Getting the illusion of roundness?

[Amit Dutta]

yprinus made good points in terms of line weight and overlap. I had a look at your comic figures. I think you are not taking accurate perspective into account when doing your figures. Perspective is actually the thing that gives us the illusion of depth.  If you don't get that right first you won't be able to foreshorten anything correctly and no amount of line weight and overlap manipulation will fix it.

The other thing is you seem to be drawing your figures thinking about contour lines as a way to outline forms rather than using lines to follow form and "wrap around" objects. I am guessing you don't follow proper construction on them, where you block in each form as a general 3D primitive, then hide the bits that aren't in view when detailing? I might be wrong, but I think you need to start doing more exercises in drawing figures/mannequins using primitives in perspective.

Loomis has sections in his figure drawing book on figures in perspective




 
Some other stuff I found off google.

Overlap and foreshortening in perspective. Look at the way the artist used lines like on a topographical map to show how the surface undulates. Using two perpendicular sets of cross-sectional lines, like on this arm drawing you can show any form in space. They are just guides. Once you are comfortable with being able to do them, you may not need to draw them in at all.



Showing volume using cross-sectional contour lines


I don't know about tutorials, I think this one is all about mileage. Practice constructing figures using simplified primitive objects in perspective.