need help with sketching

[Ummu Sufyaan]

im not sure what happens when it comes to shading in my sketches. im happy with the idea i have for a sketch and when i come to drawing it, all seems well, but something seems to go wrong when it comes to the shading part. after that, my picture is ruined.



Sketch 1: a sketch of (desert) sand dunes... the actual shape of the sand dunes themselves i guess could have been more "softer" (so rounded and not triangular) but obviously the shading doesn't look right...i cant quite figure out why though...any ideas?

Sketch 2: the following i am yet to shade in becuase i wanted some advise before attempting anything. basically, the idea behind this, is that height of the hills behind the trees and meant to be going in opposite direction to the height of the trees. then there is a full moon at the top.

i started shading the sketch when i realized 1) the trees dont really need all that much to them since (indicated by the moon) its semi-dark and 2) the trees which have to be shaded in are in front of the hills which also have to be shaded in...so the trees aren't really going to show.
im really confused about this one for several other reasons then the 2 above.






Sketch 3: this one i have tried but stopped in the middle becuase i could see, just by looking at it that it wouldn't turn out right. for this sketch, i was inspired by the following image:

i want the trees to have some type of "character" to them by shading them in...but again, im not really sure about the shading...i dont think im doing it right so far.

i have more, but i'll post later, inshaAllah.

 

[purple]

I have no advise to give you. But your work is good so far.
:wa:

 

[Amoeba]

For the sand dunes you need to specify a light source, i.e. the sun, so that all of the shadows go in the same direction.

Here's a simple example:

Top image shows how the light is cast on a source (unrealistically close to the sand dune objects I admit, in reality the sun rays would be much more parallel to one another because the sun is so far away) and how shadows are made from where the light is blocked. Light orange in the example is a sun ray cast whereas the brown lines show where a shadow is made. The lower example shows how improper lighting can make the image look confusing. My best advice would be to study how shadows are cast in real life on objects, even just looking out your window or playing with a lamp by placing certain objects near it and observing how the shadow is cast. Inshallah it should eventually click in your mind and shading should come more naturally.

As for the trees I don't know what to suggest other than more practice and copying trees from real life and from photographs. Copying is really tiresome and a bit dull but it really helps you get a feel for how trees are designed and shaped, their texture, their growth directions, etc.