ok let’s suppose that you learned everything about human anatomy (both male and female), you memorized the color table to heart (as to which colors fits with what), you know how to draw the great wilderness and the beasts as well, you mastered the art of conveying feelings through art and you know all forms of shading possible….but you know what, you could spend a whole month drawing the greatest drawing of your life and go display in front of the crowd, you would expect the people would line up and see such an amazing work you have made but no, people would just walk by, look at your art and go. you feel very disappointed and start to think as to what went wrong, you did everything right (I’ll get to the point just wait), when a passerby shows up, you ask what’s their opinion of your work, they would look long and hard and say (it is the most beautiful piece of art I have ever seen….in my head of course >_<) “its certainly amazing but its missing something”. now you would think it would be putting soul into the art or love or whatever lousy excuse any person can think of. well, the difference between art that is good but doesn’t capture attention and plain art that capture attention is the thickness of the line…..yep….your entire work depends on how well you’ve treated your lines, by now you’ve been reading everything I’ve written so far but by the time you reached to a sentence that was in bold, you suddenly slow down your reading and focus on what it says (regardless if it actually had something to do with the topic at hand). why do you think some headlines are in bold, to attract our attention, we humans are very visual-based creatures, we can be entertained through visual means (reading can be considered a visual mean since we are using our eyes to read whats on the paper).
(starting a new paragraph in case you were starting to get bored or you’re already passed that stage)if you want to have more depth, funk, power or whatever you call it, you need to learn how to very your lines. take cartoons for example, if you looked closely (not close enough to damage your eyes >_<), you would notice that the exterior lines of the character are more bold than the character’s interior lines, now why is that? because its makes the character stand out (try it yourself, make a nice drawing of a house, trees, the happy smiling sun and your character. now make your character have bold outer lines, you would immediately notice your character)not only that but manga artists seemed to have learned about this and applied to their own manga too, this caused different types of line thicknesses to appear (don’t worry, they’re not too many and they’re easy to learn), the first type which I like to call #1 the cube, which is basically the default bold outer lines thing (emphasizes on the presence of the character), #2 the ball, what you do is that you first establish where the light is coming from and where the shading will appears, you make the light parts have the thinner lines while the shaded parts have the thicker lines (this supposedly should give your art more depth), #3 the vase, this type focuses more on the curvature of the artwork (emphasizing it), to make this simpler to understand, draw a vase, when you draw the inward curves of the vase, add thick lines (just remember to add thin lines when curving outwards and thick lines when curving inwards), #4 and the final one, the block, this makes your art look a lot more close or further depending on how you adjust the lines. to do this, think of which part you want to put it close or to make further (thin lines to make look far away and thick lines to make closer) and adjust till you satisfied.
just to help you swallow this horde of information, I added a reference sheet that’ll hopefully help you understand this a lot better, I have researched this topic a lot of times and most of the times you would find something similar that I have discussed here, I suggest reading the book “how to draw manga, penning characters” (this book does contain partial nudity but not a lot to make it something for pg-18) for more information.