are intersecting;
Two planes intersecting have a line as a cutline, these three cutlines of the spheres therefore define an axis. Which axis cuts the sphere at two points. One in the front, one at the other side.

Whew! TBH I was sweating that one! Close call. Death Star to the rescue.

Oxygen is usually represented as a red sphere with two holes therefore Molecular modes use different colors to aid in understanding the atomic structure of the model. Red for Oxygen, Blue for Nitrogen. Black for Carbon, and White for Hydrogen. Hope it's help you to solve your issue.

That's kind of helpful! I did delete the irrelevant website link advert in your post.... instead of just spam banning it as I suppose might have to be done later or sooner.